The thermal relay or overload breaker protects the motor from overheating due to excessive current draw. When it trips, either the motor is drawing too much current (electrical or mechanical problem), or the voltage is too low (wiring problem).
The key diagnostic is measuring actual current with a clamp meter. If current is high, find out why. If current is normal but the relay still trips, the relay itself may be faulty.
Industrial Reciprocating Troubleshooting
Overload Relay Trips – Industrial Reciprocating Air Compressor Troubleshooting
Thermal overload or motor breaker keeps tripping—either an electrical issue or the motor is working too hard.
Industrial systems
Field-tested diagnostics
Independent & unbiased
Safety Notice
Disconnect power before testing electrical components. If measuring current with a clamp meter, keep hands away from rotating parts. Capacitors can hold charge—discharge before handling.
What this problem usually means
Check these first
5–10 minute checks before diving deeper
- Measure motor current with clamp meter—compare to nameplate FLA (full load amps)
- With power OFF, try turning the compressor by hand—should rotate freely
- Check voltage at the motor during startup and running
- Does voltage drop significantly when compressor starts?
- Is the compressor on a long extension cord or undersized wiring?
- Check for single-phasing (if 3-phase)—measure all legs
- Is the compressor trying to start against pressure? (unloader issue)
- Has the overload been tripping repeatedly? It may be heat-damaged.
Common root causes
Why this happens in industrial reciprocating compressors
Low voltage supply
Voltage drop from undersized wiring, long extension cord, or weak supply. Motor draws more current to compensate. Measure voltage under load.
Mechanical binding
Compressor hard to turn by hand indicates internal problem—stuck piston, seized bearing, or damaged connecting rod. Requires teardown.
Starting against pressure
Unloader valve not releasing pressure. Motor tries to start under load and draws excessive current. Check unloader and check valve.
Failed capacitor (single-phase)
Bad start or run capacitor causes motor to draw excessive current. Motor may hum, start slowly, or not start at all.
Worn overload relay
Thermal overloads become more sensitive with age and repeated trips. If current is normal but relay still trips, replace the relay.
What NOT to do
Don't keep resetting and restarting after multiple trips—repeated overheating damages the motor windings. If it trips more than twice in a row, stop and diagnose the problem.
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