DC Motor Maintenance—Explained Step by Step

Regular maintenance and servicing of electric motors can minimize downtime, reduce the error rate in electrical testing, and thus make a decisive contribution to the success of an industrial operation. Maintenance management and smart maintenance are therefore becoming increasingly important topics in more and more companies. However, not every plant manager has a clear understanding of the services a modern industrial service provider can offer. That’s why the experts at GP Prüfservice GmbH share insights on their blog, drawing on more than 50 years of experience in the testing and maintenance of electrical machines. Today, we’re focusing on the topic of DC motor maintenance. Using a practical example, we’ll explain step by step how a maintenance team in the industrial services sector approaches the maintenance of a DC motor.

Complex Repairs Require the Utmost Care

The design of DC motors is far more complex than, for example, that of three-phase motors. When maintaining or repairing DC motors, technical expertise combined with great care is therefore crucial. Among other things, it is important to know the motor’s operating environment, as this provides insight into typical signs of wear. With this knowledge, it is possible to carry out targeted maintenance planning. We’ll explain exactly how this works, step by step.

The General Conditions for DC Motor Maintenance

The motor to be overhauled in this case is used on a crane in a port area. There, it drives the so-called luffing mechanism—and thereby controls the angle of inclination of the boom. The DC motor is controlled by a 4Q DC controller with a three-phase 400-volt AC, 50 Hz input voltage and an output voltage ranging from 0 to 400 volts DC, as well as a 220-volt DC excitation voltage with field weakening. Feedback for control is provided by a tachogenerator with a 63-volt DC tachovoltage.

The DC controller was retrofitted as part of a retrofit project in the summer of 2016.

Motor specifications:

The cooling air in the motor is circulated via a heat exchanger, which ensures that no moist outside air enters the motor.

High levels of contamination make regular maintenance essential

DC motors are subject to a high level of contamination. This is due to the wear of the carbon brushes, which are usually made of graphite. Regular maintenance and preventive upkeep are essential. The overhaul described below takes place every two years. As part of a maintenance contract, a team ensures that the maintenance intervals are adhered to. To ensure that all important information is always available, the computer system records not only the motor’s basic data but also measurement data, logs, and detailed photos, among other things.

Step by step—this is how DC motor maintenance is carried out

1. All attached parts, bearing shields, and the brush bridge are precisely labeled.
2. The coupling is carefully pulled off the drive shaft using a three-arm hydraulic puller. To prevent scoring on the shaft, the coupling is heated evenly during the removal process.
3. After removing two forced-air fans and the heat exchanger, the technicians remove the front bearing shield (A-side).
4. On the B-side, the carbon brushes are removed. The connections on the brush bridge are marked and disconnected. It is important to record the exact position of the brush bridge relative to the field coils. This is also called the “neutral zone.” The position is marked. Later, during the test run, it is measured again and fine-tuned.
5. The rear bearing shield is removed.
6. Using a crane, the experts very carefully pull the rotor (armature) of the DC motor out of the stator. Care must be taken not to damage the motor’s windings or commutator.
7. The motor bearings are removed. The interference fits in the housing and on the shaft, as well as the shaft journal, are measured. The rotor is checked for concentricity.
8. The tachometer and forced-air fan are disassembled and measured.
9. The service technicians check the standby heater for ground faults and proper operation.
10. All PT100 resistance sensors must be calibrated and replaced if necessary.
11. The next step is to clean and dry the stator windings and the rotor winding.
12. Once dry and cold, turn-to-turn and ground fault measurements are performed.
13. All data is measured, documented, and entered into the computer system.
14. All parts and auxiliary units are thoroughly cleaned.
15. Run-in marks on the commutator are lightly machined on a lathe. A turning tool with a diamond tip is used for this purpose. Particular attention must be paid to runout and surface roughness.
16. The gaps between the fins are carefully cut out using an electric collector saw.
17. The next step is to deburr the sides of the fins.
18. Next, the rotor is balanced along with all rotating parts.
19. The two external fans are also balanced.
20. The motor is reassembled in reverse order.
21. The new carbon brushes are ground down to an 80% contact area. The dust generated during the grinding process is extracted as the grinding takes place.
22. The neutral zone of the brush bridge is adjusted and remeasured. Adjusting the neutral zone requires 100 percent accuracy, as the motor operates in reversible mode.
23. Now the test run takes place. The speedometer is already installed here, but not the auxiliary components such as the coupling, forced-air fan, and heat exchanger. All operating cycles are run through in the test cell. The experts perform all electrical and mechanical tests. The data obtained during this process is recorded and documented in the test report.
24. If the test run is satisfactory, nothing stands in the way of full assembly.
25. The remaining parts are installed. The motor undergoes a final test run.
26. In the paint shop, the DC motor is painted in the color specified by the customer.
27. Maintenance of the DC motor is complete. The fully serviced motor is delivered to the customer.

Save Costs in the Long Term with Professional Maintenance

Well-maintained motors play a crucial role in preventing malfunctions in industrial plants and avoiding defects during the DGUV V3 inspection. However, not every company has the staff and expertise to ensure professional maintenance management. Smaller businesses, in particular, are often better off entering into a maintenance and servicing contract with an industrial service specialist and also outsourcing electrical inspections to an external service provider such as GP Prüfservice.