The motor overload relay is designed to protect the motor from damage due to overcurrent. When it trips repeatedly, the motor is either drawing too much current (mechanical or electrical problem), or the relay itself has degraded and become oversensitive.
The key diagnostic is measuring actual motor current. If current is high, find out why. If current is normal but the relay still trips, the relay is likely faulty.
Rotary Screw Troubleshooting
Overload Relay Trips – Rotary Screw Air Compressor Troubleshooting
Motor overload relay keeps tripping—either the motor is drawing excessive current, or the relay itself has become too sensitive.
Industrial systems
Field-tested diagnostics
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Safety Notice
Before testing motor windings or checking connections, ensure power is locked out and tagged out. Capacitors in VFD drives can hold dangerous voltage. Only qualified personnel should perform electrical testing.
What this problem usually means
Check these first
5–10 minute checks before diving deeper
- Measure motor current with a clamp meter during operation—compare to nameplate FLA
- With compressor OFF and locked out, try turning the airend by hand—should rotate smoothly without binding
- Check motor winding insulation with a megger—should be in the megohm range
- Measure voltage at motor terminals during starting and running—significant drop indicates connection problem
- Verify all three phases are present and balanced
- Check all electrical connections, fuses, and contactors for signs of heat or corrosion
- Verify overload relay is set to correct current rating for the motor
- If current is normal and relay still trips, replace the relay (they become sensitive with age)
Common root causes
Why this happens in rotary screw compressors
Mechanical binding in airend
Worn bearings, seized airend, or internal damage causes motor to work harder. If you can't turn the airend by hand (locked out!), something is mechanically wrong.
Motor winding problem
Degraded winding insulation or shorted turns cause excessive current. Test insulation with megger—should read megohms, not kilohms.
Voltage drop or bad connection
Loose connection, corroded terminal, or undersized wiring causes voltage drop under load. Motor draws more current to compensate.
Phase loss or imbalance
Missing phase or significant phase imbalance forces remaining phases to carry extra load. Check all three phases under load.
Overload relay worn or misadjusted
Thermal overload relays become oversensitive with age and heat cycling. If motor current is normal but relay trips, replace the relay.
What NOT to do
Don't simply increase the overload relay setting or bypass it to "fix" the problem. The relay is protecting the motor from damage. If current is high, find and fix the root cause. Raising the setting just moves the failure from relay trip to motor burnout.
Full Rotary Screw Troubleshooting Manual
Step-by-step diagnostics, root cause logic, and practical fixes for oil-injected rotary screw compressors. Save time and reduce downtime with proven methods.
- Step-by-step diagnostics for common failures
- Root cause analysis techniques
- Practical fixes with parts notes
- Works across all major brands