Mode triphasé
On considère trois paires de bobines plates identiques équidistantes de l'axe du système et dont les axes font entre eux des angles de 120°. Elles sont parcourues par les courants :
I1 = Io.cos(wt)
I2 = Io.cos(wt + 2p/3)
I3 = Io.cos(wt + 4p/3)
(Courant triphasé)
Dans la simulation, on trace l'évolution au cours du temps de l'induction magnétique due à chaque bobine (trait de la couleur des bobines correspondantes) ainsi que l'induction résultante (trait noir). Le module du vecteur B reste constant mais ce vecteur tourne autour de O à la vitesse constante wt.
C'est sur ce principe que sont basés les moteurs triphasés.


Dans chaque cas,

vous pouvez modifier la fréquence de rotation,

 geler l'animation

et modifier le sens de la rotation du champ résultant.

 Pour ce faire, on permute les courants dans les bobines 1 et 2.

The Three-Phase Generator

The generators of our power supply are built in such a way that they are producing three alternating currents at the same time. The stator contains 3 coils, they are stagged at 120° and the magnet moves past the 3 coins with each full turn and induces three alternating currents, the three-phase current. Its phases differ at 120° towards each other.

Generator design, frame size 710-1250

Quality throughout
All generators are tailor made within a standard, modular concept offering great flexibility to ensure that correct generator characteristics are selected for each installation.
  • Compact and flexible design ensures a good match with the plant's requirements.
  • Rigid construction transfers dynamic and static stresses directly to the foundation.

Rotor with salient poles and solid pole plates

  • Ensures high thermal capacity and stability without the need for special damping winding.
  • Fast control and electric stability through adaptation of the air gap, design of the pole shoes and dimensioning of the pole core.
  • Solid pole shoes provide high damping without damper winding. Good overload capability and low harmonics.
  • Reliable operation and long service life ensured by large cooling surfaces and effective flow of cooling air, which also results in low, uniform rotor temperature. Class H insulation of the coils gives extra thermal margins.
  • Stiff rotor and minimized distance between bearings ensure low vibrations since the operating speed is well below the first critical speed.
  • Over-speed tests are performed as standard. The complete rotor is balanced at operating speed.

Stator design greatly influences stability and performance

  • High-grade, low-loss electrical steel increases efficiency and reduces operating costs.
  • Stiff frame transmits all forces directly to the foundation.
  • Well developed and proven methods for locking the coils into the slots and bracing the coil ends ensure long term reliability.
  • Insulation system that ensures reliability and long service life. The windings are insulated with Mica based tape. When the windings are in place, the complete stator is impregnated in a vacuum pressure impregnation (VPI) process. The windings are of insulation class F, resulting in good thermal margins.
  • Voltage drop and short circuit current limitation are achieved by designing the stator for optimized reactance values.

Bearings and bearing seats provide a reliable stator, rotor and shaft assembly

  • Bearing housing design permits easy access for inspection and maintenance. The bearing housings are insulated from the frame to eliminate circulating currents in the shaft. The shaft can be earthed. Labyrinth seals are used to prevent oil leakage.
  • Sleeve bearings are designed to be insensitive to misalignment and to permit large axial play.
  • Independent lubrication system gives high reliability. Oil rings or oil reservoir ensure lubrication during emergency rundown. Hydrostatic jacking oil systems are available for applications operating at low speed.

Exciter mounted external to the non-drive end bearing

  • Simple but highly developed design having few components and well adapted protection functions offers high reliability and easy access for maintenance.
  • Compact, brushless exciter unit is mounted on the rotor shaft outboard the bearing together with a PMG (permanently magnetized auxiliary generator). No independent support or alignment is required.
  • Improved system performance due to high field forcing possibilities. This is important when faults in the supply network arise and when increasing production of reactive power.

Efficient cooling

  • Very good cooling is obtained thanks to the the symmetrical cooling achieved with the shaft mounted fans, in combination with the design of the rotor and stator.
  • The wide line of cooling forms allows an optimum for the operating and environmental conditions of each application.



3. Features and Operating Performance


3.1. Voltage and Frequency

The generators are manufactured for all rated voltages recommended in the IEC Directives and DIN EN 60034-1 up to 6900 V at either 50 or 60 Hz. For 230 V generators please note power according to Section 3.2. Generators for special voltages and other frequencies can be supplied on request.

3.2. Continuous Power Output

The rated power outputs listed in the type summaries apply for:

  • Continuous operation S1 at 50 Hz rated frequency
  • Power factor cosφ 0.8 (over-excited)
  • Cooling air temperature 40°C
  • Installation height up to 1000 m above sea level
  • practically sine form load current
  • symmetrical phase winding load

With different power factor, cooling air temperature and installation height values the permissible continuous power output can be taken from Tables 3 & 4. If the cooling air temperature reduces with installation height by approx. 1°C per 100 m then the rated power output can be retained. With any other deviations please consult the manufacturer. This applies particularly for the static inverter load (see Section 3.12). During operation at 60 Hz the power output increases by approx. 20 % compared to the 50 Hz power output. Generators with 230 V can be supplied up to a power output of 630 kVA.


Table 3 - Changing of Rated Power Output in Dependence of Cooling Air Temperature and Installation Heigh
RT/°C 30 35 40 45 50 55 60
H/m above sea level PS/PN
up to 1000 1.06 1.03 1.00 0.96 0.92 0.88 0.84
up to 2000 1.02 0.99 0.96 0.92 0.88 0.84 0.80
up to 3000 0.97 0.94 0.91 0.87 0.83 0.79 0.75
up to 4000 0.91 0.88 0.85 0.81 0.77 0.73 0.69



Table 4 - Changing of Rated Power in Dependence of Power Factor
cosφ 0.7 0.6 0.5 0.4 0
PS/PN 0.94 0.89 0.85 0.82 0.80

3.3. Overload Capacity

At rated voltage the generators can be loaded with the 1.5 x rated current up to a period of 2 min at cosφ = 0.5 and once within 6 hours with 1.1 x rated current for 1 hour. Short term current overloads that occur when e. g. large asynchronous motors are connected are permissible. The excitation device is designed in such a way that the rated voltage is retained with a tolerance of approx. - 5 %.

3.4. Marine Classification

All generators for marine applications (Diesel generators, PTO generators, static frequency inverter for mains separation etc.) are supplied in accordance with the regulations of the following classification societies as standard. If supply in accordance with other classification societies is required please consult the manufacturer.


Table 5 - Reduction in Power Output for on Board Ship Applications
Organisation Coolant
temperature °C
Factor to PN
ABS American Bureau of Shipping 50 0.92
BV Bureau Veritas 45 0.96
CCS China Classification Society 45 0.96
DNV Det Norske Veritas 45 0.96
GL Germanischer Lloyd 45 0.96
LRS Lloyds Register of Shipping 45 0.96
MRS Maritime Register of Shipping 45 0.96
RINa Registro Italiano Navale 45 0.92

Compared to the dry land versions according to Table 3 up to Table 4 the reductions in power listed in Table 5 result from the increased cooling air temperature and the permissible winding temperatures laid down by the societies. For marginal applications please consult the manufacturer.

3.5. Efficiency

The efficiencies listed in the type summaries refer to the total generator losses including the excitation machine and the excitation equipment. The tolerances of DIN EN 60034-1 have to be taken into consideration for all values. If agreed generators with an increased efficiency can be supplied for special applications (hydro power generating plants etc.).

3.6. Voltage Form

According to DIN 60034-1 the no load voltage between two phases is practically Sine shaped:

  • max. deviation from the Sine form below 5 %;
  • distortion factor of the interlinked voltage is smaller than 3 %;
  • telephone harmonic factor THF up to 1000 kVA max. 5 %, up to 5000 kVA max. 3 %.

If the phase voltage between phase and star point has to be practically Sine shaped as well (e.g. for emergency power plant according to DIN VDE 0107) then the generatorwindings are implemented in a 2/3 chording factor. Generally this means a reduction in power of approx. 10 %.

3.7. Static Voltage Behaviour, Voltage Setting Range</<>

The desired/set value of the generator voltage will be kept constant on load from no load up to rated load at power factors 0 up to 1.0 with a tolerance of 0.5 up to 1.0 %. This accuracy applies for operation without static device independently on the warming up condition of the generator and a max. 5 % speed modification of the driven machine. The mains voltage can be modified by means of a set-point adjustment device on the regulator and/or in the control panel by ± 5 %. The setting range can be increased and this has to be agreed.
The regulator can be supplied with an input for ± 5 V for the adjustment of the voltage desired set value.

3.8. Dynamic Voltage Behaviour

Sudden changes of load are always inevitably followed by short time voltage changes (see Illustration 4).
These voltage changes depend on the level of current change, on the power factor and on specific machine parameters. Thus when the rated power is connected with a rated power factor then the voltage collapse is approx. 15 - 20 % of the rated voltage. The stabilisation period depends on the frame size and speed of the generator and is between 0.2 and 0.5 s. Through the compoundising system or selection of a high ceiling voltage of the auxiliary winding it is ensured that the rated voltage is reached again quickly.
Guide values for the voltage collapse ΔU' in dependence on the starting current and power factor can be seen in the diagram in Ill. 5. Here it has been assumed that the load is connected to the generator that is running at no-load at rated voltage and that the speed remains constant. If the load is connected on a noticeably preloaded generator then the voltage drop reduces slightly. The diagram applies for 4 pole generators at 50 Hz. For generators with a higher number of poles the values have to be multiplied by 1.1, the same applies for 60 Hz version. If agreed generators with special design for shock load connections with reduced voltage collapse can be supplied.


Illustration 4
Voltage Deviation by switching on and off of Load



Illustration 5
Voltage Dip in Dependence of Starting Current and Power Factor

Connection of Asynchronous Motors
Special dynamic loads occur when large asynchronous motors with squirrel cage rotors are connected. Through the compoundising system with higher current the excitation is also increased so that the connection of asynchronous motors is only limited by the drive and the switchgear with the connected loads. Short time current reductions up to 20 % do not cause any problems under normal circumstances.
If relatively large squirrel cage motors have to be connected directly then the following circumstances have to be taken into consideration when the plant is designed and the set is selected:

  • Is the driving machine of the generator capable of producing the increased generator drive power short time at a certain starting factor of the asynchronous motor?
  • How big can the voltage collapse become without affecting voltage dependent loads, switchgear etc. in a detrimental way?
  • Will the torque of the asynchronous motor be sufficient at voltage drops during the start?
  • Guide values for the connection of Asynchronous motors are as follows:
    • Motor Power/kW approx. 15 % of the Generator Type Power/kVA: ΔU' ca. 20 %
    • Motor Power/kW approx. 30 % of the Generator Type Power/kVA: ΔU' ca. 30 %
    • With star/delta-connection three times the motor powers can be used.

In special drive applications the following start up methods are recommended:

Frequency Start-up
If the generator is supplying an asynchronous motor that has nearly the same power then the full speed running can be achieved by connecting the load when the Diesel set is at a standstill. Subsequently the Diesel engine is started and gradually run up to its rated speed. With increasing speed excitation of the generator takes place and it takes the asynchronous motor along with it up to its rated speed.

Voltage Start-up
The de-excitation of the generator takes place at full speed (see section 3.15). The asynchronous motors is connected in a voltageless condition and another excitation of the generator takes place again. The motor starts up with increasing voltage.

In both cases the pre-requisite is a separate mains for the motor and start-up without counter-torque or with fan or pump curve and also a constant auxiliary voltage for the switchgear.

Transformer Connection
Voltage start-up should be selected for transformers with the same power in order to avoid the inrush / starting current.

3.9. U/f-Regulation

The R10-KF regulator permits a frequency proportional U/f voltage control below the rated speed.
Below a cutoff frequency the generator voltage sinks. Above this value the voltage does not depend on frequency.
When a load is connected to a Diesel set then a short collapse of the generator voltage occurs and the speed of the motor drops. Since the stabilization period for the voltage is considerably shorter than the stabilization period for the speed (approx. 1 : 10) the connected electrical load is practically constant without influencing the voltage whilst the counter-torque for the motor increases additionally because of the speed reduction.
Contrary to that the frequency dependent voltage control removes the load from the motor during the stabilization phase and reduces the stabilization period. Ill. 6 shows the progression of the voltage, torque and speed with and without U/f voltage control. When large loads in proportion to the driving motor power are connected - specifically with motors with turbo loaders - there is the possibiliy to improve the load take-over behaviour of the Diesel sets and to increase the permissible connection power. The R10-KF regulator can be fitted onto the SE Types subsequently without modifications.


Illustration 6
Voltage, Torque and Speed Behaviour by Switching on without (A) and with (B) U/f Voltage Control

3.10. Short Circuit Behavior

The shock short circuit current complies with DIN VDE 60034-1. The required short circuit protection of the generators is ensured. Please consult us if you require generator data for the short circuit calculation. With a three phase terminal short circuit the generator produces a stable continous (permanent) short circuit current of at least 3.5 x IN.
The continuous short circuit current has to be switched off after a maximum of 5 s.
The two phase short circuit current is approx. 1.5 times and the one phase approximately 2 times of the continuous short circuit current.
The shock short circuit current dies away quickly and transforms into the continuous short circuit current after approx. 100 - 150 ms. In order to adhere to the protective measures a continuous short circuit current of up to 5 x IN may be necessary in special cases. In these cases please consult us for special agreement.

Short circuit currents are absolute values that refer to the rated powers; please take into consideration at power deratings!

3.11. Asymmetric Load

It is permissible to supply asymmetric mains as long as the current does not exceed the rated value in any phase. Here it has to be taken into consideration that the voltage deviation, voltage form and the power data no longer reach the rated values. Thus the voltage asymmetry is approx. ± 5 % with a single phase load with rated current and two no-load phases or with two phase rated current and one no-load phase. To ensure optimum operating conditions an as even as possible division of the currents onto the three phases should always be aimed at.

3.12. Static Inverter Load

Static inverters are non-linar loads the connection of which leads to a distortion of the voltage curve, which results in increased generator losses and which possibly impaires the functionality of the other connected loads. In order to keep the consequences of the static inverter load as small as possible the generators are designed in a special way for this. An effective damper winding, 2/3 chording, particularly small subtransient reactances (xd'') and according to the type of load also a larger generator type ensure an optimum operation.
Pre-requisite for the sizing is the knowledge of the plant that is being designed. If possible the following should therefore be stated when placing the order:

  • share / portion of the static inverter load of the total load
  • current distortion factor
  • Inverter type (3, 6, 12 phase inverter)
  • type of the static inverter load (Drives, communicationequipment,battery loading etc.)

Permissible voltage distortion factor of the plant Guide values for the sizing of the generator are as follows:

  • Inverter 12 pulse approx. 80 %,
  • Inverter 6 pulse approx. 70 %,
  • Inverter for variable speed approx. 50 % of the rated power with standard winding.

3.13. Parallel Operation

All generators have a damper cage and are designed for parallel operation. Through the fitted statics device the voltage curves in dependency on load current and power factor receive a slighly falling characteristic to the power proportional reactive load distribution. The voltage statics can be set up to a max. of 6 % (refered to cosφ = 0.8). The prerequisite for a stable parallel operation with power proportional active load division is that the speed regulator of the driving machines is designed accordingly.

3.13.1. Synchronisation

The parallel connection of the generators can be performed using the known synchronisation methods. The voltage, frequency and phase position have to be brought in line.
Following deviations before the connection are permissible:

  • voltage difference max. 10 % of UN
  • frequency deviation max. 2 % of fN
  • error angle max. 15° referred to 180° between two zero transits of the voltage float/hover

For hand synchronisation a better frequency alignment is required!

Since mechanical damage can occur on the generator and the set please ensure that false synchronisations are avoided at all times.

3.13.2. Starting Synchronisation

Certain mains replacement plant must be capable of operation for max. of 15 s (VDE 01108), i.e. after mains failure the emergency power supply must take over the complete power output within this period of time. For plant with several sets this period of time is not sufficient for the driving machines to reach the full speed, for excitation to take place and for the subsequent synchronisation to be carried out. This can be shortened considerably by using start-up synchronisation. With generators of the SE series the start-up synchronisation for the same generators is possible. (With different types please consult the manufacturer.) In this case the generators are connected parallel by means of an equalization lead prior to the start (s. 3.13.4.), are run up to speed jointly, during which they pull themselves in a synchronous phase position at excitation.

3.13.3. Parallel Operation of the identical AEM Generators


Table 6 - Actual Operating Diagram of Gensets
  Effect in single operation Main effect in parallel operation
Voltage set point of generator rise Generator voltage rise Generator supply more reactive power
Voltage set point of generator drop Generator voltage drop Generator supply less reactive power
Speed set point of drive rise Frequency rise Generator supply more active power
Speed set point of drive drop Frequency drop Generator supply less active power

Parallel operation of generators of any type is ensured with the aid of the statics device. The statics of approx. 2 % (referred to the no-load voltage; at rated current and power factor cosφ = 0.8) that is set in the factory ensures a correct parallel operation. Thus the required even distribution of the reactive load onto the individual generators is achieved. The statics can be set using the relevant potentiometer on the regulator. No additional measures are required on the switchgear.

3.13.4. Parallel Operation of the identical AEM Generators

This is possible without any problems using an equalization lead. For this the stator windings of the excitation machines have to be joined after synchronisation. A proportional active load division leads inevitably to a corresponding reactive load distribution. The voltage tolerance remains unchanged compared to single operation. The statics are not necessary.

3.13.5. Parallel Operation with Generators of other Manufactures

For this the generators must have a damper cage and the excitaiton device must be suitable for parallel operation. A correct distribution of the reactive load requires that all generators have a statics device and practically corresponding falling voltage curves can be set.

If the star points are connected then through differences in the voltage form it may be necessary to fit a choke in the star point conductor.

3.13.6. Mains Parallel Operation

To ensure a stable parallel operation of a Genset with rigid mains it is necessary to avoid an overload at changing mains conditions and at the same time utilise the power to the highest possible extent. As far as the active load is concerned this is done by means of the speed regulator or an electronic load distribution. As far as the reactive load is concerned it can be carried out either by means of the statics device or by means of a power factor dependent regulator (cosφ regulator/please take the power limitation according to Table 4 into consideration! )
Regarding the limitation of the current in the start point conductor see Section 2.11.

  • Mains Parallel Operation with Statics
    In order to prevent reactive power overload of a generator at the mains the generator voltage must be lowered by the statics device at increasing reactive load (also see Section 3.13.).
  • Mains Parallel Operation with cosφ Regulator (R10-KC, R9-C Regulator)
    At constant operation at mains a special cosφ regulator can be supplied for reactive current regulation. Apart from the normal voltage regulation it also allows the regulation of the power factor cosφ independently on the active load of the Genset and on the changes of the mains voltage at the infeed point. A cosφ regulator can also be fitted subsequently without any modifications being required.

3.14. Self Excitation

This occurs through remanence in the magnetic circuit of the excitation machine. On start up the excitation of the generator to the rated voltage is carried out through this within approx. 2 - 5 s. In special cases the build-up can be shortened by e.g. special construction or when an external voltage is applied short time to the stator winding of the excitation machine.

3.15. De-excitation

The generator with running engine de-excite if:

  • on the SE types with compoundising device the lead-out terminals (+,-) of the stator winding of the excitation machines are short circuited;
  • on the SH types with direct regulation the auxiliary winding are switched off on the terminals in the terminal box.

On the generator terminals there remains only a residual voltage at the rate of the remanence voltage.

3.16. Overvoltage Protection and Emergency Manual Operation

  • The compoundisation principle with step down regulation that is used for the excitation of the SE types has the advantage that in the event of the regulator failing the excitation current is limited by the choke and the voltage cannot exceed the 1.1 times rated value. If a spare regulator is not available then an emergency operation at a voltage deviation of approx. ±3 % that is not limited in time can be implemented. To do this a variable resistance (approx. 150 Ω, 2 Amp) has to be connected in parallel to the stator winding of the excitation machine after the regulator has been disconnected; this can be used to set the required voltage. Fur-ther instructions are contained in the Operating Instructions for the Excitation Device.
  • With the generator types with direct regulation (SH-types) a fuse in the regulator protects the generators against an excessive voltage increase in the event of the regulator failing. The generator is de-energised when the fuse responds. An emergency manual control can be implemented by connecting a settable external voltage of approx. 50 V DC.

3.17. Behaviour at Underspeed

Underspeed of the driving machine (e.g. warm up running, measurements on motor) is possible without any time limitations:

  • On the SE types the choke limits the excitation current so that even without U/f regulation a speed dependent terminal voltage appears. A load application onto the generator at partial speed is possible with limitations due to the reduced ventilation.
  • With the SH types with direct regulation the regulator contains a U/f function that limits the excitation

3.18. Vibrations

In the standard version the Generators comply with vibration grade N according to DIN ISO 2373. Version in Grades R or S can be agreed. The permissible vibration load through the drive and the installation site is as follows:


10 Hz     s ≤ 0,4 mm
10 - 100 Hz     Veff ≤ 18 mm/s
> 100 Hz     b ≤ 1,6 g


Please consult us if higher values occur.

3.19. Vibrations

The limit values according to DIN EN 60034-9 are complied with.

3.20. Interference Suppression

In the standard version the Interference Suppression Grade N according to DIN VDE 0875 is guaranteed. Interference Suppression Grade K against enquiry.


From Wikipedia, the free encyclopedia

Three-phase power transformer which is the sole transfer point for electricity to a suburban shopping mall in Canada.  Note the four wires used for the 208 V/120Y service: one is for the neutral, and the other three are for the X, Y, and Z phases.

Three-phase power transformer which is the sole transfer point for electricity to a suburban shopping mall in Canada. Note the four wires used for the 208 V/120Y service: one is for the neutral, and the other three are for the X, Y, and Z phases.

Three Phase Electric Power Transmission

Three Phase Electric Power Transmission

Three-phase is a common method of electric power transmission. It is a type of polyphase system used to power motors and many other devices.

This article deals with where, how and why "three phase" is used. For information on the basic mathematics and principles of three phase see three-phase. For information on testing three phase equipment (kit) please see three-phase testing.

Three phase systems may or may not have a neutral wire. A neutral wire allows the three phase system to use a higher voltage while still supporting lower voltage single phase appliances. In high voltage distribution situations it is common not to have a neutral wire as the loads can simply be connected between phases (phase-phase connection).

Three phase has properties that make it very desirable in electric power systems. Firstly the phase currents tend to cancel one another (summing to zero in the case of a linear balanced load). This makes it possible to eliminate the neutral conductor on some lines. Secondly power transfer into a linear balanced load is constant, which helps to reduce generator and motor vibrations. Finally, three-phase systems can produce a magnetic field that rotates in a specified direction, which simplifies the design of electric motors. Three is the lowest phase order to exhibit all of these properties.

Most domestic loads are single phase. Generally three phase power either does not enter domestic houses at all, or where it does, it is split out at the main distribution board.

The three phases are typically indicated by colors which vary by country. See the table for more information.






Generation and distribution

Condiitions for connections of two generators

same rotating phases

same frequence

same voltage

phases inside of a defined window at closing

Animation of three-phase power flowAnimation of three-phase power flow

Animation of three-phase power flow

At the power station, an electrical generator converts mechanical power into a set of alternating electric currents, one from each electromagnetic coil or winding of the generator. The currents are sinusoidal functions of time, all at the same frequency but with different phases. In a three-phase system the phases are spaced equally, giving a phase separation of 120°. The frequency is typically 50 Hz in Europe and 60 Hz in the US and Canada (see List of countries with mains power plugs, voltages and frequencies).

Generators output at a voltage that ranges from hundreds of volts to 30,000 volts. At the power station, transformers "step-up" this voltage to one more suitable for transmission.

After numerous further conversions in the transmission and distribution network the power is finally transformed to the standard mains voltage (i.e. the "household" voltage). The power may already have been split into single phase at this point or it may still be three phase. Where the stepdown is 3 phase, the output of this transformer is usually star connected with the standard mains voltage (120 V in North America and 230 V in Europe) being the phase-neutral voltage. Another system commonly seen in North America is to have a delta connected secondary with a centre tap on one of the windings supplying the ground and neutral. This allows for 240 V three phase as well as three different single phase voltages (120 V between two of the phases and the neutral, 208 V between the third phase (known as a wild leg) and neutral and 240 V between any two phases) to be made available from the same supply.



Single-phase loads

Single-phase loads may be connected to a three-phase system, either by connecting across two live conductors (a phase-to-phase connection), or by connecting between a phase conductor and the system neutral, which is either connected to the center of the Y (star) secondary winding of the supply transformer, or is connected to the center of one winding of a delta transformer (Highleg Delta system). (see Transformer#Polyphase transformers and Split phase ) Single-phase loads should be distributed evenly between the phases of the three-phase system for efficient use of the supply transformer and supply conductors.

The line-to-line voltage of a three-phase system is √3 times the line to neutral voltage. Where the line-to-neutral voltage is a standard utilization voltage, (for example in a 240 V/415 V system) individual single-phase utility customers or loads may each be connected to a different phase of the supply. Where the line-to-neutral voltage is not a common utilization voltage, for example in a 347/600 V system, single-phase loads must be supplied by individual step-down transformers. In multiple-unit residential buildings in North America, lighting and convenience outlets can be connected line-to-neutral to give the 120 V utilization voltage, and high-power loads such as cooking equipment, space heating, water heaters, or air conditioning can be connected across two phases to give 208 V. This practice is common enough that 208 V single-phase equipment is readily available in North America. Attempts to use the more common 120/240 V equipment intended for three-wire single-phase distribution may result in poor performance since 240 V heating equipment will only produce 75% of its rating when operated at 208 V.

Where three phase at low voltage is otherwise in use, it may still be split out into single phase service cables through joints in the supply network or it may be delivered to a master distribution board (breaker panel) at the customer's premises. Connecting an electrical circuit from one phase to the neutral generally supplies the country's standard single phase voltage (120 VAC or 230 VAC) to the circuit.

The power transmission grid is organized so that each phase carries the same magnitude of current out of the major parts of the transmission system. The currents returning from the customers' premises to the last supply transformer all share the neutral wire, but the three-phase system ensures that the sum of the returning currents is approximately zero. The delta wiring of the primary side of that supply transformer means that no neutral is needed in the high voltage side of the network.



Connecting phase-to-phase

Connecting between two phases provides √3 or 173% of the single-phase voltage (208 VAC in US; 400 VAC in Europe) because the out-of-phase waveforms add to provide a higher peak voltage in the resulting waveform. Such connection is referred to as a line to line connection and is usually done with a two-pole circuit breaker. This kind of connection is typically used for high-power appliances, because it can provide nearly twice as much power for the same current. This allows more power to be supplied for a given wire size. This may also allow loads to be served that would otherwise be so large as to exceed the capability of the building's wiring. Existing wiring can be reconnected to provide the higher voltage to the load. In USA, for example, where the single-phase voltage is 120 V, a 2 kW 208 volt electric baseboard heater could require use of a phase-phase connection where only standard wiring exists. (Note that electrical codes typically would require wire color coding to be readjusted in this case.)



Three-phase loads

The rotating magnetic field of a three-phase motor.

The rotating magnetic field of a three-phase motor.

The most important class of three-phase load is the electric motor. A three phase induction motor has a simple design, inherently high starting torque, and high efficiency. Such motors are applied in industry for pumps, fans, blowers, compressors, conveyor drives, and many other kinds of motor-driven equipment. A three-phase motor will be more compact and less costly than a single-phase motor of the same voltage class and rating; and single-phase AC motors above 10 HP (7.5 kW) are uncommon. Three phase motors will also vibrate less and hence last longer than single phase motor of the same power used under the same conditions.

Large air conditioning equipment (for example, most York units above 2.5 tons (8.8 kW) cooling capacity) use three-phase motors for reasons of efficiency , economy and longevity.

Resistance heating loads such as electric boilers or space heating may be connected to three-phase systems. Electric lighting may also be similarly connected. These types of loads do not require the revolving magnetic field characteristic of three-phase motors but take advantage of the higher voltage and power level usually associated with three-phase distribution.

Large rectifier systems may have three-phase inputs; the resulting DC current is easier to filter (smooth) than the output of a single-phase rectifier. Such rectifiers may be used for battery charging, electrolysis processes such as aluminum production, or for operation of DC motors.

An interesting example of a three-phase load is the electric arc furnace used in steelmaking and in refining of ores.

In much of Europe stoves are designed to allow for a three phase feed. Usually the individual heating units are connected between phase and neutral to allow for connection to a single phase supply where this is all that is available.



Phase converters

Occasionally the advantages of three-phase motors make it worthwhile to convert single-phase power to three phase. Small customers, such as residential or farm properties may not have access to a three-phase supply, or may not want to pay for the extra cost of a three-phase service, but may still wish to use three-phase equipment. Such converters may also allow the frequency to be varied allowing speed control. Some locomotives are moving to multi-phase motors driven by such systems even though the incoming supply to a locomotive is nearly always either DC or single phase AC.

Because single-phase power is interrupted at each moment that the voltage crosses zero but three-phase delivers power continuously, any such converter must have a way to store energy for the necessary fraction of a second.

One method for using three-phase equipment on a single-phase supply is with a rotary phase converter, essentially a three-phase motor with special starting arrangements and power factor correction that produces balanced three-phase power. When properly designed these rotary converters can allow satisfactory operation of three-phase equipment such as machine tools on a single phase supply. In such a device, the energy storage is performed by the mechanical inertia (flywheel effect) of the rotating components.

Another method often attempted is with a device referred to as a static phase converter. This method of running three phase equipment is commonly attempted with motor loads though it only supplys 2/3 power and can cause the motor loads to run hot and in some cases overheat. This method will not work when any circuitry is involved such as cnc devices, or in induction and rectifier type loads.

Some devices are made which create an imitation three-phase from three-wire single phase supplies. This is done by creating a third "subphase" between the two live conductors, resulting in a phase separation of 180° − 90° = 90°. Many three-phase devices will run on this configuration, but at lower efficiency.

It can be valuable to look up the various ratings of 3 phase converter technology with the US Phase Converter Standards Organization. They regulate the standards of phase converters manufactured in the US and provide ratings on various technologies used to convert single phase power to three phase power.

Variable frequency drives (also known as solid-state inverters) are used to provide precise speed and torque control of three phase motors. Some models can be powered by a single phase supply. VFDs work by converting the supply voltage to DC and converting the DC to a suitable three phase source for the motor. The drives usually include large capacitors to smooth out supply variations and zero crossing states.



Small scale applications

While most three-phase motors are very big (>750w), there are small (<50w) three-phase motors. The most common example is a computer fan. An inverter circuit inside the fan converts DC to three-phase AC. This is done to decrease noise (as the torque from a three-phase motor is very smooth compared to that from a single phase motor or a brushed DC motor) and increase reliability (as there are no brushes to wear out, unlike a brushed DC motor).



Alternatives to three-phase

  • Two phase power, like three phase, gives constant power transfer to a linear load. For loads which connect each phase to neutral, assuming the load is the same power draw , the two wire system has a neutral current which is greater than neutral current in a three phase system. Also motors aren't entirely linear and this means that despite the theory motors running on three phase tend to run smoother than those on two phase. The generators at Niagara Falls installed in 1895 were the largest generators in the world at the time and were two-phase machines. True two-phase power distribution is essentially obsolete. Special purpose systems may use a two-phase system for control. Two-phase power may be obtained from a three-phase system using an arrangement of transformers called a Scott T.
  • Monocyclic power was a name for an asymmetrical modified two-phase power system used by General Electric around 1897 (championed by Charles Proteus Steinmetz and Elihu Thomson; this usage was reportedly undertaken to avoid patent legalities). In this system, a generator was wound with a full-voltage single phase winding intended for lighting loads, and with a small (usually 1/4 of the line voltage) winding which produced a voltage in quadrature with the main windings. The intention was to use this "power wire" additional winding to provide starting torque for induction motors, with the main winding providing power for lighting loads. After the expiration of the Westinghouse patents on symmetrical two-phase and three-phase power distribution systems, the monocyclic system fell out of use.
  • High phase order systems for power transmission have been built and tested. Such transmission lines use 6 or 12 phases and design practices characteristic of extra-high voltage transmission lines. High-phase order transmission lines may allow transfer of more power through a given transmission line right-of-way without the expense of a HVDC converter at each end of the line.


Color codes

Conductors of a three phase system are usually identified by a color code, to allow for balanced loading and to assure the correct phase rotation for induction motors. Colors used may adhere to old standards or to no standard at all, and may vary even within a single installation. However, the current National Electrical Code (2005) does not require any color identification of conductors other than that of the neutral (white or white with a color stripe), the ground (green or green with a yellow stripe), and, in the case of a High Leg Delta system, the High Leg ("shall be durably and permanently marked by an outer finish that is orange in color or by other effective means"). (NEC 110.15).

  L1 L2 L3 Neutral Ground
North America Black Red Blue White Green
North America (newer 277/480 installations) Brown Orange Yellow White Green
UK until April 2006 (colours in brackets are Harmonised colours) Red (Brown) Yellow (prev. white) (Black) Blue (Grey) Black (Blue) Green/yellow striped (green on very old installations, approx. before 1970)
Europe (including UK) from April 2004 Brown Black Grey Blue Green/yellow striped
Previous European (varies by country) Brown or black Black or brown Black or brown Blue Green/yellow striped
Europe, for busbars Yellow Green Purple
Australia Red White (prev. yellow) Blue Black Green/yellow striped (green on very old installations)

Note that in the U.S. a green/yellow striped wire typically indicates an Isolated ground.










Variable setup and basic definitions

One voltage cycle of a three-phase system, labelled 0 to 360° ( 2 π radians) along the time axis.   The plotted line represents the variation of instantaneous voltage (or current) with respect to time. This cycle will repeat 50 or 60 times per second, depending on the power system frequency. The colours of the lines represent the American color code for three-phase. That is black=VL1 red=VL2 blue=VL3

One voltage cycle of a three-phase system, labelled 0 to 360° ( 2 π radians) along the time axis. The plotted line represents the variation of instantaneous voltage (or current) with respect to time. This cycle will repeat 50 or 60 times per second, depending on the power system frequency. The colours of the lines represent the American color code for three-phase. That is black=VL1 red=VL2 blue=VL3


x=2\pi ft\,\!

where t is time and f is frequency.

Using x = ft the waveforms for the three phases are

V_{L1}=A\sin x\,\!
V_{L2}=A\sin \left(x-\frac{2}{3} \pi\right)
V_{L3}=A\sin \left(x-\frac{4}{3} \pi\right)

where A is the peak voltage and the voltages on L1, L2 and L3 are measured relative to the neutral.



Balanced loads

Generally, in electric power systems the loads are distributed as evenly as practical between the phases. It is usual practice to discuss a balanced system first and then describe the effects of unbalanced systems as deviations from the elementary case.

To keep the calculations simple we shall normalise A and R to 1 for the remainder of these calculations



Star connected systems with neutral



Constant power transfer

An important property of three-phase power is that the power available to a resistive load, P = V I = \frac{V^2}R, is constant at all times.

Using : R = 1 and A = 1

P_{TOT}=\sin^{2} x+\sin^{2} (x-\frac{2}{3} \pi)+\sin^{2} (x-\frac{4}{3} \pi)

Using angle subtraction formulae

P_{TOT}=\sin^{2} x+\left(\sin x\cos\left(\frac{2}{3} \pi\right)-\cos x\sin\left(\frac{2}{3} \pi\right)\right)^{2}+\left(\sin x\cos\left(\frac{4}{3} \pi\right)-\cos x\sin\left(\frac{4}{3} \pi\right)\right)^{2}
P_{TOT}=\sin^{2} x+\left(-\frac{1}{2}\sin x-\frac{\sqrt{3}}{2}\cos x\right)^{2}+\left(-\frac{1}{2}\sin x+\frac{\sqrt{3}}{2}\cos x\right)^{2}
P_{TOT}=\sin^{2} x+\frac{1}{4}\sin^{2} x+\frac{\sqrt{3}}{2}\sin x\cos x +\frac{3}{4}\cos^{2} x+\frac{1}{4}\sin^{2} x-\frac{\sqrt{3}}{2}\sin x\cos x +\frac{3}{4}\cos^{2} x
P_{TOT}=\frac{6}{4}\sin^{2} x+\frac{6}{4}\cos^{2} x
P_{TOT}=\frac{3}{2}(\sin^{2} x+\cos^{2} x)

Using the Pythagorean trigonometric identity


since we have eliminated x we can see that the total power does not vary with time. This is essential for keeping large generators and motors running smoothly.



No neutral current

The neutral current is the sum of the phase currents.

Using : R = 1 and A = 1

I_{N}=\sin x+\sin \left(x-\frac{2}{3} \pi\right)+\sin \left(x-\frac{4}{3} \pi\right)

Using angle subtraction formulae

I_{N}=\sin x+\sin x\cos\left(\frac{2}{3} \pi\right)-\cos x\sin\left(\frac{2}{3} \pi\right)+\sin x\cos\left(\frac{4}{3} \pi\right)-\cos x\sin\left(\frac{4}{3} \pi\right)
I_{N}=\sin x-\frac{1}{2}\sin x-\frac{\sqrt{3}}{2}\cos x-\frac{1}{2}\sin x+\frac{\sqrt{3}}{2}\cos x


Star connected systems without neutral

Since we have shown that the neutral current is zero we can see that removing the neutral core will have no effect on the circuit, provided the system is balanced. In reality such connections are generally used only when the load on the three phases is part of the same piece of equipment (for example a three-phase motor), as otherwise switching loads and slight imbalances would cause large voltage fluctuations.


Three-phase AC is not hard to understand if you use a phasor diagram


Most alternating-current (AC) generation and transmission, and a good part of use, take place through three-phase circuits. If you want to understand electric power, you must know something about three-phase. It is rather simple if you go at it the right way, though it has a reputation for difficulty.

Phase is a frequently-used term around AC. The word comes from Greek fasis, "appearance," from fanein, "to appear." It originally referred to the eternally regular changing appearance of the moon through each month, and then was applied to the periodic changes of some quantity, such as the voltage in an AC circuit. Electrical phase is measured in degrees, with 360° corresponding to a complete cycle. A sinusoidal voltage is proportional to the cosine or sine of the phase.

Three-phase, abbreviated 3φ, refers to three voltages or currents that that differ by a third of a cycle, or 120 electrical degrees, from each other. They go through their maxima in a regular order, called the phase sequence. The three phases could be supplied over six wires, with two wires reserved for the exclusive use of each phase. However, they are generally supplied over only three wires, and the phase or line voltages are the voltages between the three possible pairs of wires. The phase or line currents are the currents in each wire. Voltages and currents are usually expressed as rms or effective values, as in single-phase analysis.

When you connect a load to the three wires, it should be done in such a way that it does not destroy the symmetry. This means that you need three equal loads connected across the three pairs of wires. This looks like an equilateral triangle, or delta, and is called a delta load. Another symmetrical connection would result if you connected one side of each load together, and then the three other ends to the three wires. This looks like a Y, and is called a wye load. These are the only possibilities for a symmetrical load. The center of the Y connection is, in a way, equidistant from each of the three line voltages, and will remain at a constant potential. It is called the neutral, and may be furnished along with the three phase voltages. The benefits of three-phase are realized best for such a symmetrical connection, which is called balanced. If the load is not balanced, the problem is a complicated one one whose solution gives little insight, just numbers. Such problems are best left to computer circuit analysis. Three-phase systems that are roughly balanced (the practical case) can be analyzed profitably by a method called symmetrical components. Here, let us consider only balanced three-phase circuits, which are the most important anyway.

The key to understanding three-phase is to understand the phasor diagram for the voltages or currents. In the diagram at the right, a, b and c represent the three lines, and o represents the neutral. The red phasors are the line or delta voltages, the voltages between the wires. The blue phasors are the wye voltages, the voltages to neutral. They correspond to the two different ways a symmetrical load can be connected. The vectors can be imagined rotating anticlockwise with time with angular velocity ω = 2πf, their projections on the horizontal axis representing the voltages as functions of time. Note how the subscripts on the V's give the points between which the voltage is measured, and the sign of the voltage. Vab is the voltage at point a relative to point b, for example. The same phasor diagram holds for the currents. In this case, the line currents are the blue vectors, and the red vectors are the currents through a delta load. The blue and red vectors differ in phase by 30°, and in magnitude by a factor of √3, as is marked in the diagram.

Suppose we want to take two phase wires and neutral to make a three-wire household service supplying 120 V between each hot wire and ground. The neutral will become the grounded conductor, the two phases the hot conductors. Then, the wye voltage is 120, so the delta voltage will be √3 x 120 = 208 V. This is the three-phase line voltage necessary in this case. Note that the two 120 V sources are not opposite in phase, and will not give 240 V between them. On the other hand, suppose we do want a 240 V service. Then this must be the line voltage, and the voltages to neutral will be 139 V, not 120 V. A 120 V three-phase service will give only 69 V from line to neutral. Note that √3 appears everywhere, and that the differences in phase explain the unexpected results.

If the load consists of general impedances Z, the situation is described by current and voltage phasors connected by V = IZ, both in magnitude and phase. The diagrams are similar in shape, and rotated by the phase angle between voltage and current in each impedance. Remember that the line voltages are the red vectors, while the line currents are the blue vectors. Z relates either the line voltages and delta currents, or the wye voltages and the line currents, depending on the connection. Z does not relate the line current and line voltage, which are different in phase by 30° even for unity power factor (pure resistance load).

This comes out more clearly when we consider the power P delivered to the load. For a resistive delta load, P = 3 VlineIdelta = √3 VlineIline, since Idelta = √3 Iline. For a wye load, P = 3 VwyeIline = √3 VlineIline. This is, of course, the same expression. For other than unity power factor, this must be multiplied by cos θ, which is the angle of Z, not the phase difference between the line voltage and line current. This means, most emphatically, that our usual rule for finding the power from phasors does not apply to three-phase!

If you write out the three phase currents as explicit functions of time, Imaxcos ωt, Imaxcos (ωt - 120°) and Imaxcos (ωt + 120°), square them, multiply by the resistance R, and add, the result is the constant (3/2)Imax2R = 3 I2R. The power is applied steadily as in DC circuits, not in pulses as in single-phase AC circuits. This is a great advantage, giving three-phase machines 48% greater capacity than identical single-phase machines.

In Germany and Switzerland, where three-phase power was originated and developed, it is known as Drehstrom, "rotating current" for this property of constant power. Ordinary AC is called Wechselstrom, or "change current." Nikola Tesla, the discoverer of polyphase currents and inventor of the induction motor, employed two-phase current, where the phase difference is 90°. This also can be used to create a rotating magnetic field, and is more efficient than single-phase, but is not quite as advantageous as three-phase. Two-phase power was once rather common in the United States, where Tesla was important in the introduction of AC, but has now gone completely out of use.

Two-phase can be supplied over three wires, but there is no true neutral, since the phases are not symmetrical. However, it is always easy to double the number of phases in a transformer secondary by making two secondary windings and connecting them in opposing phases. Four-phase does have a neutral, like three-phase, but requires four wires. In fact, three-phase is more economical than any other number of phases. For applications like rectifiers and synchronous converters where DC is produced, it is most efficient to use six-phase AC input, which is easily produced from three-phase in a transformer.

If you are transmitting a certain amount of power single-phase, adding one more conductor operated at the same line voltage and current and using three-phase will increase the power transmitted by 72% with only a 50% increase in the amount of copper and losses. The advantage is obvious. Under certain conditions, transmitting a certain amount of power by three-phase only requires 75% of the copper of single-phase transmission. This is not the major advantage of three-phase, but it does play a factor.

Three wires are usually seen in high-voltage transmission lines, whether on towers or poles, with pin or suspension insulators. Some high-voltage lines are now DC, since solid state devices make it easier to convert to and from AC. The DC lines are free of the problems created by phase, as well as eliminating the skin effect that reduces the effective area of the conductors. It is not nearly as easy to manage long-distance electrical transmission as might be thought.




Unbalanced systems

Practical systems rarely have perfectly balanced loads, currents, voltages or impedances in all three phases. The analysis of unbalanced cases is greatly simplified by the use of the techniques of symmetrical components. An unbalanced system is analyzed as the superposition of three balanced systems, each with the positive, negative or zero sequence of balanced voltages.



Revolving magnetic field

Any polyphase system, by virtue of the time displacement of the currents in the phases, makes it possible to easily generate a magnetic field that revolves at the line frequency. Such a revolving magnetic field makes polyphase induction motors possible. Indeed, where induction motors must run on single-phase power (such as is usually distributed in homes), the motor must contain some measure to produce a revolving field, otherwise the motor cannot generate any stand-still torque and will not start. The field produced by a single-phase winding can provide energy to a motor already rotating, but without auxiliary functions the motor will not accelerate from a stop when energized.



Conversion to other phase systems

Provided two voltage waveforms have at least some relative displacement on the time axis, other than a multiple of a half-cycle, any other polyphase set of voltages can be obtained by an array of passive transformers. Such arrays will evenly balance the polyphase load between the phases of the source system. For example, balanced two-phase power can be obtained from a three-phase network by using two specially constructed transformers, with taps at 50% and 86.6% of the primary voltage. This Scott T connection produces a true two-phase system with 90° time difference between the phases. Another example is the generation of higher-phase-order systems for large rectifier systems, to produce a smoother DC output and to reduce the harmonic currents in the supply.

When three-phase is needed but only single-phase is readily available from the utility company a phase converter can be used to generate three-phase power from the single phase supply. The US Phase Converter Standards Organization conducts independent three phase tests on the various phase converter technologies and publishes the results.

If the frequency (HZ) of the three-phase power supplied does not match the frequency needed to run the machines or equipment a Frequency converter can be used




The power of alternating current (AC) fluctuates. For domestic use for e.g. light bulbs this is not a major problem, since the wire in the light bulb will stay warm for the brief interval while the power drops. Neon lights (and your computer screen) will blink, in fact, but faster than the human eye is able to perceive. For the operation of motors etc. it is useful, however, to have a current with constant power.
Voltage Variation for Three Phase Alternating Current
3-Phase AC graph It is indeed possible to obtain constant power from an AC system by having three separate power lines with alternating current which run in parallel, and where the current phase is shifted one third of the cycle, i.e. the red curve above is running one third of a cycle behind the blue curve, and the yellow curve is running two thirds of a cycle behind the blue curve.
As we learned on the previous page, a full cycle lasts 20 milliseconds (ms) in a 50 Hz grid. Each of the three phases then lag behind the previous one by 20/3 = 6 2/3 ms.
Wherever you look along the horizontal axis in the graph above, you will find that the sum of the three voltages is always zero, and that the difference in voltage between any two phases fluctuates as an alternating current.
On the next page you will see how we connect a generator to a three phase grid.
© Copyright 1997-2003 Danish Wind Industry Association
Updated 12 May 2003
Synchronous Generators
3-Phase Generator (or Motor) Principles
All 3-phase generators (or motors) use a rotating magnetic field.
In the picture to the left we have installed three electromagnets around a circle. Each of the three magnets is connected to its own phase in the three phase electrical grid.
As you can see, each of the three electromagnets alternate between producing a South pole and a North pole towards the centre. The letters are shown in black when the magnetism is strong, and in light grey when the magnetism is weak. The fluctuation in magnetism corresponds exactly to the fluctuation in voltage of each phase. When one phase is at its peak, the other two have the current running in the opposite direction, at half the voltage. Since the timing of current in the three magnets is one third of a cycle apart, the magnetic field will make one complete revolution per cycle.
Synchronous Motor Operation
The compass needle (with the North pole painted red) will follow the magnetic field exactly, and make one revolution per cycle. With a 50 Hz grid, the needle will make 50 revolutions per second, i.e. 50 times 60 = 3000 rpm (revolutions per minute).
In the picture above, we have in fact managed to build what is called a 2-pole permanent magnet synchronous motor. The reason why it is called a synchronous motor, is that the magnet in the centre will rotate at a constant speed which is synchronous with (running exactly like the cycle in) the rotation of the magnetic field.
The reason why it is called a 2-pole motor is that it has one North and one South pole. It may look like three poles to you, but in fact the compass needle feels the pull from the sum of the magnetic fields around its own magnetic field. So, if the magnet at the top is a strong South pole, the two magnets at the bottom will add up to a strong North pole.
The reason why it is called a permanent magnet motor is that the compass needle in the centre is a permanent magnet, not an electromagnet. (You could make a real motor by replacing the compass needle by a powerful permanent magnet, or an electromagnet which maintains its magnetism through a coil (wound around an iron core) which is fed with direct current).
The setup with the three electromagnets is called the stator in the motor, because this part of the motor remains static (in the same place). The compass needle in the centre is called the rotor, obviously because it rotates.
Synchronous Generator Operation
If you start forcing the magnet around (instead of letting the current from the grid move it), you will discover that it works like a generator, sending alternating current back into the grid. (You should have a more powerful magnet to produce much electricity). The more force (torque) you apply, the more electricity you generate, but the generator will still run at the same speed dictated by the frequency of the electrical grid.
You may disconnect the generator completely from the grid, and start your own private 3-phase electricity grid, hooking your lamps up to the three coils around the electromagnets. (Remember the principle of magnetic / electrical induction from the reference manual section of this web site). If you disconnect the generator from the main grid, however, you will have to crank it at a constant rotational speed in order to produce alternating current with a constant frequency. Consequently, with this type of generator you will normally want to use an indirect grid connection of the generator.
In practice, permanent magnet synchronous generators are not used very much. There are several reasons for this. One reason is that permanent magnets tend to become demagnetised by working in the powerful magnetic fields inside a generator. Another reason is that powerful magnets (made of rare earth metals, e.g. Neodynium) are quite expensive, even if prices have dropped lately.
Wind Turbines With Synchronous Generators
Wind turbines which use synchronous generators normally use electromagnets in the rotor which are fed by direct current from the electrical grid. Since the grid supplies alternating current, they first have to convert alternating current to direct current before sending it into the coil windings around the electromagnets in the rotor.
The rotor electromagnets are connected to the current by using brushes and slip rings on the axle (shaft) of the generator.
© Copyright 1997-2003 Danish Wind Industry Association
Updated 19 September 2003
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Three-phase Y and Δ configurations

Three-phase Y and Δ configurations

Initially we explored the idea of three-phase power systems by connecting three voltage sources together in what is commonly known as the "Y" (or "star") configuration. This configuration of voltage sources is characterized by a common connection point joining one side of each source:

If we draw a circuit showing each voltage source to be a coil of wire (alternator or transformer winding) and do some slight rearranging, the "Y" configuration becomes more obvious:

The three conductors leading away from the voltage sources (windings) toward a load are typically called lines, while the windings themselves are typically called phases. In a Y-connected system, there may or may not be a neutral wire attached at the junction point in the middle, although it certainly helps alleviate potential problems should one element of a three-phase load fail open, as discussed earlier:

When we measure voltage and current in three-phase systems, we need to be specific as to where we're measuring. Line voltage refers to the amount of voltage measured between any two line conductors in a balanced three-phase system. With the above circuit, the line voltage is roughly 208 volts. Phase voltage refers to the voltage measured across any one component (source winding or load impedance) in a balanced three-phase source or load. For the circuit shown above, the phase voltage is 120 volts. The terms line current and phase current follow the same logic: the former referring to current through any one line conductor, and the latter to current through any one component.

Y-connected sources and loads always have line voltages greater than phase voltages, and line currents equal to phase currents. If the Y-connected source or load is balanced, the line voltage will be equal to the phase voltage times the square root of 3:

However, the "Y" configuration is not the only valid one for connecting three-phase voltage source or load elements together. Another configuration is known as the "Delta," for its geometric resemblance to the Greek letter of the same name (Δ). Take close notice of the polarity for each winding in the drawing below:

At first glance it seems as though three voltage sources like this would create a short-circuit, electrons flowing around the triangle with nothing but the internal impedance of the windings to hold them back. Due to the phase angles of these three voltage sources, however, this is not the case.

One quick check of this is to use Kirchhoff's Voltage Law to see if the three voltages around the loop add up to zero. If they do, then there will be no voltage available to push current around and around that loop, and consequently there will be no circulating current. Starting with the top winding and progressing counter-clockwise, our KVL expression looks something like this:

Indeed, if we add these three vector quantities together, they do add up to zero. Another way to verify the fact that these three voltage sources can be connected together in a loop without resulting in circulating currents is to open up the loop at one junction point and calculate voltage across the break:

Starting with the right winding (120 V ∠ 120o) and progressing counter-clockwise, our KVL equation looks like this:

Sure enough, there will be zero voltage across the break, telling us that no current will circulate within the triangular loop of windings when that connection is made complete.

Having established that a Δ-connected three-phase voltage source will not burn itself to a crisp due to circulating currents, we turn to its practical use as a source of power in three-phase circuits. Because each pair of line conductors is connected directly across a single winding in a Δ circuit, the line voltage will be equal to the phase voltage. Conversely, because each line conductor attaches at a node between two windings, the line current will be the vector sum of the two joining phase currents. Not surprisingly, the resulting equations for a Δ configuration are as follows:

Let's see how this works in an example circuit:

With each load resistance receiving 120 volts from its respective phase winding at the source, the current in each phase of this circuit will be 83.33 amps:

So, the each line current in this three-phase power system is equal to 144.34 amps, substantially more than the line currents in the Y-connected system we looked at earlier. One might wonder if we've lost all the advantages of three-phase power here, given the fact that we have such greater conductor currents, necessitating thicker, more costly wire. The answer is no. Although this circuit would require three number 1 gage copper conductors (at 1000 feet of distance between source and load this equates to a little over 750 pounds of copper for the whole system), it is still less than the 1000+ pounds of copper required for a single-phase system delivering the same power (30 kW) at the same voltage (120 volts conductor-to-conductor).

One distinct advantage of a Δ-connected system is its lack of a neutral wire. With a Y-connected system, a neutral wire was needed in case one of the phase loads were to fail open (or be turned off), in order to keep the phase voltages at the load from changing. This is not necessary (or even possible!) in a Δ-connected circuit. With each load phase element directly connected across a respective source phase winding, the phase voltage will be constant regardless of open failures in the load elements.

Perhaps the greatest advantage of the Δ-connected source is its fault tolerance. It is possible for one of the windings in a Δ-connected three-phase source to fail open without affecting load voltage or current!

The only consequence of a source winding failing open for a Δ-connected source is increased phase current in the remaining windings. Compare this fault tolerance with a Y-connected system suffering an open source winding:

With a Δ-connected load, two of the resistances suffer reduced voltage while one remains at the original line voltage, 208. A Y-connected load suffers an even worse fate with the same winding failure in a Y-connected source:

In this case, two load resistances suffer reduced voltage while the third loses supply voltage completely! For this reason, Δ-connected sources are preferred for reliability. However, if dual voltages are needed (e.g. 120/208) or preferred for lower line currents, Y-connected systems are the configuration of choice.

Three-phase power systems

Three-phase power systems

Split-phase power systems achieve their high conductor efficiency and low safety risk by splitting up the total voltage into lesser parts and powering multiple loads at those lesser voltages, while drawing currents at levels typical of a full-voltage system. This technique, by the way, works just as well for DC power systems as it does for single-phase AC systems. Such systems are usually referred to as three-wire systems rather than split-phase because "phase" is a concept restricted to AC.

But we know from our experience with vectors and complex numbers that AC voltages don't always add up as we think they would if they are out of phase with each other. This principle, applied to power systems, can be put to use to make power systems with even greater conductor efficiencies and lower shock hazard than with split-phase.

Suppose that we had two sources of AC voltage connected in series just like the split-phase system we saw before, except that each voltage source was 120o out of phase with the other:

Since each voltage source is 120 volts, and each load resistor is connected directly in parallel with its respective source, the voltage across each load must be 120 volts as well. Given load currents of 83.33 amps, each load must still be dissipating 10 kilowatts of power. However, voltage between the two "hot" wires is not 240 volts (120 ∠ 0o - 120 ∠ 180o) because the phase difference between the two sources is not 180o. Instead, the voltage is:

Nominally, we say that the voltage between "hot" conductors is 208 volts (rounding up), and thus the power system voltage is designated as 120/208.

If we calculate the current through the "neutral" conductor, we find that it is not zero, even with balanced load resistances. Kirchhoff's Current Law tells us that the currents entering and exiting the node between the two loads must be zero:



So, we find that the "neutral" wire is carrying a full 83.33 amps, just like each "hot" wire.

Note that we are still conveying 20 kW of total power to the two loads, with each load's "hot" wire carrying 83.33 amps as before. With the same amount of current through each "hot" wire, we must use the same gage copper conductors, so we haven't reduced system cost over the split-phase 120/240 system. However, we have realized a gain in safety, because the overall voltage between the two "hot" conductors is 32 volts lower than it was in the split-phase system (208 volts instead of 240 volts).

The fact that the neutral wire is carrying 83.33 amps of current raises an interesting possibility: since it's carrying current anyway, why not use that third wire as another "hot" conductor, powering another load resistor with a third 120 volt source having a phase angle of 240o? That way, we could transmit more power (another 10 kW) without having to add any more conductors. Let's see how this might look:

A full mathematical analysis of all the voltages and currents in this circuit would necessitate the use of a network theorem, the easiest being the Superposition Theorem. I'll spare you the long, drawn-out calculations because you should be able to intuitively understand that the three voltage sources at three different phase angles will deliver 120 volts each to a balanced triad of load resistors. For proof of this, we can use SPICE to do the math for us:


120/208 polyphase power system  
v1 1 0 ac 120 0 sin     
v2 2 0 ac 120 120 sin   
v3 3 0 ac 120 240 sin   
r1 1 4 1.44     
r2 2 4 1.44     
r3 3 4 1.44     
.ac lin 1 60 60 
.print ac v(1,4) v(2,4) v(3,4)  
.print ac v(1,2) v(2,3) v(3,1)
.print ac i(v1) i(v2) i(v3)  


freq        v(1,4)      v(2,4)      v(3,4)   
6.000E+01   1.200E+02   1.200E+02   1.200E+02


freq        v(1,2)      v(2,3)      v(3,1)  
6.000E+01   2.078E+02   2.078E+02   2.078E+02 


freq        i(v1)       i(v2)       i(v3)     
6.000E+01   8.333E+01   8.333E+01   8.333E+01


Sure enough, we get 120 volts across each load resistor, with (approximately) 208 volts between any two "hot" conductors and conductor currents equal to 83.33 amps. At that current and voltage, each load will be dissipating 10 kW of power. Notice that this circuit has no "neutral" conductor to ensure stable voltage to all loads if one should open. What we have here is a situation similar to our split-phase power circuit with no "neutral" conductor: if one load should happen to fail open, the voltage drops across the remaining load(s) will change. To ensure load voltage stability in the even of another load opening, we need a neutral wire to connect the source node and load node together:

So long as the loads remain balanced (equal resistance, equal currents), the neutral wire will not have to carry any current at all. It is there just in case one or more load resistors should fail open (or be shut off through a disconnecting switch).

This circuit we've been analyzing with three voltage sources is called a polyphase circuit. The prefix "poly" simply means "more than one," as in "polytheism" (belief in more than one deity), polygon" (a geometrical shape made of multiple line segments: for example, pentagon and hexagon), and "polyatomic" (a substance composed of multiple types of atoms). Since the voltage sources are all at different phase angles (in this case, three different phase angles), this is a "polyphase" circuit. More specifically, it is a three-phase circuit, the kind used predominantly in large power distribution systems.

Let's survey the advantages of a three-phase power system over a single-phase system of equivalent load voltage and power capacity. A single-phase system with three loads connected directly in parallel would have a very high total current (83.33 times 3, or 250 amps:

This would necessitate 3/0 gage copper wire (very large!), at about 510 pounds per thousand feet, and with a considerable price tag attached. If the distance from source to load was 1000 feet, we would need over a half-ton of copper wire to do the job. On the other hand, we could build a split-phase system with two 15 kW, 120 volt loads:

Our current is half of what it was with the simple parallel circuit, which is a great improvement. We could get away with using number 2 gage copper wire at a total mass of about 600 pounds, figuring about 200 pounds per thousand feet with three runs of 1000 feet each between source and loads. However, we also have to consider the increased safety hazard of having 240 volts present in the system, even though each load only receives 120 volts. Overall, there is greater potential for dangerous electric shock to occur.

When we contrast these two examples against our three-phase system, the advantages are quite clear. First, the conductor currents are quite a bit less (83.33 amps versus 125 or 250 amps), permitting the use of much thinner and lighter wire. We can use number 4 gage wire at about 125 pounds per thousand feet, which will total 500 pounds (four runs of 1000 feet each) for our example circuit. This represents a significant cost savings over the split-phase system, with the additional benefit that the maximum voltage in the system is lower (208 versus 240).

One question remains to be answered: how in the world do we get three AC voltage sources whose phase angles are exactly 120o apart? Obviously we can't center-tap a transformer or alternator winding like we did in the split-phase system, since that can only give us voltage waveforms that are either in phase or 180o out of phase. Perhaps we could figure out some way to use capacitors and inductors to create phase shifts of 120o, but then those phase shifts would depend on the phase angles of our load impedances as well (substituting a capacitive or inductive load for a resistive load would change everything!).

The best way to get the phase shifts we're looking for is to generate it at the source: construct the AC generator (alternator) providing the power in such a way that the rotating magnetic field passes by three sets of wire windings, each set spaced 120o apart around the circumference of the machine:

Together, the six "pole" windings of a three-phase alternator are connected to comprise three winding pairs, each pair producing AC voltage with a phase angle 120o shifted from either of the other two winding pairs. The interconnections between pairs of windings (as shown for the single-phase alternator: the jumper wire between windings 1a and 1b) have been omitted from the three-phase alternator drawing for simplicity.

In our example circuit, we showed the three voltage sources connected together in a "Y" configuration (sometimes called the "star" configuration), with one lead of each source tied to a common point (the node where we attached the "neutral" conductor). The common way to depict this connection scheme is to draw the windings in the shape of a "Y" like this:

The "Y" configuration is not the only option open to us, but it is probably the easiest to understand at first. More to come on this subject later in the chapter.


Technische Stromarten: Gleichstrom, Wechselstrom und Drehstrom

Technische Stromarten:



Im einfachsten Fall fließt ein zeitlich konstanter Strom. Einen solchen Strom nennt man Gleichstrom (engl. direct current).

techn. und phys. Stromrichtung

techn. und phys. Stromrichtung

Zu beachten ist die Technische Stromrichtung: Vereinbarungsgemäß wird eine Stromrichtung von Plus nach Minus angenommen. Diese Stromrichtung geht auch in alle physikalischen Gleichungen ein, die den Strom als solchen betreffen. Eine elektrische Spannungsdifferenz ist jedoch immer von Plus nach Minus positiv. Daher ist die technische Stromrichtung sinnvoll und wird üblicherweise verwendet, damit die Richtung von Strom und Spannung identisch ist. Die technische Stromrichtung ist nicht zu verwechseln mit der Flussrichtung der Elektronen (negative Ladungträger), die entgegen der technischen Stromrichtung fließen. Siehe auch Technische und physikalische Stromrichtung.

Physikalische Stromrichtung: Um den Mechanismus des Stromflusses zu verstehen und bestimmte elektrische Eigenschaften von Materialien herzuleiten, betrachtet man die wirkliche Bewegung der Ladungsträger. In Metallen bewegen sich in der Regel Elektronen, also negative Ladungsträger, die vom Minus-Pol zum Plus-Pol fließen, denn am Minus-Pol herrscht ein Überschuss an Elektronen, und/oder am Plus-Pol ein Mangel, der durch den elektrischen Strom ausgeglichen wird, sobald der Stromkreis geschlossen wird.

In elektrisch leitfähigen Flüssigkeiten sind gegebenenfalls positive und negative Ladungsträger oder reduzierbare und oxidierbare Stoffe vorhanden, die sich zu den jeweiligen Polen hinbewegen. An den Polen werden sie reduziert bzw. oxidiert, nehmen also an einem Pol Elektronen auf und geben Elektronen an dem anderen Pol ab und überbrücken dadurch die Übertragung von Elektronen im Stromkreis.

In einem Experiment mit einer wässrigen Lösung zur Feststellung der Stromrichtung wurde die physikalisch falsche, technische Stromrichtung ermittelt, da nur die positiven Ladungsträger sichtbar waren, die sich allerdings auf den Minus-Pol zubewegen.

Ein anderer Fall tritt bei p-dotierten Halbleitern auf: Hier verhalten sich fehlende Elektronen (so genannte Löcher oder Defektelektronen) wie positive Ladungsträger mit Masse. Da in der Löcherleitung die Elektronen die Löcher füllen wandern tatsächlich die Elektronen und hinterlassen an ihrem vorherigen Ort ein Loch. Daher wandern die Löcher in die entgegengesetzte Richtung der Elektronen.

Als Gleichspannungsquelle kommen galvanische Zellen (Batterien), entsprechende Dynamos (zum Teil mit nachgeschalteter Gleichrichtung), photovoltaische Zellen (Solaranlagen) oder Schaltnetzteile in Frage. In der Technik häufig anzutreffen ist auch eine Kombination von Transformator und Gleichrichter.

Fällt bei gleich bleibender Stromrichtung die Spannung (und damit, sofern ein Verbraucher angeschlossen ist, die Stromstärke) periodisch stark ab, so spricht man von einer pulsierenden Gleichspannung. Gleichrichter liefern beim Umwandeln von Wechselspannung in Gleichspannung meist pulsierende Gleichspannung, sofern die Spannung nicht durch Kondensatoren oder andere Maßnahmen geglättet wird.



Drehstrom - die wundersame Dreieinigkeit von drei verketteten Wechselströmen

Neben der Möglichkeit des Ferntransports von elektrischer Energie bietet Wechselstrom noch weitere Vorteile, wenn er als "Drehstrom" angewendet wird. Schon bei der ersten Fernübertragung elektrischer Energie, die 1891 anläßlich der Frankfurter Elektrizitätsausstellung stattfand, wurde davon Gebrauch gemacht. Auf der Drehstromtechnik basiert heute die gesamte Stromwirtschaft.

Bei Drehstrom handelt es sich um drei Wechselströme, die im selben Generator erzeugt werden. Die "Phasen" dieser Wechselströme - also das Auf und Ab der Sinuskurve bei jeder Drehung der Generatorachse um 360 Winkelgrade - sind um jeweils 120 Winkelgrade gegeneinander versetzt. So entsteht ein dreiphasiger Wechselstrom, dessen Sinuskurven sich gleichmäßig überlagern. Betrachtet man die miteinander verketteten Phasen des Drehstroms auf dem Diagramm rechts, wird man feststellen, daß sich die Ausschläge der Sinuskurven in den positiven oder negativen Bereich in jedem Augenblick zur Gesamtsumme null ergänzen.

Drehfeld für robuste Motoren

Der Drehstrom verdankt seinen Namen der Anwendung für den Betrieb von Elektromotoren. Er erzeugt nämlich in den Ständerwicklungen dieser Motoren ein magnetisches Drehfeld, das den Rotor mitnimmt und so die Drehbewegung des Motors erzeugt. Dadurch entfällt der verschleißträchtige Stromwender, der beim Gleichstrommotor auf mechanische Weise für die Stromversorgung und Umpolung des Magnetfeldes im Rotor sorgen muß. Drehstrommotoren brauchen nicht einmal Bürsten und Schleifringe. Ihre Rotoren können sich völlig kontaktfrei drehen, wenn sie den Strom für den Aufbau des Rotorfelds auf induktivem Wege aus dem Magnetfeld des Ständers beziehen ("Käfigläufer"). Deshalb sind Drehstrommotoren äußerst robust und leistungsfähig. Dasselbe gilt für Drehstromgeneratoren.

Stern-und Dreieck-Schaltung

Drehstrom benötigt insgesamt drei Leitungen. Das mag auf den ersten Blick überraschen, da schon für einphasigen Wechselstrom zwei Leitungen erforderlich sind. Demnach bräuchte man für drei Phasen eigentlich sechs Leitungen. Die Rückleitungen können jedoch bei Drehstrom entfallen, da sich die drei Phasen in jedem Augenblick zu null ergänzen. Voraussetzung ist, daß der elektrische Verbraucher an alle drei Phasen angeschlossen wird und diese gleichmäßig belastet. Jedes Drehstrom-Gerät setzt sich deshalb aus drei gleichgearteten elektrischen Teil-Verbrauchern zusammen. Für den Anschluß dieser Teil-Verbraucher an die drei Phasen gibt es zwei Möglichkeiten:

Stern-Schaltung: Man legt jede Eingangsklemme der drei Teil-Verbraucher an eine der Phasen und verbindet die Ausgangsklemmen untereinander.

Dreieck-Schaltung: Die drei Teil-Verbraucher werden jeweils so zwischen zwei Phasen gelegt, daß jede Ausgangsklemme mit einer Eingangsklemme verbunden ist.


Bei Stern-Schaltung werden die Eingänge der drei Teilverbraucher des Drehstromgeräts (rot) mit den Leitern R, S, T und die Ausgänge sternförmig miteinander verbunden. Im Niederspannungsnetz erhält dadurch jeder der drei Teilverbraucher die "Strangspannung" von 230 Volt, die zwischen jedem der drei Leiter und dem gemeinsamen Sternpunkt besteht.



Bei Dreieck-Schaltung werden die Ein- und Ausgänge der drei Teilverbraucher in Form eines Dreiecks miteinander verbunden. Zugleich wird jede "Spitze" des Dreiecks an eine der drei Phasen gelegt. Im Niederspannungsnetz erhält so jeder der Teilverbraucher die "Leiterspannung" von 400 Volt, die jeweils zwischen zwei der drei Phasen-Leiter besteht.


Je nach Art der Schaltung verketten sich die Stromstärken der drei Phasen oder deren Spannungen. So läßt sich zum Beispiel aus demselben Drehstrom durch Sternpunkt-Schaltung eine Spannung von 230 Volt oder durch Dreieck-Schaltung eine Spannung von 400 Volt gewinnen.

Die Transformatoren, die der Strom auf seinem Weg vom Kraftwerk zur Steckdose passieren muß, sind entweder im Stern oder Dreieck geschaltet. Zum Beispiel verwendet man beim Übergang von der Mittel- auf die Niederspannung üblicherweise die Dreiecksschaltung für die Oberspannungs-Seite des Transformators (30 kV oder 10 kV) und die Sternschaltung für die Niederspannungs-Seite (400/230 Volt).



Wenn elektrische Geräte im Stern oder im Dreieck an die drei Drehstrom-Leiter R, S und T angeschlossen werden, ist eine Rückleitung nicht erforderlich, da R, S und T gleichmäßig (symmetrisch) belastet werden. Voraussetzung ist natürlich, daß die drei Teilverbraucher des Drehstromgeräts exakt dieselben Leistungswerte haben.





Einphasige Wechselstromgeräte können auf diese Weise an die drei Drehstrom-Leiter angeschlossen werden. Allerdings werden dadurch R, S und T ungleichmäßig belastet. Damit die Unsymmetrien keine störenden Auswirkungen haben, ist der "Neutralleiter" N als Rückleiter erforderlich. Er ist mit der Stromquelle (Generator oder Transformator) in deren Sternpunkt verbunden und wurde deshalb früher auch als "Mittelpunktleiter" bezeichnet. Je nach Belastung der drei Drehstrom-Leiter fließt im Neutralleiter ein Ausgleichsstrom.

Der Endverbraucher kann seine elektrischen Geräte wahlweise in Sternschaltung (mit 230 Volt) oder in Dreiecksschaltung (mit 400 Volt) betreiben. Die dafür erforderlichen Steckdosen erkennt man an den drei Kontakten für die drei Phasen-Leiter (außerdem verfügen sie über den üblichen Schutzkontakt). Eine besondere Führungs-"Nase" gewährleistet, daß die Phasen beim Einstöpseln des Steckers übereinstimmen. Beim Vertauschen der Phasen laufen Drehstrommotoren nämlich rückwärts, was unliebsame Folgen haben kann.

Drehstrom-Steckdosen und drehstromtaugliche Geräte findet man aber normalerweise nur in Gewerbe- und Industriebetrieben. In Haushalten sind sie auf Ausnahmefälle beschränkt, etwa auf einen besonderen "Kraft"-Anschluß für Waschmaschinen oder leistungsstarke Heizgeräte.

Das Prinzip der Drehstrom-Übertragung ist seit über hundert Jahren unverändert


Diese Abbildung zeigt das Prinzip der Drehstromübertragung, wie es F. A. Haselwander 1888 in seiner Patentanmeldung skizziert hat: Links der Generator, rechts der Motor; dazwischen die beiden Trafos, die den Strom für den Transport über die Fernleitungen (Strichel-Linien in der Mitte) erst hoch- und dann wieder heruntertransformieren. Dieses Prinzip ist bis heute unverändert gültig.

Übrigens hatte Haselwander mit seiner Patentanmeldung nicht viel Glück: Zunächst verschlampte sein Anwalt die Anmeldung, dann wurde sie wegen angeblicher Unklarheiten in der Beschreibung beanstandet und schließlich aus formalen Gründen zurückgewiesen. Als es ihm später doch noch gelang, seine Erfindung schützen zu lassen, wurde das Patent auf die Klage eines Elektrounternehmens hin für ungültig erklärt.

YOUR EXPERT for Resistance Grounding Experts - Sponsored by I-Gard

Tony Locker

Tony Locker
Director, Business Development
(888) 737-4787
Tony Locker received a BSEE from Rose-Hulman Institute of Technology and a MSEE from Georgia Institute of Technology. He is presently Director, Business Development with I-Gard, Cincinnati, Ohio, where he is responsible for providing technical seminars and application assistance on resistance grounding and ground fault protection in the USA. Prior to joining I-Gard in 2005, he was Vice President – Engineering with Post Glover Resistors. Previously, he was Director of Engineering at Power Engineering Technology. His background includes leading design teams and managing installations of numerous multi-million dollar control and power systems for data centers, industrial plants, cogeneration facilities, and utilities. Before Power Engineering Technology, he was a R&D engineer for Square D Company.

Mr. Locker is active in the Industry Applications and Power Engineering Societies of IEEE, where he serves on several Subcommittees and Working Groups, as well as NSPE. He is a registered Professional Engineer in Ohio and has two patents.

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regarding the neutral and ground
July 18, 2006 @ 5:05 am
Q. When a current of 100A is passing through the phase, what will be the status of neutral and ground, will the same current will be passing even from them,,,,,
A. I am assuming that you are referring to a single-phase circuit (because ideally no current would be flowing in a three-phase circuit a solidly grounded system. In a single-phase circuit, the “flowing” current must flow to the load via the phase conductor and return via the neutral conductor. So, the current flow must return via the neutral conductor (in your case, 100A). The ground conductor should not have any current. In fact, this is how a GFCI works on a single-phase circuit. Generally speaking, both the phase and neutral conductors are fed thru a window CT. The current (from the source) in the phase conductor (100A) produces a flux in the CT in one direction, and the current in the neutral (traveling the other way back to the source) produces a flux in the CT in the other direction. The net result … 0 flux IF the currents are the same. If the currents are not, the flux will not be the same and a voltage will be generated by the CT, which can be detected by a relay. The most common way for a difference in current is current returning back to source via ground conductor. So, if the phase conductor is 100A and the neutral is 95A, 5A must be traveling back to source via ground conductor. (This will be detected by the CT and relay.) Zero-sequence CT’s will the same way, except all three phase conductors are fed thru one window CT, which measures the net flux. If the current traveling down one conductor returns thru the other two, then the net flux is 0. If some current returns thru ground conductor, then the net flux is NOT 0 and a ground fault relay will alarm.


Neutral connection during paralleling process
July 13, 2006 @ 4:23 am
Q. Although you recommend that only one generator need to be grounded during parallel operation, do we need to connect the follower generator’s ground during paralleling process?, if we do, what the reason for that?
A. I do not recommend that only one generator be grounded during paralleling operation, I would rather see either the paralleling bus have a common ground or each generator have a resistance-ground to avoid circulating currents. Please see my responses dated on July 7th and 8th.


Correction to Previous Q&A
July 12, 2006 @ 9:10 am
Q. Tony,
One of our astute readers read your Q&A item on “using a GFCI in lieu of a grounding conductor” and then sent me an NEC reference that allows for replacement of a non-grounding type receptacle with a GFCI. I think it would be valuable if you added this reference and some additional text to your existing answer to help clarify this situation.
See 406.3(D)(3)(b) of the 2005 NEC.
A. The original question was: can a gfci be used in lieu of a grounding conductor? is there a specific NEC paragraph that addresses this? It is my understanding of the question that the questioner has a choice, either to install a grounding conductor or gfci. I do not recommend doing this. Whenever possible, install a grounding conductor. When giving a choice, I am not aware of a NEC paragraph that addresses this issue. NEC 406.3 (D) (3) (b) addresses replaces an old-style non-grounding type receptacle with a new gfci. This is permissible given that a grounding conductor is not available as the gfci must be labeled “No Equipment Ground”.


Circuit breaker triped .
July 12, 2006 @ 6:36 am
Q. Dear Sir,
In over network Main Circuit breaker Tripped due to one small Circuit Photo Cell Short Circuit occurred. In this circuit having 05 NOS Circuit Breakers having different rating like 20Amps, 50Amps, 100Amps,350Amps,1600Amps and 2000Amps. So, Main 2000Amps tripped on Ground Tripped set 1200Amps set as @ 0.4 and time 0.1 sec, We can change/increase this setting? On safe side. Our Natural & Ground is same. Not Repeat this trip or no?
A. Unfortunately this is a common problem with solidly grounded systems. Per the National Electric Code 230-95, Ground Fault Protection is required for solidly grounded systems <600V and >1000A. The maximum setting is 1200A and 1sec. If I understand your question, you have a 1200A GFI set at 0.4, or 480A at 0.1sec. With this setting, you are hoping that a downstream circuit breaker (CB) clears the fault before the main circuit breaker (MCB) does. Since I do not know the manufacturer and type of CB’s, I am going to use IEC 60898-1 to check coordination. The instantaneous trip at 0.1sec can range from 5 to 20 times rated current. So, let’s look at each downstream CB that you mentioned: 20A * 5 = 100A 20A * 20 = 400A (20A CB will trip prior to MCB) 50A * 5 = 250A 50A * 20 = 1000A (50A CB may or may not trip prior to MCB) 100A * 5 = 500A 100A * 20 = 2000A (100A CB will not trip prior to MCB) 350A * 5 = 1750A 350A * 20 = 7000A (350A CB will not trip prior to MCB) As you can see, your system does not appear to be coordinated i.e. a 100A CB will not clear a low level ground fault before the MCB. What causes a low level ground fault? An arcing fault is resistive in nature, so an arcing ground fault is ~35% of a bolted fault. Often times, the fault current will not be high enough to for the CB to react quickly to trip. The result is a low magnitude fault current that lasts until the MCB clears as shown above ONLY IF you are lucky. If not, the fault escalates into a severe arc flash / blast incident. It is estimated that 85-95% of all electrical faults are ground faults, which typically start out as an arcing fault. There are several ways to solve your problem. 1) You could increase your current setting and/or time. Even though this may help with coordination, your system has become less safe. Your exposure to Arc Flash / Blast will significantly increase. The base equation below shows that an increase in time is directly proportional to an increase in energy and an increase in current is proportional by the square. Energy = Current * Current * Resistance * time 2) A better solution is to install Zone-Selective Interlocking Protection (see our Application Guide on MGFR relays This will prevent the MCB from unwanted tripping on a ground fault downstream AND significantly reduces your exposure to Arc Flash / Blasts.


ELCBs in parallel circuits
July 12, 2006 @ 6:05 am
Q. In a circuit for Emergency lighting fixtures (one with built-in battery) there are two ELCBs installed, one in lighting power and one in charger circuit. Both circuits are single phase and are derived from common main busbars.
The problem is that these fixtures have a common Neutral terminal for both supplies, thus neccesiating to terminate neutral of both circuits together. When one circuit is switched on, it is fine, but when the second ELCB is turned on, the neutral current gets divided and ELCBs trip.
Request explain what practice should be followed when there are 2 ELCBs and loads having common neutral.
A. You have three possibilities to consider. 1) Separate the neutrals and feed them through the elcb to each load separately. This may not be possible with outopening the fixture and changing the wiring. 2) Use 1 phase 120/240 V circuit. This comprises of phase to phase voltage of 240V and you will have a two pole elcb with common neutral. This is common in North America and may not be possible in your distribution system. 3) Change the two breakers to normal mcbs and add elcb serving both loads on the line side. This will use the common neutral and will not cause the trip.