1.

Does the torque capability of a motor drop when it is operated at a higher speed than its rated speed using a variable-frequency drive?

2.

How can an induction motor be used as an induction generator?

1.

Every motor has a rated mechanical power (hp) that it can deliver, given by

where T _rated is the rated torque and speed _rated is the rated speed. If a variable-frequency drive is used to drive the motor at a speed greater than its rated speed, the torque developed by the motor will be less than the rated value. This is due to the fact that the motor cannot deliver larger mechanical power than its rated mechanical power.

2.

An induction motor becomes an induction generator when it is driven by a prime mover beyond its synchronous speed. In variable-speed drive applications, the variable-speed drive can vary the synchronous speed of the motor by varying the frequency in the power supply to the motor. This can be done within a short time. Thus, if a motor is driving a heavy flywheel and the variable-speed drive lowers its synchronous speed below its running speed, the motor will operate as a generator. It will start converting the mechanical energy in the flywheel to electric energy. During this process of "power generation," the motor (now acting as a generator) starts applying a countertorque on the shaft to convert the mechanical energy to electric energy. The...