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Table 1-1.-Troubleshooting Chart-Continued
4. Place the voltmeter probes at C and F. A voltage
When starting a three-phase motor and the motor
reading with the start button closed would indicate a
fails to start and makes a loud hum, you should stop the
good OL1, but also would indicate an open OL3, an open
motor immediately by pushing the stop button. These
relay coil, or an open connection to line 3.
symptoms usually mean that one of the phases to the
motor is not energized. You can assume that the control
5. Place the voltmeter probes at points C and G
and close the start switch. A no-voltage reading indicates
circuit is good since the main operating coil has operated
the OL3 contacts are open.
and the maintaining contacts are holding the main
operating contactor in. Look for trouble in the power
A faulty holding relay contact will be indicated
circuit (the main contacts, overload relays, cable, and
when the system operates as long as the start switch is
held in, but stops when the start switch is released.
motor).
1-21

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