# Induction generator

An induction generator or asynchronous generator is a type of AC electrical generator that uses the principles of induction motors to produce power. Induction generators operate by mechanically turning their rotor in generator mode, giving negative slip. In most cases, a regular AC asynchronous motor is used as a generator, without any internal modifications.

## Principle of operation

Induction generators and motors produce electrical power when their rotor is turned faster than the synchronous frequency. For a typical four-pole motor (two pairs of poles on stator) operating on a 60 Hz electrical grid, synchronous speed is 1800 rotations per minute. The same four-pole motor operating on a 50 Hz grid will have a synchronous speed of 1500 RPM.
In normal motor operation, stator flux rotation is faster than the rotor rotation. This causes the stator flux to induce rotor currents, which create a rotor flux with magnetic polarity opposite to stator. In this way, the rotor is dragged along behind stator flux, at a value equal to the slip.
In generator operation, a prime mover (turbine, engine) drives the rotor above the synchronous speed. The stator flux still induces currents in the rotor, but since the opposing rotor flux is now cutting the stator coils, an active current is produced in stator coils, and the motor now operates as a generator, sending power back to the electrical grid

## Excitation

Equivalent circuit of induction generator
Note that a source of excitation current for magnetizing flux (reactive power) for stator is still required, to induce rotor current.
Induction generators are not, in general, self-exciting, meaning they require an electrical supply, at least initially, to produce the rotating magnetic flux (although in practice an induction generator will often self start due to residual magnetism.) The electrical supply can be supplied from the electrical grid or, once it starts producing power, from the generator itself. The rotating magnetic flux from the stator induces currents in the rotor, which also produces a magnetic field. If the rotor turns slower than the rate of the rotating flux, the machine acts like an induction motor. If the rotor is turned faster, it acts like a generator, producing power at the synchronous frequency.

## Active power

Active power delivered to the line is proportional to slip above the synchronous speed. Full rated power of the generator is reached at very small slip values (motor dependent, typically 3%). At synchronous speed of 1800 rpm, generator will produce no power. When the driving speed is increased to 1860 rpm, full output power is produced. If the prime mover is unable to produce enough power to fully drive the generator, speed will remain somewhere between 1800 and 1860 rpm range.

## Required capacitance

A capacitor bank must supply reactive power to the motor when used in stand-alone mode. The reactive power supplied should be equal or greater than the reactive power that the machine normally draws when operating as a motor. Terminal voltage will increase with capacitance, but is limited by iron saturation.

## Grid and stand-alone connections

Typical connections when used as a standalone generator
In induction generators the magnetizing flux is established by a capacitor bank connected to the machine in case of stand alone system and in case of grid connection it draws magnetizing current from the grid.
For a grid connected system, frequency and voltage at the machine will be dictated by the electric grid, since it is very small compared to the whole system.
For stand-alone systems, frequency and voltage are complex function of machine parameters, capacitance used for excitation, and load value and type.

## Use of induction generators

Induction generators are often used in wind turbines and some micro hydro installations due to their ability to produce useful power at varying rotor speeds. Induction generators are mechanically and electrically simpler than other generator types. They are also more rugged, requiring no brushes or commutators.
Induction generators are particularly suitable and usually used for wind generating stations as in this case speed is always a variable factor, and the generator is easy on the gearbox.

## Example application

We must use 10 hp, 1760 r/min, 440 V, 3 phase induction motor as an asynchronous generator. Full-load current of the motor is 10 A and full-load power factor is 0.8.
Required capacitance per phase if capacitors are connected in delta:
Apparent power S = √3 E I = 1.73 * 440 * 10 = 7612 VA
Active power P = S cos θ = 7612 * 0.8 = 6090 W
Reactive power Q = $\sqrt{S^2-P^2}$ = 4567 VAR
For machine to run as an asynchronous generator, capacitor bank must supply minimum 4567 / 3 phases = 1523 VAR per phase. Voltage per capacitor is 440 V because capacitors are connected in delta.
Capacitive current Ic = Q/E = 1523/440 = 3.46 A
Capacitive reactance per phase Xc = E/I = 127 Ω
Minimum capacitance per phase:
C = 1 / (2*π*f*Xc) = 1 / (2 * 3.141 * 60 * 127) = 21 microfarads.
If load also absorbs reactive power, capacitor bank must be increased in size to compensate.
Prime mover speed should be used to generate frequency of 60 Hz:
Typically, slip should be similar to full-load value when machine is running as motor, but negative (generator operation):
Slip = 1800 - 1760 = 40 rpm
Required prime mover speed N = 1800 + Slip = 1840 rpm.

## References

• Electrical Machines, Drives, and Power Systems, 4th edition, Theodore Wildi, Prentice Hall, ISBN 0-13-082460-7, pages 311-314.
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