Induction motor as generator in grid-connected mode

A 3-phase induction motor connected to the grid works as an induction (asynchronous) generator when its rotor is driven by a prime mover above synchronous speed. Slip becomes negative and the machine feeds active power into the grid.

Principle

Slip $s = \dfrac{N_s - N}{N_s}$.

• As a motor, $N < N_s$, $s > 0$: the field moves faster than the rotor, rotor emf and current produce motoring torque.
• If a turbine drives the rotor at $N > N_s$, the rotor conductors cut the field in the opposite direction. Rotor emf, rotor current and torque reverse; $s < 0$.
• Air-gap power $P_{ag} = 3I_2^2 R_2/s$ becomes negative: power flows from the rotor across the air gap to the stator and out to the grid. The electromagnetic torque now opposes the prime mover.

 T  motoring (s>0)
 |     .--.
 |   .'    '.
 |__/________\________________ N
 0          Ns\         .
               \      .'
                '.__.'
           generating (s<0)

Grid-connected operation

 Turbine/   +--------+   P (active) -->  3-phase
 wind  =====| Ind.   |=================  grid
 N > Ns     | machine|   <-- Q (VAr)     (V, f fixed)
            +--------+

1. Starting: the machine is started as a motor from the grid (or run up by the turbine) to near $N_s$, then the turbine input is increased so that $N$ goes slightly above $N_s$.
2. Excitation from grid: the machine has no field winding. It draws its magnetizing (lagging reactive) current from the grid, which acts as an infinite bus. Capacitors are not essential; some are added only for power-factor correction.
3. Voltage and frequency are fixed by the grid, independent of the exact rotor speed. No synchronizing equipment or exciter/governor for frequency is needed.
4. Power output rises roughly in proportion to the negative slip in the normal range (about 1–5% above $N_s$). If the turbine drives too fast (beyond the maximum generating torque), the machine runs away, so overspeed protection is needed.
5. Power factor: the machine delivers active power to the grid but absorbs reactive power from it, so in generator convention its power factor is leading.

Advantages

• Simple, rugged cage rotor; low cost and maintenance.
• No synchronization or hunting problems; tolerates speed changes of the turbine.
• Short circuit at terminals collapses the excitation, limiting fault current.

Disadvantages

• Needs reactive power from the grid, burdening it; poor power factor.
• Cannot control terminal voltage; cannot work alone without capacitors.

Applications

Wind turbines (fixed-speed cage and doubly-fed induction generators), small and mini hydro plants connected to the national grid, and energy recovery in drives (e.g. regenerative braking of lifts and cranes when the load overhauls the motor).