Why must an induction generator rotate slightly faster than its synchronous speed?

An induction generator must rotate faster than its synchronous speed to function as a generator rather than a motor.

In an induction machine, the stator creates a rotating magnetic field that moves at a fixed synchronous speed determined by the supply frequency and the number of poles. When the rotor is driven by a prime mover at a speed exceeding this synchronous speed, the rotor conductors move faster than the magnetic field. This relative motion induces currents in the rotor that interact with the magnetic field to produce a torque opposing the prime mover.1

Under these conditions, the machine acts as a generator, delivering electrical power into the supply network. The amount of power generated is approximately proportional to the "slip"—the difference between the rotor speed and the synchronous speed—which is typically 0.5 to 2 percent of the rated speed at full load. Because the machine lacks its own source of magnetic field, it generally requires connection to an external electric supply network to operate.1