How to Choose & Maintain Electric Motors: 6 Key Steps

"Selecting a High-Quality Motor" – Remember the Six Key Steps

• Inspect (Look): Check the motor’s appearance
  The motor’s surface should have a smooth, even paint finish. The nameplate must be properly installed with complete and clear markings, including: model number, serial number, rated power, rated current, rated voltage, allowable temperature rise, connection method, speed, noise level, frequency, protection rating, weight, standard code, duty type, insulation class, manufacturing date, and manufacturer. For enclosed motors, the frame’s cooling fins should be intact and undamaged, with all accessories present.

• Rotate: Manually turn the motor shaft
  A quality motor should rotate smoothly and freely without resistance or unusual noises. It should have good inertia, and there should be minimal axial movement (endplay).

• Listen: Listen to the motor’s sound during operation
  Power the motor and run it for 15–25 minutes. A healthy motor produces a steady, light, and smooth sound—uniform and harmonious. You should hear only a soft "humming" (electromagnetic noise) and faint "rustling" (mechanical noise). Sharp, dull, rubbing, or vibrating noises indicate poor motor quality.

• Touch: Feel the motor after test operation
  After running and stopping the motor, touch the motor frame and end shields. They should not feel excessively hot, and bearing temperature should be normal. Check carefully for any oil leakage or oil throw.

• Check: Open the terminal box and inspect wiring
  Ensure phase wire labels are clear and complete. All connecting links should be securely tightened with nuts in place. A grounding bolt must be provided.

• Test: Measure insulation resistance and current
  Use a 500V megohmmeter to measure insulation resistance between phases and between each phase and the frame. A qualified motor should have insulation resistance greater than 0.5 MΩ. During operation, use a clamp meter to measure no-load current in each phase. Any single phase current should not deviate by more than 10% from the average of the three phases. No-load current should be 25%–50% of the rated current.

Importance of Daily Motor Inspection and Maintenance
The normal operation of machinery heavily depends on the reliable performance of electric motors. Therefore, motor maintenance is crucial. Many people neglect maintenance or don’t know how—only realizing its importance when the motor fails and requires costly repairs that also cause work delays. Proper maintenance is a critical discipline.

Motor Maintenance
The key to motor maintenance is preventing burnout. The following methods are proven effective:

• Maintain the starter equipment in good condition
  Most burned-out motors fail due to difficult or faulty startup, such as phase loss caused by poor starter performance. Arcing or sparking contacts can cause large voltage and current fluctuations. To keep starter equipment in good condition: regularly inspect, clean, and tighten components. Dirty or oxidized contactor contacts increase contact resistance, causing overheating and arcing, which may lead to phase loss and burnout of windings. Rust or dust on the contactor coil core can prevent proper engagement, causing loud noise and increased coil current, eventually burning out the coil. Therefore, electrical control panels should be installed in dry, well-ventilated, and accessible locations. Regularly clean dust and inspect contacts. Add rust prevention to the coil core. Frequently tighten all connections and ensure contactor contacts make good contact. Mechanical operations must be flexible and accurate—these are essential for smooth motor startup.

• Keep the motor clean and ensure good cooling
  The motor’s air inlet must be kept clean at all times. No dust, oil, straw, or debris should be near the intake, as these can be drawn into the motor, causing internal short circuits, damaging winding insulation, or blocking airflow and causing overheating and burnout.

• Operate the motor within rated current; avoid overload
  Overloading causes reduced speed, increased current, and rising temperature. Causes include excessive load, low voltage, or mechanical jamming. During overload, the motor draws excessive power, causing current to spike and temperature to rise. Prolonged high temperatures accelerate insulation aging and lead to winding burnout—the primary cause of motor failure. Therefore: regularly check the transmission system for smooth and reliable operation; avoid prolonged overloading of machinery; and maintain stable voltage—never operate under low voltage.

• Maintain balanced phase currents

• Keep motor temperature and temperature rise within normal limits
  During operation, regularly check the temperature of bearings, stator, and housing. This is especially critical for motors without overload protection. If bearings lack lubrication or are damaged, temperature will rise—particularly near the bearing area. Immediately stop the motor and inspect. Try adding lubricant; if ineffective, remove and inspect the bearing. Replace if there are cracks, scratches, or damage on rolling elements or raceways, if clearance is excessive, or if the inner ring rotates on the shaft. Any of these conditions can lead to severe failures like rotor-stator rubbing (scraping). To monitor temperature, insert a thermometer into the motor’s vent and secure it with cotton—this allows continuous monitoring. The temperature difference between inside and outside the housing is typically around 1°C.

• Promptly identify and address abnormalities
  During operation, the motor should exhibit no vibration, unusual noise, or odors. These are key signs of abnormal operation and potential major failures. Detecting and resolving issues early is essential to prevent fault escalation and motor burnout.