A General Guide to DC Motor Maintenance

June 30, 2011  |  Guides

A D.C. motor maintenance program is a preventative and corrective maintenance schedule that covers inspections, cleaning, testing, replacement and lubrication tasks that are necessary to ensure the proper operation of D.C. motors and associated equipment. A maintenance program is fairly easy to develop and implement. 1 Once regular maintenance checks are incorporated into a shop’s work schedule, they soon become transparent yet will reap cost-savings well beyond the investment in time and materials spent in maintaining them.

Overview

In brief, a D.C. motor maintenance program (Table 1) begins with reviewing a motor’s service history. 2 This review may reveal on-going problems that are both integral to the motor itself as well as external conditions (e.g., overloads, unbalances, misapplications) that are adversely affecting the motor’s normal operating condition. 3 After a service history review, a visual inspection should be performed to identify any obvious wear, blockages to cooling fans, or environmental contamination (moisture or corrosion). This inspection should also be conducted after the motor has been disassembled to discover evidence of failed components, such as burnt windings, broken leads, etc. Next, the motor’s windings should be tested. For most maintenance shops, this is a ground insulation test. 4 Since the commutator and brush assembly are high-wear parts of a D.C. motor, extra time should be spent on inspecting, repairing or replacing these vital components. Finally, an inspection of the bearings should be performed; worn out or noisy bearings require replacement. In the event that sealed, non-lube bearings are not used, the motor should be lubricated and then reassembled. 5 An operational test should be performed prior to the motor being shipped out of the repair shop.

Table 1: D.C. Motor Maintenance Program 6

Wipe off dust, dirt, oil, etc. Monthly
Clean vent screens and fans Quarterly
Lubricate bearings (if applicable) Semi-annual
Vacuum or blow out interior Semi-annual
Check commutator, brushes leads Semi-annual
Check brush spring tension Semi-annual
Test field coils Semi-annual
Test armature windings Semi-annual
Check electrical connections Semi-annual

Maintenance Guide Summary

This maintenance guide will discuss standard maintenance procedures for maintaining most D.C. motors. To determine the maintenance requirements of a specific motor, the maintenance technician should refer to the manufacturer’s technical documentation prior to performing maintenance. This guide is divided into the following sub-sections:

  • Reviewing the Service History
  • Noise and Vibration Inspections
  • Visual Inspection
  • Windings Tests
  • Brush and Commutator Maintenance
  • Bearings and Lubrication

Reviewing the Service History

D.C. motor maintenance, as with all types of industrial maintenance, requires pre-maintenance planning and scheduling. This starts with reviewing the motor’s service history usually contained within an equipment maintenance log or, if the log is not available, interviewing the customer, operator or responsible party to determine what type of maintenance is required, preventative or corrective (failure repair). 7 The goal is to determine:

  • What kind of maintenance is required.
  • What maintenance personnel are needed to perform the maintenance (skill level).
  • What parts are needed to complete the maintenance (i.e., bearings, brushes, etc.)
  • What kind of scheduling or coordination with other departments is required to perform the maintenance (downtime or off-hours scheduling)8
  • What kind of safety hazards exist that would interfere with the maintenance.
  • If there are problems other than the motor itself that caused the motor breakdown.

Noise and Vibration Inspections

Prior to disconnecting the motor and sending it to the shop or a repair facility for maintenance, a noise and vibration inspection should be conducted. This requires the motor to be connected to its driven load, energized and operated normally (if possible). The existence of mechanical noises or vibrations can indicate a variety of problems, such as mechanical and/or electrical imbalances, misalignments, brush chattering, bad bearings, bent shafts, mechanically loose windings (shaken lose by excessive vibrations, for instance) or simply a loose cooling fan or something stuck inside the vents or shroud. If the windings are loose, after it is disassembled, check for insulation and lead damage. 9 Vibrations can also be the root cause for excessive heat and brush sparking.

Noises and vibrations are not limited to mechanical problems or imbalances; electrical imbalances, such as open or shorted windings or uneven airgaps, can cause noises or vibrations. The easy way to troubleshoot an electrical from an mechanical imbalance is to first power up the motor, then disconnect power. If the noise/vibration exists while it is unpowered, the problem is mechanical; if the noise stops while power is disconnected, the problem is usually electrical. 10

Visual Inspections

Before disassembly, refer to the manufacturer’s technical documentation on recommended inspection tests or procedures. This documentation will provide valuable information for conducting visual inspections. 11

A visual inspection is meant to observe and record anomalies about the physical condition of the motor in a de-energized state. A motor that appears dirty, corroded or has the “beat up” look indicates that it was operated in a rough environment and may have more problems than usual. This inspection should include the “smell” test. Is there a burned odor coming from the motor windings? The burnt smell is coming from the insulation varnish of the motor windings. 12 If so, this suggests an overheating problem. Motor winding damage is possible under these conditions so winding tests should be conducted.

Overheating problems may not necessarily be internal to the motor; rather, they could be the result of mechanical overloads such as jams in the driven load or a cold oil that is being pumped via a motor drive 13, running the motor at low speeds such that there’s inadequate cooling airflow, electrical noise from DC drives overheating the windings or it could simply be the result of a dirty environment. Dirt acts like a heat insulator and heat damage is the weakness of normal motor operation. Inspect the cooling fan and passages to ensure they are operational and free of blockage, respectively. Clean all surfaces with a rag and blow out or vacuum passages with a shop vac. Corrosion can damage motor windings as well as create high resistance wiring connections. If the corrosion is chronic, a motor rewind may be merited if the winding tests verify that winding damage has occurred. Re-lugging the motor connection box terminals also may be required.

Motor Winding Tests

Once the motor is disassembled, and a thorough inspection of internal components has been conducted, testing the motor windings is done. This is where a maintenance history can prove its value. What kind of service history is on record relative to the winding failures or abnormalities? Has the motor ever been rewound? If so, what was the cause of the failure? This information suggests what kind of motor winding tests are necessary. In some situations, motor winding tests beyond the ground insulation (megger) test may need to be conducted. Is there any evidence of overheating of the windings? This may appear as burn marks, cracks, or, if catastrophic, exposed wire. Severe damage would require rewinding the motor.

Once again, look at the physical condition of the windings. If they are dirty or corroded, clean the windings with a brush, hot water and detergents and a vacuum. 14 Check the manufacturer’s documentation before using any solvents or detergents to ensure they will not damage the insulation. Avoid using pressurized air because the force of the air may propel particles into the winding insulation and damage it. Is there moisture on the windings? If so, prior to conducting any winding tests, the windings must be thoroughly dried out. Moist or wet windings will generally give false readings when conducting insulation tests so the windings must be dried first. This is done by baking the motor windings in an oven until the insulation resistance is at least 10 megohms. Refer to the manufacturer’s technical documentation on specific requirements. If this does not work, consider revarnishing the motor first. 15 If the motor passes the insulation tests, this is an adequate solution. If not, a rewind will be necessary, a job that is beyond the capabilities of most general maintenance shops.

The standard way to test winding insulation is the megger test 16 which applies a D.C. voltage, usually 500 or 1000 volts, to the motor and measures the resistance of the insulation. The minimum insulation resistance to ground is 1 megohm per kv of rating plus 1 megohm at 40 degrees Celsius ambient 17. Measurements of 50 megohms or more are common. 18 Resistance readings depend on the motor size, type of wire, etc. Refer to the manufacturer’s documentation for the specific values of ground insulation resistance. One caveat about testing ground insulation with a megger: the values can vary so conduct several tests over a period of time. Low readings indicate a problem that needs to be investigated. A ground insulation test is not a comprehensive motor insulation test; it does not, for instance, test the insulation’s resistance between turns of the windings. To test coil-to-coil or turn-to-turn insulation failures, a high surge test, the Hipot test, would be required and requires special test equipment 19

Brush and Commutator Maintenance

The brushes and commutator are integral to the normal operation of a D.C. motor. The brushes 20 ride or slide on the rotating commutator of the armature; there should be little brush noise, chatter or sparking when the motor is powered up. 21 Excessive brush wear or chipping are signs that the motor is not commutating properly, which can be caused by a variety of factors. While de-energized, rotate the armature by hand to see if the brushes are free to ride on the commutator and there’s adequate spring tension 22 to keep them hugging the commutator. 23 A good brush should have a polished surface which indicates that it has been seated properly. 24 Check the brush connections to ensure they are tight and clean. Determine if the brushes are aligned properly. Misalignment from neutral can cause sparking (armature reaction). The brushes should have equidistant spacing around the commutator and parallel to the bars. Clean any debris around the brushes. Compare the brushes to a new set of brushes to gauge the amount of wear. If excessive or, if you don’t think they will last until the next maintenance time, replace them.

The commutator 25 should have a smooth, polished, brown appearance. There should be no grooves, scratches or scores. If there is any blackened, rough areas on the commutator, it’s probably caused by brush sparking. If a commutator has a brassy appearance, there’s excessive wear that could be caused by the wrong type of brush or the wrong spring tension. Check the manufacturer’s technical documentation to verify the correct brushes are installed. Carbon dust and debris from the brushes can cause sparking and damage the commutator. If the commutator is rough and the bars are uneven, it will need to be turned on a lathe to restore its roundness. To clean the commutator, use a commutator cleaning brush (fiberglass) and some electric motor cleaner. Never use emery paper because it has metal particles in it that if rubbed off could cause electrical shorts. Remove the brush springs, slide the brush across the commutator hood and spray. When done, blow out the motor so it is dry and clean.26

Bearings: Replacement and Lubrication

There are different types of bearings 27 and the required maintenance on them will depend on the type of bearing, operating environment and the motor application. There are lubed-for-life, sealed bearings used in low horsepower motors that do not require lubrication.28

Lubrication is only one of three maintenance tasks involved with motor bearings. Cleaning, removal and replacement are the other tasks. In the noise and vibration inspections, the bearings should have been inspected for abnormal noises, vibrations or hot bearings. The “feel” and “sound” tests are simple methods to gauge bearing condition. 29 For the “feel” test, with the motor running, touch the bearing housing. If it is very hot to the touch, the bearing is probably malfunctioning. In the “sound” test, listen for thumping or grinding noises. If they exist, the bearings need a closer look and possible replacement. For most types, the sources of bearing failures are:

  • Insufficient oil or grease.
  • Too much grease causing churning and overheating.
  • Worn bearings (i.e., broken balls or rough races, etc.)
  • Hot motor or external environment.

If the service history demonstrates repeated bearing failures, check the manufacturer’s specifications to determine if the correct bearing has been installed. If that’s not the case, then an external factor could be the cause. Prior to bearing removal or replacement, clean the housing with solvents or flushing oils. The bearings should be cleaned with a lint free rag. Take a lot of care to keep dirt out of the bearing. When bearings need to be replaced, remove them with the proper tool. Hammers should never be used since they can damage the bearing races. The bearing puller’s claws should be attached to the sidewall of the inner ring or an adjacent part.

The lubrication schedule depends on the bearing and the motor application. Small-to-medium motors with ball bearings (except sealed) are greased every 3-6 years under normal conditions. A wet, corrosive or high temperature environment may require more frequent lubrication. The proper lubricant is critical to proper lubrication; check with the manufacturer on oil/grease recommendations. Prior to lubrication, remove the relief plug from bottom of the housing in order to prevent excessive pressure during lubrication. After completing greasing, run the motor 5-10 minutes until grease flowing out of the grease hole. This will expel the excess grease.30

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