Electric motors are rotating machines composed mainly of a stator and a rotor which through the action of magnetic fields generated in their coils (located in the stator) convert electrical energy into mechanical.

These machines do not have a constant speed, have high performance (approximately 75%), have high and constant torque (depending on the type of motor), do not need external cooling or external ventilation (are self-ventilated), and in general, they are more efficient than their combustion equivalents.

Knowing their importance in the industry, it is important to know some of the considerations needed when performing maintenance on these motors. Let’s see them below.

The maintenance in any machine is of vital importance since it guarantees its correct operation, prevents some type of faults and even allows to increase its useful life. It can be classified as preventive and corrective.

The preventive maintenance is performed periodically in order to verify its operation, as well as perform the procedures (adjustments, lubrication, revision, etc.) and replace parts recommended by the manufacturer based on the workload of the motor. This procedure is performed according to the activities of maintenance planning and should interfere as little as possible in its routine operation.

On the other hand, corrective maintenance is performed as a consequence of a failure in the motor, therefore it is random in nature and usually causes interruption of the production process, it is important to minimize its effect to have the main spare parts needed for repair, as well as having qualified personnel for the task.

For the conservation of the motors, both the electrical and mechanical components must be covered, since they are subject to continuous use, so they wear out and become contaminated with external elements.

The electrical components (windings, brushes, etc.) have a routine of maintenance that consists on the revision of the connections, the change of the carbon brushes and the periodic measurement. For this task, it is required to use various instruments such as megohmmeter, micro-ohmmeter, multimeter, oscilloscope, and others. They allow obtaining values of insulation, continuity, frequency, current, voltage, power and waveforms.

Meanwhile the mechanical components such as: bearings, shaft and housing, have a conservation program that consists of cleaning the mechanical components, reviewing the bases and nuts of the motor, as well as taking measurements that allow evaluating the speed, the balance of the rotor, the torque applied by the load and the temperature.

Preventive Maintenance checks

 Below is a list of some basic preventive maintenance checks;

Perform Visual Inspection

A quick visual inspection can reveal some important details about the electric motor. Take a look at its physical condition and record your observations. If the electric motor operates in a rugged environment, you will see signs of corrosion and dirt buildup on individual components. Observe the motor’s windings to detect any hint of overheating, such as a burnt odor. Ensure relays and contacts are dust-free and aren’t rusted. All these factors may cause an internal problem as the debris may pose a threat to the efficient performance of the equipment.

Perform a brush and commutator Inspection

Regular maintenance checks help ensure that electric motors won’t experience inconsistencies or stop working abruptly. Look for signs of wear and tear; any hint of excessive wear leads to commutation problems with the motor. This means you need to change the brush in order to regain the integrity of the equipment’s function. Also keep a check on the commutator to ensure it doesn’t have any dents, grooves, or scratches. These rough spots indicate brush sparking. Additionally, inspect the motor mount, rotor, stator and belts thoroughly. Replace all worn out parts.

Conduct a motor winding test

Once you have inspected the various machine components, you need to test the motor’s windings. This test helps you identify any anomalies or failures in the windings. If you see any burn marks or cracks or smell a burning odor, conduct a mandatory motor winding test. The test involves disassembling the motor to determine the abnormalities of the motor. If the windings are overheated, the chance of serious damage is higher. Rewinding the motor and testing the wind insulation, which reveals information on the resistance level, are also critical parts of the test.

Check the Bearings

Check the bearings for noise and vibration as they indicate potential problems, like poor lubrication, dirt buildup, and wear and tear. If the bearing’s housing is too hot to touch, it may mean the motor is getting overheated or there is an insufficient amount of grease. The maintenance requirements for bearings may vary, depending on where the equipment is situated. You need to be aware of the different kinds of bearings being used in the plant and what their repair requirements are.

Perform Vibrations Test

Sometimes, excessive vibrations are difficult to detect manually. But, if not detected on time, vibration can reduce the life span of an electric motor, which then eventually leads to motor bearing failure or failure of windings. In most cases, the cause of vibration is mechanical in nature, such as a faulty sleeve or ball bearings, too much belt tension, or improper balance. The electric motor can be tested by removing the belts or by disconnecting the load and then operating the motor. Sometimes, even electrical problems can give rise to vibrations. A few tests, such as field vibration analysis that is conducted by mobile instruments that measure exact frequency and amplitude of vibrations, can help in detecting the exact cause of vibrations.

Use Infrared thermography in Predictive maintenance

Recently, this method of inspection has become popular with predictive maintenance due to its desired outcome. With infrared thermography, an infrared camera is used to capture thermal images without interfering with the motor’s operation. These images provide a temperature profile of the electric motor by giving heat patterns at several points throughout the motor simultaneously. All mechanical systems produce a particular amount of thermal energy, therefore, they have normal thermal patterns along with a maximal temperature at which the motor can work. In case any problem exists, such as insufficient air flow, insulation failure, or degradation in the stator, the infrared camera will immediately detect the unstable voltage in the form of a thermal image, helping you find its cause and solution.

Document Your findings

Documentation is extremely important. Keep detailed records of all preventive maintenance schedules, tests performed and their results. Maintain records of all repairs and replacements, as well. Doing so allows you to have a better understanding of the equipment, identify which issues need to be addressed, or determine which parts have to be replaced or repaired. Your records also will be helpful for future audits and inspections.

Precautions to take while performing maintenance checks

• Only assign electric motor maintenance tasks to those individuals who are well-trained in handling electrical components. Those who perform this task need to be aware of hazardous situations.
• Qualified personnel who perform maintenance checks should be equipped with protective gear, along with dielectric tested gloves and approved electrical test devices.
• Employees must make sure that pulleys and belts are in proper alignment and ensure operating parts are moving easily and without excess friction. Contactors and relays can be checked by hand for binding and sticking parts.
• Employees must be encouraged to regularly perform a maintenance task that keeps their surrounding environment dust-free and clean to avoid creating an unwanted path for electric current to flow.

To ensure better maintenance of electric motors, all maintenance procedures and tests should be conducted systematically in order to pinpoint potential problems and correct them before they result in undesired downtime. This approach not only improves the motor’s operation, but also increases its life span.

Different electrical materials2 have different maintenance requirements, so regular inspections must be scheduled per their needs. With electric motors, it’s a matter of understanding what they need and implementing those measures to enhance their productivity and the company’s profitability.

Activities of Preventive maintenance

• Inspect the electric motor at regular time intervals.
• Remove deposits of dust, oil, and dirt in the fan cover to maintain good ventilation and allow proper cooling of the motor.
• Observe the behavior of the seals.
• Observe the electrical, mechanical connections and the fixing bolts.
• Check the bearings paying attention to anomalous noises and vibrations.
• When there is a lot of bearing wear, replace them using a tool called extractor, then connect the new ones in cold or hot (oil baths) and lubricate them.
• Test the insulation resistance with a megohmmeter when the motor is without power and has a considerable time at rest.
• If it is necessary to disassemble the motor and access its internal parts, it requires the intervention of qualified personnel using appropriate tools and working methods.
• Clean windings with dielectric solvent and bake in infrared light when there is a lot of dirt in the coils to remove the corrosive substances, all in order to obtain better insulation resistance.
• Perform start-up tests once thorough maintenance has been carried out to verify its operation.

Activities of Corrective maintenance

• The motor does not start: Check the voltages of the grid, fuses, contacts, correct connections (star or delta according to the terminal board and the load), voltages in the rotor, contact of the brushes, the circuit of the starting resistors and insulation of the windings.
• The current absorbed during operation is excessive: Check the load and replace the motor if it is small, if this is not the case, check the insulation, rings, brushes, resistance circuit and rotor winding to repair or rewind as the case may be.
• The motor overheats: Check load, clean grilles, ventilation slots, terminal board connections, and the stator winding.
• The motor smokes and burns: Check windings and always keep the ventilation circuits clean to repair or rewind as necessary.

Maintenance Activities according to Frequency

Frequent checks (3 Months or Less)

• Clean motor of any dust or oil.
• Check oil rings turn with shaft.
• Check oil level in bearings.
• Visually check for oil and grease from bearings.
• Technician to examine the starter switch, fuses and tighten loose connections.

Every 6 months

• Clean motor, blowing out dirt from windings, and wipe commutator and brushes.
• Visually inspect commutator clamping ring.
• Check and replace brushes that are more than half worn.
• Examine brush holders, and clean them if dirty.
• Check brush pressure and position.
• Remove, clean out, and replace oil in sleeve bearings.
• Check grease in ball or roller bearings.
• Check operating speed or speeds.
• Technician to examine and tighten loose connections.
• Test current input and compare it with normal.
• Visually check drive, for smooth running, absence of vibration.
• Check motor foot bolts.

Annually

• Remove and renew grease in ball or roller bearing.
• Test insulation.
• Clean out magnetic dirt that may be attached to poles.
• Check clearance between shaft and journal boxes of sleeve bearing motors.
• Check the commutator for smoothness and slot damage.
• Examine connections of commutator and armature coils.
• Inspect armature bands.