Excitation capacitors in isolated and grid-connected induction generator

An induction motor becomes an induction generator when a prime mover drives its rotor above synchronous speed. Slip becomes negative and the machine converts mechanical power into electrical power.

Motor used as generator

• Slip $s = (N_s - N)/N_s$. For $N > N_s$, $s < 0$.
• The rotor now moves faster than the rotating field, so the rotor conductors cut the field in the opposite direction. Rotor emf, rotor current and torque all reverse.
• The torque opposes the prime mover; air-gap power $P_{ag} = 3I_2^2R_2/s$ is negative, i.e. power flows from the rotor to the stator and to the load or supply.
• The useful range is a small negative slip (about −1% to −5%); beyond the maximum generating torque the machine overspeeds.

 T
 |  motor (0<s<1)
 |    .--.
 |  .'    '.
 |_/________'.________ s
 |        0  '.   .'
 |             '-'  generator (s<0)

Why capacitors are needed in isolated mode

1. An induction machine has no field winding. Its magnetic field is created by a magnetizing current that lags the voltage by nearly 90°, i.e. it needs reactive power whether motoring or generating.
2. In isolated (stand-alone) operation there is no grid to supply this reactive power. A capacitor bank connected across the stator terminals draws a leading current, which is the same as supplying lagging reactive power to the machine.
3. Self-excitation: residual magnetism gives a small emf when the rotor is driven. This emf sends current through the capacitors, which increases the flux, which raises the emf further. The voltage builds up until the magnetization curve crosses the capacitor line $V = I_c/(\omega C)$.
4. Without capacitors (or with too small a capacitance, i.e. capacitor line steeper than the air-gap line) the voltage cannot build up.
5. The capacitors must also supply the reactive power of lagging loads; voltage and frequency vary with speed, load and capacitance.

 Turbine ==[IG]==+=====> Load
                 |
                === C (3-phase)

Why capacitors are not required in grid-connected mode

1. The grid (infinite bus) has many synchronous generators that supply reactive power. The induction generator simply draws its magnetizing current from the grid.
2. The grid fixes the terminal voltage and frequency, so no voltage build-up process is needed and residual magnetism is irrelevant.
3. The machine delivers active power to the grid while absorbing reactive power from it.
4. Capacitors may still be installed, but only for power-factor correction to reduce the reactive burden on the grid — not for excitation.