Induction Generator
Aim
To understand the operation of an induction machine as a generator in grid connected and standalone
mode.
Theory
Induction machine operates as a motor when its speed (ωm ) lies between 0 and synchronous speed (ωs ),
i.e. 0 < ωm < ωs . If the machine speed crosses ωs then the machine operates with negative slip. This
means that the rotor field is rotating “ahead of” the stator field. The torque produced is negative and
hence the machine is generating power. The complete torque speed characteristics can be seen in the
Fig.1
Figure 1: Torque Speed characteristics of induction machine: both in motoring and generating mode
The rotor currents in the induction machine are due to the induced emf in the rotor. There is no
separate excitation supplied to the rotor. The motor can enter generating mode only of receives reactive
power in the form of field supply. Two such cases have been considered here based on the source of
reactive power.
Grid-connected Induction Machine
Consider an induction machine connected to the grid. The machine can run as a motor by connecting
it to a three phase supply. Note, the machine speed (in motoring mode) is less than the synchronous
speed (ωm < ωs ). However, if the speed is increased by means of a prime mover to a value greater than
ωs then the machine acts like a generator with power being supplied to the grid. The reactive power
needed is drawn from the grid.
Standalone Induction Generator
The standalone induction generator can operate at any speed. Start the induction machine by using
a prime mover.The reactive power needed for generator operation is provided using capacitor banks.
Switch in capacitor banks across the machine terminals that will excite the induction machine and
1
will enter generating mode. The value of capacitor bank needed for self excitation depends on the
parameters of the induction machine.
Procedure
Self-excited Induction Generator
1. Connect the circuit as shown in the Fig. 2.
2. Fix the speed of the induction machine to any value using the prime mover (DC Motor).
3. Switch on the capacitor banks one by one by closing switches S1, S2, S3.
4. Close the switch S4. You can see the bulbs glowing as power is supplied to them by the induction
generator.
5. What is the power delivered to the bulbs?
3 phase resistive
(light) load
Single phase supply
230 V, 50 Hz
S4 3 phase
induction
DC
machine # motor
mechanical
S1 S2 S3 coupling
controlled rectifier
field winding
Single phase supply
230 V, 50 Hz
Figure 2: Independently operating (self excited) induction generator
Grid-connected Induction Generator
1. Make circuit connections as shown in Fig.3
2. Start the induction machine using an autotransformer and note the direction of rotation. Reduce
the voltage to zero to stop the machine.
3. Start the DC motor. Note the direction of rotation and turn off the dc supply.
4. If the two directions are the same go to the next step. Otherwise, interchange the armature
supply terminals of the DC machine. (The same effect can be achieved by interchanging any two
of the phase terminals of the induction machine; why? ).
5. Close the switch S1 and start the induction machine with the autotransformer. Apply full rated
voltage.
6. Note the speed of the machine and the sign of power in the power analyzers.
7. Start the DC machine. Increase its speed greater than the synchronous speed of the machine.
Note the sign of power readings in the power analyzers.
This experiment/chapter was prepared by R M Ramkumar (Research Assistant, 2014-17 batch).
2
3 phase supply
Single phase supply
440 V, 50 Hz
230 V, 50 Hz
S1 3 phase
3 phase Power induction
DC
autotransformer Analyzer
machine # motor
mechanical
coupling
controlled rectifier
field winding
Single phase supply
230 V, 50 Hz
Figure 3: Grid connected induction generator
