From Electrical Equipment Handbook: Troubleshooting and Maintenance

INTRODUCTION TO MACHINERY PRINCIPLES (CHAP. 2)

1.

How can a generator act as a motor?

2.

What is the sum of the torques on the shaft of a turbine generator or a pump motor assembly during normal operation?

3.

What will happen when a generator is suddenly disconnected from the grid?

4.

Why are the cores used in transformers, generators, and motors made from steel?

5.

Are all cores laminated?

Answers

1.

A generator is exactly the same machine as a motor. The only difference between them is that the flow of energy in the machine is reversed. A generator converts mechanical energy (in the form of torque and speed) delivered by a prime mover (e.g., a turbine) to electric energy. A motor converts electric energy to mechanical energy.

While a generator is synchronized to a grid (i.e., it is operating in parallel with hundreds of other generators), if the torque delivered by the prime mover is interrupted (e.g., if the steam valves to the turbine fail shut), the generator will continue to rotate at the same speed in the same direction. In this mode of operation, it is taking power from the grid to continue to rotate at the same speed. This is called motoring. The generator is taking enough power from the grid to overcome the friction and windage losses. The friction losses are caused by the resistance at the bearings. The windage losses are caused by the resistance that the air imposes on the rotor (the...

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