DC Motor: Maintenance and Sources of Failure

Maintenance Program

A general dc motor maintenance program starts with service history review followed by maintenance activity followed by functional tests. The historical data can reveal information about current state of the motor as well as external conditions in which motor has operated. Any mechanical dimension or electrical parameter that is out of the allowable tolerance range can potentially have an adverse effect on motor’s nominal operations and hence on the overall system’s performance as well as the safety.

Maintenance activity

A general dc motor maintenance activity starts with cleaning of the individual components. Some components require dry air or water based cleaning while others require special chemical agents. The purpose is to get rid of the unwarranted material (e.g., dust particles, worn out material, oxides formed on the surfaces, and other contaminations). Then comes the inspection of the individual components. Here the idea is to observe and estimate obvious damages such as amount of wear and tear, cracks, etc. Then comes the testing of individual components or sub-systems. There is a certain limitation to human observation and estimation capability. And so advanced and sometimes automated inspection systems are employed. These also offer certain benefits such as quality control and scalability in using advanced instruments (e.g., microscopes) and techniques (non-destructive testing). The final task of the maintenance activity is to repair what is repairable, replace what is unrepairable (e.g., worn out brushes and bearings, burnt windings, broken wires, coating of metal surfaces and lubrication at the interfaces of mechanical components with relative motion. The last step in the maintenance program is operation or fit-to-use test.

This ensures that the equipment delivers expected performance and is safe to operate.

Corrective Vs. Preventive Maintenance

A maintenance program can be corrective or preventive. A maintenance activity that is held post a failure is called corrective whereas the one held prior to a failure is called preventive. A preventive maintenance program is augmented with a maintenance schedule. Due to many mechanical components exposed to the environment, cleaning is the most frequency activity. It involves wiping off the metal particles, oil, dust, and other contaminations from the environment. This is performed once in a month. All the other activities are necessary and sufficient when done twice in a year. This is a general guide; however, the exact schedule will depend on the exact type of the motor, the application, remaining useful life and usage. ?Service history plays critical role in planning and scheduling of maintenance activities whether corrective or preventive.

Common sources of failures

1.?????Noise and vibrations resulting from mechanical or electrical imbalances e.g., shaft mass imbalance, misalignments, shaft bending, damaged bearing, brush wear and tear, shorts in the windings, uneven airgaps, unwarranted resonance due to electrical parameter change, sparks etc. Systematic vibrations can cause heat generation and hence increased operating temperature.

2.?????Dirt (dust, oil, moisture and contaminations): Dirt is considered one of the major sources of motor damage. It can cause corrosion in wires, lugs, brushes, commutation ring, shafts and bearings. It can also damage insulations. It also acts as a heat insulator and can cause unnecessary increase in operating temperature. Dirt can block the air vents and cause failure of cooling mechanisms. If oil film is formed on the commutator surface, then arcs can cause black spots on the commutator surface due to burning of the oil film.

3.?????Wear and tear of brush-commutator system

4.?????When operated out of the nominal operating range especially for longer durations

References:

1.?????www.ohioelectricmotors.com

2.?????www.plantengineering.com

3.?????DC Machines DMI - Operating and Maintenance Instructions, ABB?