Trade of Electrician Motor Control COURSE NOTES - eCollege

FAS Electrical Course Notes – Module 2.4.2
Power Circuit
The control circuit may be functioning correctly, but the motor is not running. In this case the
driven machinery should first be checked to ensure that it is free to rotate. The motor will not
start against a mechanically jammed load. Remember that the loss of one phase to a loaded
three phase motor will result in failure to start. As with the control circuit, the assumption is
made that full power was present when the faulty operation was discovered.
Having proven that the fault is electrical, the following items should be checked out:-
Motor isolator closed
Motor protective device(s) closed
Contactor main contacts closing correctly
Motor winding resistance correct
All interconnecting wiring correct
The problem could be caused by any one or more of the following list of faults:-
Isolator contact open circuit
Motor protective device open circuit
Contactor main contact(s) not closing when operated
High resistance or open circuit termination
Open circuit motor winding(s)
Short circuit motor winding(s)
Delta connected motor actually connected in Star
Physical damage to wiring or components
Circuit wiring incorrect
Insulation Resistance Faults
Remember that an insulation resistance fault will also trip an MCB or rupture a fuse. A fault of
this nature is most likely to occur on the following items:-
Cables
Motor windings,
Relay or contactor coils
Signal lamp units
These faults may occur between current carrying conductors or between current carrying
conductors and earth. The use of an Insulation Resistance Tester is required to locate
problems of this nature.
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Revision 4. Feb 2009