How to Troubleshoot DC Motor Controllers

DC motor controllers are used to control the speed of a direct current motor while still maintaining the torque or force at the rotating shaft. Typically a control signal is fed into the controller from an auxiliary source. The variation of the control signal is what manipulates the speed of the motor. Some DC motors also use a tachometer to send another electrical feedback signal into the controller. This feedback signal confirms the motor's speed so an accurate RPM, rotations per minute, can be maintained.

Things You'll Need

• Screwdriver
• Voltmeter
• DC controller specifications

Instructions

1. Check all electrical connections on the motor controller with the screwdriver. Screw terminals can come loose from the connectors. These loose terminals can cause failure in signal transmission from the control circuits to the main power being fed into the controller and the wires leading to the motors.

2. Inspect all electrical relay contactors in the electrical system. These relays and contactors are used for remotely switching main power on and off. Smaller relays can also be used for limit switches that are a part of the safety circuits.

3. Monitor the control voltage on the DC controller itself. Use the voltmeter to test the incoming voltage of not only the control speed circuit, but also the main power. Check with the controller specifications as to the span of variable control voltage that is needed to operate the controller for outputting a signal to the motors.

4. Measure the tachometer voltage if one is connected to the DC motors shaft. You may need to physically disconnect the motor shaft from any mechanical device it operates. The tachometer can cause the motor to "runaway" when being tested. This runaway operation can occur from interrupting the tachometers normal output to the controller.

5. Clean the interior of the controller. Dirt and dust can build up on the components of the motor controller. The excess material can cause the electronics of the controller to over heat and malfunction.

6. Maintain the DC motors bearings and carbon brushes. Dirt and carbon debris inside the motor itself can cause a situation in which failure at the DC controller is mimicked. The controller may exhibit problems but check out fine. All the while the motor itself is the cause of the problem.

Tips & Warnings

• Confirm the existence of proper earth grounds on the electrical system. A floating or nonexistent ground can create a static electrical build up in a DC motor control system. This build up of static power can cause errant behavior in the controller. Confirm that all electrical shielding is used on the control signal wires and the tachometer feedback signals. Control wires that do not use a wire shield can exhibit malfunctions from errant voltage inputs that leak into the control wires.

References

• Quick Guide to DC Controllers
• Industrial Motor Control; Stephen L. Herman, Walter N. Alerich; Delmar Publishing; 1985

Resources

• Repair Manuals and Publications