Induction generator in isolated and grid-connected modes

An induction generator is an ordinary induction machine driven by a prime mover above synchronous speed, so that slip becomes negative and the machine converts mechanical power into electrical power.

Motor to generator action

Slip $s = (N_s - N)/N_s$.

• $N < N_s$: $s$ positive, rotor conductors cut the field in one direction, machine works as a motor.
• $N = N_s$: $s = 0$, no emf in rotor, no torque.
• $N > N_s$: $s$ negative. The rotor now cuts the rotating field in the opposite direction, so rotor emf, rotor current and torque reverse. The torque opposes the prime mover, and the air-gap power $P_{ag} = 3I_2^2R_2/s$ becomes negative: power flows from rotor to stator to the supply. The machine is a generator.

 T
 |   motoring
 |      .--.
 |    .'    '.
 |___/________'.______ N
 |  Ns   (s<0)
 | '.  .'  generating
 |   ''    (T negative)

The induction generator cannot produce its own magnetizing current. It always needs reactive power (lagging VAr) to set up the rotating field. Where this comes from decides the two modes.

Grid-connected mode

 Prime    +--------+     3-phase grid
 mover ===| IG     |=====(fixes V and f,
 N > Ns   +--------+      supplies VAr)

1. The stator is connected to the grid (infinite bus). The grid fixes voltage and frequency and supplies the magnetizing (reactive) current.
2. The machine is first run up near $N_s$ (or started as a motor), then driven above $N_s$ by the turbine.
3. Active power delivered rises with negative slip (typically 1–5% above $N_s$). Frequency stays equal to grid frequency even if turbine speed varies a little, so no synchronizing is needed.
4. Used in wind farms and small hydro plants connected to the grid.

Isolated (self-excited) mode

 Prime    +--------+
 mover ===| IG     |===+=====> Load
 N > Ns   +--------+   |
                     ===== C (delta/star
                           capacitor bank)

1. There is no grid, so a capacitor bank across the stator terminals supplies the lagging VAr (capacitors take leading current = machine's lagging magnetizing current).
2. Voltage build-up: residual magnetism in the rotor gives a small emf when it is driven. This drives a small current through the capacitors, which increases the flux, which raises the emf further. The process continues until the magnetizing curve of the machine meets the capacitor line $V = I_c X_c$ — the operating voltage.
3. Voltage and frequency depend on speed, capacitance and load. Higher speed or larger C raises voltage; a load needing VAr lowers it. Regulation is poor, so voltage/frequency controllers or electronic load controllers are used.
4. Used in micro-hydro and stand-alone wind systems in remote areas.