Induction generator (s < 0)

Reference notes

Use Next → on the narrator above to see how the same induction machine operates as a generator when its rotor is driven above synchronous speed.

Negative slip → generator mode

The slip equation s = (n_s − n)/n_s doesn't care about sign. Drive the rotor above synchronous speed (n > n_s) and s goes negative. From the equivalent circuit, R₂/s also becomes negative — and a "negative resistor" is the mathematical signal that real power is flowing OUT of the machine, not in. The same physical hardware that was a motor at s > 0 becomes a generator at s < 0.

Power flow reverses

• Mechanical: shaft now delivers torque to the rotor (prime-mover drives it above n_s).
• Air-gap: P_g flows backwards — from rotor to stator.
• Stator: P delivers real power into the grid.
• Reactive: the machine still consumes lagging VAR — the magnetising current has nowhere else to come from.

The induction generator is therefore unusual: it generates real power but consumes reactive. It cannot self-start its own excitation when isolated unless something else supplies the magnetising current.

Two flavours of induction generation

1. Grid-connected (asynchronous generator): the grid supplies the magnetising current and locks the stator frequency. The prime mover drives the rotor slightly above n_s; the resulting negative slip delivers real power to the grid. Used in older fixed-speed wind turbines and microhydro installations.
2. Self-excited induction generator (SEIG): a capacitor bank on the stator provides the magnetising VAR. Residual rotor magnetism kicks off an initial voltage; capacitors and rotor flux build it up by positive feedback until the iron saturates and the voltage stabilises. Standalone — no grid needed — but voltage and frequency depend on load and speed. Used in remote microhydro, emergency standby generation.

Why use an induction generator at all (vs synchronous)?

• Robust, cheap, maintenance-free — same squirrel-cage rotor as the standard induction motor.
• No synchronisation ritual — connect to grid, accelerate above n_s, real power flows. No careful matching of V, f, phase.
• Inherent overload protection — as load grows, slip grows; if the prime mover can't keep up, slip drops below zero and the machine becomes a motor (drains energy rather than damaging itself).

Disadvantages: poor voltage and frequency regulation (depends on grid for both); needs external reactive support.

Modern variant: DFIG (Doubly-Fed Induction Generator)

The dominant wind-turbine generator topology in the 2000s and 2010s. A slip-ring rotor is connected to the grid through a back-to-back power converter. The stator feeds the grid directly; the rotor exchanges slip power with the grid through the converter. This lets you:

• Run at variable rotor speed (typically ±30 % of n_s) while still synthesising 50 Hz on the stator.
• Independently control real and reactive power, much like a synchronous generator.
• Use a converter rated at only ~30 % of total power (only the rotor slip power flows through it), saving cost vs full-rated converters.

DFIG has largely been displaced by direct-drive permanent-magnet synchronous machines in newer wind installations, but billions of dollars worth of DFIG turbines are still in service.

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