Below some basic excitation of induction generator.
1. Provide a prime mover to the induction motor
a. Engine
b. Wind Turbine
c. Hydro Turbine
d. deccelaration during breaking for electric vehicle.
2. Provide an excitation to the induction motor stator winding, this will energize the squirell cage rotor and will produce a leading magnetic field around the rotor, due to the speed of the prime mover.
a. 3 Phase Induction generator speed =(120 * Frequency)/ # of Poles, Stator frequency variation depends on the rotor shaft speed to produce the amount of power.
b. Provide a negative slip ( positive slip for motoring application, Set the stator winding excitation speed to provide a negative slip ( means the speed of the mechanical rotation is faster than the speed of the electrical excitation) thus converting the indcution motor as a generator.
b1: Induction Motor Positive Slip = (Stator Speed - Rotor Speed)/ Stator Speed * 100%
Stator speed > Rotor Speed to produce the amount of Torgue.
For a 10HP Induction motor with a maximum slip of 10%, will have a starting current around 300%-600% without a variable speed drive.
b2: Induction Generator Negative Slip = Stator speed - Rotor Speed ( rotation of Prime Mover)* 100%
Rotor Speed > Stator speed, thus providing a negative slip to the induction motor to act as a generator. Increasing the rotor mechanical speed, will increase the negative slip to produce higher output power.
For a 10HP Induction motor with a maximum slip of 10%, can produce 300 % to 600% of the actual power rating.
10HP * .746 = 7.46 Kwatts @ ~5% negative slip
10HP *.746 = 7.46 * 3 (300%) = 22.38 Kwatts, Negative slip should be controlled to limit the power dessipation on the Stator winding, by increasing the frequency of the stator winding.
3. Sample Induction generator design.
3Phase 2 Pole 10HP Induction generator
Prime Mover Speed (Wind) : 120RPM - Rotor Mechanical Speed
Assume a Negative Slip of -5% to provide the maximum power rating of the Induction generator ~10HP * .746 = 7.46Kwatts output.
Solve for Stator Electrical Speed.
Negative % Slip = ((Stator Speed - Rotor Speed)/ Stator Speed)* 100
-5% = ((Stator speed - 120RPM)/Stator speed)*100
Stator Speed = 12000/105
Stator Speed = 114.28 RPM
Solve for Stator Frequency.
Stator Speed = (120 * Frequency)/ # of Pole
57.14 RPM = (120 * F)/2
F = (2 * 114.28RPM)/120
F = 2HZ
This Design example illustrate the capability of an induction generator to produce maximum amount of power at very low speed. Changing the Stator frequency with respect to the mechanical speed ( Negative slip), induction generator is capable from 1HZ to 120HZ 0r 60RPM - 7200 of a 2 Pole Induction generator.
Variable Speed induction generator keep tracks of the wind speed then adjust the Stator electrical speed to have a negative slip of 3% - 5%.
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If no load applied to the generator, the DC Bus voltage will keep on increasing, at No load, Negative Slip should be adjusted to around -0.5 to - 1%, thus minimum power generation will take place, care should be take not to increase the stator speed higher than the Rotor speed, this will make the Induction Generator to act as a motor.
