THE POWER SUPPLY
DC power supply of a synchronous motor can be pro- vided by several methods. The most common of these methods is either a small DC generator mounted to the shaft of the motor, or an electronic power supply that converts the AC line voltage to DC voltage.
TESTING THE SYNCHRONOUS MOTOR
The procedure for testing the stator winding of a synchronous motor is the same as that described for testing the stator of a squirrel-cage induction motor. The rotor can be tested with an ohmmeter for an open winding or a grounded winding. To test the rotor for an open winding, connect one of the ohmmeter leads to each of the slip rings on the rotor shaft, Figure 17–5. Since the rotor winding of a synchronous motor is intended for DC current, the resistance of the wire will be high as compared with the wire resistance of a wound rotor motor. Owing to the fact that alternating current flows in the rotor of a wound rotor motor, the current is limited by the inductance of the coil and not its resistance.
To test the rotor for a grounded winding, connect one ohmmeter lead to the shaft of the motor, and the other lead to one of the slip rings. There should be no continuity between the winding and the motor shaft, Figure 17–6.
Because the resistance of the rotor is relatively high, a shorted winding can often be found with
the ohmmeter. In coils designed for DC current, the resistance of the wire is used to limit the flow of current. For example, assume the specifications of a synchronous motor indicate that the DC excitation voltage should be 125 volts, and that maximum rotor current should be 10 amps. The resistance of the rotor can now be calculated by using Ohm’s law:
If the ohmmeter measures a rotor resistance close to 12.5 ohms, the rotor is good. If the ohmmeter measures a much lower resistance, however, the rotor is shorted.