Induction Motor - Equivalent Circuit

Equivalent Circuit

The equivalent circuit of an induction motor has the equivalent resistance of the stator on the left, consisting of the copper and core resistance in series, as . During operation, the stator induces reactance, represented by the inductor . represents the effect of the rotor passing through the stator's magnetic field. The effective resistance of the rotor, is composed of the equivalent value of the machine's power and the ohmic resistance of the stator windings and squirrel cage.

The induction motor equivalent circuit when idle is approximately, which is mostly reactive. Induction motors generally have a poor power factor, which can be improved by a compensation network.

The idle current draw is often near the rated current, due to the copper and core losses existing without load. In these conditions, this is usually more than half the power loss at the rated load. If the torque against the motor spindle is increased, the active current in the rotor increases by . Due to the construction of the induction motor, the two resistances induce magnetic flux, in contrast to synchronous machines where it is induced only by the reactive current in the stator windings.

The current produces a voltage drop in the cage factor of and a slightly higher one in the stator windings. Hence, the losses increase faster in the rotor than in the stator. and the copper factor of both cause losses, meaning the efficiency improves with increasing load and reduces with temperature.

gets smaller with smaller frequency and must be reduced by the delivered drive voltage. Thus, increases engine power losses. In continuous operation, this is an approximation because a nominal torque generated by the cooling of the rotor and stator is not included in the calculation. Above the rated speed or frequency, induction motors are more effective at higher voltages. Today, and are measured automatically and thus can be used on a motor to automatically configure itself and thus protect it from overload. Holding torques and speeds close to zero can be achieved with vector controls. There can be problems with cooling here, since the fan is usually mounted on the rotor.