When an induction machine operates at a speed higher than its synchronous speed, its behavior changes significantly. Let's analyze this operating condition.

Induction Machine Operation Fundamentals

An induction machine typically functions as a motor when the rotor speed ($N_r$) is less than the synchronous speed ($N_s$). In this mode, the machine draws electrical power from the supply and converts it into mechanical power. The difference between the synchronous speed and the rotor speed is crucial and is quantified by slip ($s$).

The formula for slip is given by:

$s = \frac{N_s - N_r}{N_s}$

For a motor operating below synchronous speed, the rotor speed $N_r$ is less than $N_s$, resulting in a positive slip value ($0 < s < 1$).

Operation Above Synchronous Speed

Now, consider the case when the induction machine's rotor is driven mechanically at a speed ($N_r$) that is greater than the synchronous speed ($N_s$). In this scenario:

• The term $(N_s - N_r)$ becomes negative because $N_r > N_s$.
• Consequently, the slip ($s$) becomes negative: $s = \frac{N_s - N_r}{N_s} < 0$.

A negative slip indicates that the relative speed between the rotating magnetic field (determined by the supply frequency and number of poles) and the rotor conductors is such that the rotor conductors are cutting the magnetic flux in the opposite direction compared to motor operation. This induces currents in the rotor that create a magnetic field opposing the rotation. The interaction between the stator's magnetic field and the rotor's magnetic field produces a torque that opposes the external driving torque, causing the machine to deliver electrical power back to the supply system.

Matching Characteristics to Options

Let's evaluate the given options based on this understanding:

• Synchronous motor: A synchronous motor runs exactly at synchronous speed and requires a DC excitation for its rotor. It does not operate above synchronous speed in the manner described.
• Induction motor: An induction motor operates below synchronous speed and consumes power. Its characteristic does not match operation above synchronous speed where power is generated.
• Induction generator: An induction machine operating above synchronous speed, driven by an external prime mover, acts as an induction generator. It feeds electrical power back into the grid. This perfectly matches the described characteristic.
• DC motor: A DC motor operates on direct current and functions on a different electromagnetic principle, unrelated to synchronous speed or slip in AC machines.

Therefore, when an induction machine is allowed to run above synchronous speed, it exhibits the characteristics of an induction generator.