To successfully fault find on even simple motor control circuits, one must understand the system and all the components involved in it. All relevant information such as diagrams, drawings, record sheets should be available and used to assist in solving the problem. Company safety procedures must be adhered to. Phase 2 apprentices are not allowed to fault find on live projects. The approximate resistance of all of the following items must be known or available to compare against actual readings at the time of the fault.
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:-
· 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.