Maintenance and Troubleshooting for Three-Phase Synchronous Motors

Question 1

List four problems with the field circuit that could cause the motor to fail to synchronize.

Answer 1

• Open in the field coils
• No exciter voltage
• Open in the field rheostat
• Rheostat resistance set too high (field current too low)

Question 2

How can too high or too low of a rotor excitation current cause a synchronous motor to overheat?

Answer 2

Too high or too low a field current will cause a large reactive current in the stator windings, which will cause excessive heat in the windings

Question 3

If the manufacturers specifications for brush pressure are not available, what is a safe rule to use?

Answer 3

2psi (1.38kPa)

Question 4

Is it safer to have the brush pressure too high or too low?

Answer 4

too high

Question 5

At what point should a brush be replaced?

Answer 5

replace when the brush is down to one-quarter of its original useful length

Question 6

A synchronous motor is connected up and runs with the wrong rotation. What can be done to remedy this situation?

Answer 6

Interchange any two of the line leads to the stator