Motor Failure Library

Understanding Motor Failures

Electric motors are the most common prime mover in industry. Understanding failure modes helps extend motor life and prevent unplanned downtime.

Bearing Failure

Bearings are the most common motor failure mode, accounting for approximately 50% of failures. Motor bearings fail from wear, contamination, inadequate lubrication, and electrical damage.

• Symptoms: Increasing vibration and noise, rising bearing temperature, grease leakage, and eventual seizure or locked rotor.
• Root causes: Inadequate or contaminated lubrication, over-greasing (causes overheating), water or dirt ingress, misalignment, excessive belt tension (overhung load), and bearing currents from VSDs (EDM pitting).
• Prevention: Follow the manufacturer's lubrication schedule with the correct grease, avoid over-greasing (use a grease gun with pressure relief), install VSD output filters (dV/dt or sine wave) to prevent bearing currents, use insulated bearings for large VSD-driven motors, verify alignment, and maintain correct belt tension.

Winding Insulation Breakdown

Insulation failure is the second most common motor failure. Windings short to ground or to each other when insulation degrades from heat, contamination, or voltage stress.

• Symptoms: Earth fault trip, overcurrent trip, burnt smell, smoke, reduced insulation resistance (measured with megger), and eventual winding failure.
• Root causes: Thermal ageing (insulation degrades with time and temperature — every 10°C above rated halves the insulation life), contamination (moisture, oil, dust, chemicals), voltage stress (transients from switching or VSDs), mechanical vibration, and overloading.
• Prevention: Keep motors within rated temperature (do not overload), keep motors clean and dry, use space heaters for motors in humid environments that are stopped, install surge protection, use VSD-rated motors (inverter duty) with VSDs, and perform insulation resistance testing.

Overheating

Overheating is both a failure mode and a root cause of other failures (insulation breakdown, bearing failure). It is caused by overloading, inadequate cooling, or high ambient temperature.

• Symptoms: High frame temperature, tripped thermal protection, burnt paint smell, reduced efficiency, and shortened motor life.
• Root causes: Overloading (current above rated), inadequate cooling (fan blocked, filter clogged, enclosure too confined), high ambient temperature, low voltage (increases current for the same load), and unbalanced voltage (causes negative sequence currents and heating).
• Prevention: Size motors correctly for the load, maintain cooling (clean fans and filters, ensure adequate ventilation), keep ambient temperature within design, verify voltage is balanced (within 1% phase-to-phase), and install thermal protection.

Single-Phasing

Single-phasing occurs when one phase of a three-phase supply is lost. The motor continues to run on two phases but draws excessive current, overheating the windings.

• Symptoms: Humming sound, excessive current on two phases, motor overheating, reduced torque, and eventual burnout of the affected windings.
• Root causes: Blown fuse on one phase, loose connection, broken wire, contactor contact failure, or transformer failure.
• Prevention: Install phase loss protection (phase failure relay) that trips the motor when a phase is lost, maintain contactors and fuses, check connections during maintenance, and use motor protection relays with single-phase detection.

Contamination

Contamination by dust, moisture, oil, or chemicals degrades insulation, corrodes components, and blocks cooling paths.

• Symptoms: Reduced insulation resistance, corrosion on internal components, blocked cooling passages, and premature winding or bearing failure.
• Root causes: Inadequate enclosure for the environment (open motor in a dirty area), failed seals and breathers, washdown in food/pharma environments, and chemical fumes.
• Prevention: Select the correct enclosure (TEFC or TENV for dirty environments, washdown-duty for wet areas), install and maintain seals and breathers, use space heaters in humid environments, and keep the motor clean.