Fault & Noise Diagnosis

Typical servo motor faults, their causes and remedies, plus why servo systems are noisy.

Why servo systems are noisy

Noise and instability usually come from a mismatch between the very fast servo response and a slower mechanical transmission (e.g. elastic V-belt). Fix by increasing mechanical rigidity and lowering inertia, replacing belt with direct screw or gearbox, or reducing servo gain/bandwidth and using resonance suppression / low-pass filtering. The motor itself is rarely the cause.

Insulation resistance (idle motor):

R > UN/(1000 + P/1000) [MΩ] ; UN = rated winding voltage (V), P = power (kW). For UN = 380 V, R > 0.38 MΩ.

Common faults

Fault	Cause / Remedy
Shaft current	Cause: asymmetric field, supply harmonics, rotor eccentricity, split stator. Harm: bearing pitting, overheating. Cure: AC reactor on drive output; insulate bearing seats.
Not for high altitude	Altitude raises temperature rise and reduces output; DC commutation worsens. Cure: derate or anti-corona; choose proper brush material.
Avoid light-load running	Light load lowers power factor and efficiency; uneconomic. Size the motor to the real load.
Overheating	Cause: overload, phase loss, blocked airflow, long low-speed run, supply harmonics. Improve cooling and balance the load.
Idle motor pre-commissioning	Measure insulation; dry by no-load run, low-voltage AC, or heated air. Clean fan; renew grease.
Cold-environment start	Insulation cracks, grease freezes. Keep warm; check windings/bearings before running.
Three-phase current imbalance	Cause: unbalanced voltage, bad joints, short circuits, wrong wiring. Restore balance.
60 Hz motor on 50 Hz supply	Lower frequency raises flux and current, overheating. Do not connect mismatched-frequency motors.
Phase loss	Supply: bad switch, broken line, blown fuse. Motor: loose terminals, bad solder, broken winding.
Abnormal vibration / sound	Mechanical: bad/under-lubricated bearing, loose screws, debris. Electromagnetic: overload, imbalance, phase loss, shorted winding, broken rotor bar.
Before starting	Check insulation (>0.5 MΩ low-voltage), voltage, starter, fuse, grounding, transmission, environment.
Bearing overheating	Motor: tight fit, poor tolerances, wrong bearing, bad grease, shaft current. Use: bad alignment, over-tight belt, poor maintenance.
Low insulation resistance	Cause: damp/wet winding, dust/oil, aged insulation, damaged leads. Dry/clean/replace.