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​Trouble Shooting Motor Control Circuits

​Trouble Shooting Motor Control Circuits
2 years ago 1645 Views No comments

Trouble shooting motor control circuits requires the ability to read a schematic or understand basics of control circuits as well as a multi-meter. Remember safety first, wear safety glasses and rubber soled shoes. Do not check continuity or resistance on energized circuits.

Using your meter or simple voltage tester check for equal line voltage across all incoming "Line" contacts of your motor switch, magnetic, or soft starter. If you do not have voltage, go ahead of the line and check your fused switch or circuit breaker. A short or overload may have caused the breaker or fuse to trip or fail. If this is the case you probably have a short in the motor. Use procedures in my blog on "Trouble Shooting Motors" to correct the problem.

If all the above are in working order than you may have a problem with the motor starter such as a blown fuse on a control transformer, a burnt out contactor coil, or open in the control circuit caused by faulty Start/Stop buttons, bad momentary contacts, limit switches, proximity sensors, or other devices or control boards in the control circuit. Make sure you check the fuse on the control transformer with the power turned off. With the power turned on check that there is the correct voltage coming out of the control transformer. Many control transformers are fed by 440 or 480 volts which can be deadly or cause explosions if shorted out, so be very careful. With the power turned off check for continuity on the magnetic starter coil, it should read close to zero. So now you have checked the control transformer. Some simple systems may use the line voltage for control also, so there would be no transformer.

If everything checks so far such as incoming power, a good contactor coil, and voltage coming from the transformer, you will need to check the overload relays, they should be normally closed, except if they are tripped, a continuity tester across the line and load will work for this step. If they bare open they will need to be reset or replaced if they will not reset.

Your next step will be time consuming as you will have to go through all the switches in series will the control circuit and check for continuity. If their wires come into a terminal strip you can check continuity if they are normally closed, otherwise you will have open every switches little terminal box and check for continuity along the line with the power turned off. You may need a schematic of the control circuit to determine which switches should be on normally on or off when the machine is operating.

Good luck and remember safety first.

Posted in: Automation Controls