Motor Maintenance
In general, motors are machines that are known for their reliability and low maintenance requirements. However, to ensure that a typical electric motor has the longest possible service life, it is important to perform regular maintenance on it.
SCHEDULE PERIODIC INSPECTIONS
To minimize motor problems, it is important to perform scheduled routine inspection and service. It is also recommended to keep records of all maintenance schedules and procedures performed. The frequency and procedures of routine service vary widely between applications. Periodic inspection of motors is necessary to ensure that shaft alignment, motor base tightness, and belt condition and tension are in good condition.
BRUSH AND COMMUTATOR CARE
To maintain DC motors, it is recommended to remove the covers and perform checks on brush wear, spring tension, and commutator wear or scoring. If there is any chance that the brushes will not last until the next inspection date, it is advisable to replace them. The commutator should be clean, smooth, and have a polished brown surface where the brushes ride. While the motor is running, observe the brushes to ensure that they ride on the commutator smoothly with little or no sparking and no brush chatter.
TESTING WINDING INSULATION
Twice yearly, test winding and winding-to-ground resistance to identify insulation problems. Motors that have been flooded or have low megger readings should be thoroughly cleaned and dried before being energized. The following are typical minimum motor insulation resistance values:
KEEP YOUR MOTORS CLEAN
Wipe, brush, vacuum, or blow accumulated dirt from the frame and air passages of the motor. Dirty motors run hot when thick dirt insulates the frame and clogged passages reduce cooling airflow. Heat reduces insulation life and eventually causes motor failure.
KEEP YOUR MOTORS DRY
Motors that are used continuously are not prone to moisture problems. It is the intermittent use or standby motor that may have difficulties. Try to run the motor for at least a few hours each week to drive off moisture. Be careful that steam and water are not directed into open drip-proof motors.
CHECK LUBRICATION
Lubricate motors according to manufacturer specifications. Apply high-quality greases or oils carefully to prevent contamination by dirt or water.
CHECK FOR EXCESSIVE HEAT, NOISE, AND VIBRATION
Feel the motor frame and bearings for excessive heat or vibration. Listen for abnormal noise. All indicate a possible system failure. Promptly identify and eliminate the source of the heat, noise, or vibration.
EXCESSIVE STARTING IS A PRIME CAUSE OF MOTOR FAILURES
The high current flow during start-up contributes a great amount of heat to the motor. For motors 200 hp and below, the maximum acceleration time a motor connected to a high-inertia load can tolerate is about 20 seconds. The motor should not exceed more than about 150 “start-seconds” per day.
Troubleshooting Motors
Electrical testing and maintenance of motors can be dangerous, and complacency can lead to injury. To reduce the risk of injury, always:
- Disconnect power to the motor and lock out/tag out before performing any service or maintenance.
- Discharge all capacitors before servicing the motor.
- Keep hands and clothing away from moving parts.
- Make sure all required safety guards are in place before starting equipment.
Electrical contact accounts for one-fifth of all construction deaths. Never work on energized equipment unless absolutely necessary. If you must work on energized equipment, always wear the appropriate personal protective equipment (PPE) and use appropriate tools and equipment. Follow the “buddy rule” and never work on energized equipment alone. Always have a partner working with you, in case of emergency.
- Digital multimeter (DMM)—most often used for precise voltage measurement, as well as circuit continuity and motor winding resistance. Multi-functional DMM meters types include capacitor capacitance testing and frequency measurement functions.
- Clamp-on ammeter—used for monitoring of motor current. The probe clamps around the outside of the conductor, thus avoiding having to open the circuit and connect the meter in series to measure the current.
- Megohmmeter—used for checking the motor’s insulation resistance to ground resistance. Measures insulation resistance in millions of ohms.
- Infrared (IR) thermometer—used for noncontact motor surface temperature measurements. The laser sight will pinpoint high-temperature problems on any surface of the motor.
- Tachometer—used to check the speed of motors, from no load to full load.
- Oscilloscope—draws a graph of an electrical power or control voltage. Is commonly used for troubleshooting problems associated with electronic AC and DC motor drives.
The motor system is composed of the power supply, controller, motor, and driven load. In the event of a motor problem, it is important to identify which part of the system is at fault. Power supplies and controllers can fail, as can the motor itself. Mechanical loads can increase due to an increase in the size of the load that the motor is driving or due to failure of bearings or coupling mechanisms. Mechanical overloading is a major cause of motor failure.
TROUBLESHOOTING GUIDES
Once it has determined that the motor is at fault, you can proceed to locate the problem with the motor. A troubleshooting guide outlines a comprehensive variety of motor problems. Generally the categories are arranged according to symptoms, offering brief suggestions concerning what to look for when investigating motor failures and often providing advice on how to correct the problem once it has been identified. The following is an example of a troubleshooting guide that outlines fault symptoms common to most types of motors.
1/ Symptom: The motor fails to start. Possible causes
Blown fuse or open-circuit breaker. Check the voltage at the input and output of the overcurrent protection device. If voltage is measured at the input but not at the output, the fuse is blown or the circuit breaker is open. Check the rating of the fuse or circuit breaker. It should be at least 125 percent of the motor’s full-load current.
Motor overload relay on starter tripped. Allow overload relay to cool and reset it. If the motor causes the overload relay to open after a short period, check for motor short circuits and grounds. Check the full-load current of the motor and compare it to the setting of the overload relay.
Low voltage or no voltage applied to the motor. Check the voltage at the motor terminals. The volt-age must be within 10 percent of the motor nameplate voltage. Determine the cause of the low voltage. Loose fuse clips and connections at the terminals of the disconnect switch or circuit breaker can result in low voltage at the motor.
Mechanical overload. Rotate the motor shaft to see if a binding load is the problem. Check for frozen bearings. Check the air gap between the stator and the rotor. Reduce the load or try operating the motor with no load applied.
Defective motor windings. Make resistance checks of the motor windings for opens and shorts in coil windings and coils shorted to ground faults. An ohm-meter reading of infinity across a set of coil windings means that there is an open somewhere—sometimes it is at one end of the coil and accessible for repair. A short circuit in only a few turns of a coil, while difficult to detect, will still result in a motor overheating. One way to test for a short-circuited coil winding is to compare its resistance reading with that of a known good identical coil.
Burnt-out motor. If one or more of the motor windings looks blackened and smells burnt, it is most likely burnt out and needs to be replaced.
2/ Symptom: The motor overheats. Possible causes
Load. A basic rule is that your motor should not get too hot to touch. Check ammeter reading against full-load current rating of motor. For a higher-than-normal current reading, reduce the load or replace motor with a larger sized one.
Insufficient cooling. Remove any buildup of debris in or around the motor.
Ambient temperature. Higher-than-normal ambient temperatures. Take steps to improve the motor’s ventilation and/or lower the ambient temperature.
Bearings and alignment. Bad bearings or poor coupling alignment can increase friction and heat.
Source voltage. If the operating voltage is too high or too low, the motor will operate at a higher temperature. Correct voltage to within 10 percent of the motor’s rating.
3/ Symptom: Excessive motor noise and vibration. Possible causes:
Bearings. With the motor stopped, try gently moving the shaft up and down to detect bearing wear. Use a stethoscope to check the bearings for noise. When the handle of a screwdriver is placed to the ear and the blade to the bearing housing, the screwdriver will amplify the noise, like a stethoscope. Replace worn or loose bearings. Replace dirty or worn-out oil or grease.
Coupling mechanism. Check for bent shaft on motor or load. Straighten if necessary. Measure the alignment of the couplings. Realign if necessary.
Loose hardware. Tighten all loose components on the motor and load. Check fasteners on the motor and load mounts. Motors with centrifugal mechanisms, brushes, slip rings, and commutators can cause noise due to wear and looseness of the mechanisms.
4/ Symptom: Motor produces an electric shock when touched. Possible cause:
Grounding. Broken or disconnected equipment grounding conductor. Motor windi
ng short-circuited to frame. Check motor junction box for poor connections, damaged insulation, or leads making electrical connection with the frame.
5/ Symptom: Motor overload protector continually trips. Possible cause:
Load. Load too high. Verify that the load is not jammed. Remove the load from the motor and measure the no-load current. It notably should be less than the full-load rating stamped on the nameplate.
Ambient temperature too high. Verify that the motor is getting air for proper cooling.
Overload protector may be defective. Replace the motor’s protector with one of the correct rating.
Winding short-circuited or grounded. Inspect windings for defects and loose or cut wires that may cause a path to ground.
TROUBLESHOOTING CHARTS
Troubleshooting charts may be used to quickly identify common problems and possible corrective courses of action. The following are examples that pertain to specific motor types.
Single – Phase Motors
| Problem | Probable Cause and Course of Action |
| Split-phase motor hums, and it will run normally if started by hand. | Centrifugal switch is not operating properly. Disassemble the mechanism. Clean the contacts. Adjust spring tension. Replace switch. |
| Capacitor-start motor hums, and it will run normally if started by hand. | Centrifugal switch (same as for split-phase motor). Defective capacitor. Test capacitor. If defective replace. |
| Start capacitors continuously fail. | – The motor is not coming up to speed quickly enough as a result of not being sized properly. – The motor is being cycled too frequently. Capacitor manufacturers recommend no more than twenty 3-second starts per hour. – Starting switch may be defective, preventing the motor from opening the start winding circuit. |
| Run capacitor fails. | – Ambient temperature too high. – Possible power surge to motor caused by high transient voltage. If a common problem install a surge protector. |
| Universal motor sparks. | – New brushes not properly seated. Seat brushes with fine sandpaper to fit contour of commutator. – Worn or sticky brushes. Replace brushes or clean brush holder. – Open- or short-circuited armature coils. Replace armature. |
Three – Phase Motors
| Problem | Probable Cause and Course of Action |
| Single-phasing—one phase of the three-phase system is lost. Motor will not start, but if in operation may continue to operate at increased current and diminished capacity. Unique high-pitched sound from motor. | A fuse is blown or one leg of a circuit breaker is open. Check each of the three-phase power lines for correct voltage. |
| Unbalanced three-phase voltage—the voltages of all phases of a three-phase power supply are not equal. A voltage imbalance of 3.5% between phases will cause a temperature rise of 25°C in the motor. Motor operates at a higher-than-normal temperature and reduced efficiency. | – Blown fuse on power factor correction capacitor bank—find and replace fuse. – Uneven single-phase loading—distribute single-phase loads more evenly on the three-phase circuit. – Utility unbalanced voltages—if the incoming voltages are substantially unbalanced, contact the utility and ask them to correct the problem. – Harmonic distortion—The presence of harmonic distortion in the applied voltage to a motor will increase motor temperature, which could result in insulation damage and possible failure. – Locate the sources of the harmonics and use harmonic filters to control or reduce harmonics. |
| Wound-rotor induction motor fails to start or starts and runs erratically. | External rotor resistors—Look for failed components in the resistor bank when troubleshooting. Clean slip rings and check brushes for wear and proper pressure |
| Synchronous motor experiences increased start-up times or erratic acceleration. | Damaged or defective amortisseur windings—Historical inrush testing that records the stator’s current during start-up can greatly assist in determining if these windings have degraded over the life of the motor. |
Direct Current Motors
| Problem | Probable Cause and Course of Action |
| Excessive arcing at brushes. | – Worn or sticky brushes. Replace brushes or clean brush holder. – Incorrect brush position with respect to neutral plane. Rotate brush rigging to the correct position to aid in commutation. – Overload. Measure current to the motor and compare to full-load current rating. If necessary, reduce motor load. – Dirty commutator. The commutator surface should be clean and bright; slight scratches and discoloring can be removed with emery paper. Deep scratches/ridges require the commutator to be machined and mica-undercut. – Armature faults. Test for open- and short-circuited windings in the armature and correct or replace motor. – Field-winding faults. Test for short circuits, open circuits, and ground faults and correct or replace motor. |
| Rapid brush wear. | – Wrong brush material, type, or grade. Replace with brushes recommended by manufacturer. – Incorrect brush tension. Adjust brush tension so that the brush rides freely on the commutator. Replace brush springs if tension measured by a scale is insufficient. |