The supply voltage drops under load because of undersized cables, loose connections, or a weak supply. The motor compensates by drawing more current -- and the overload trips.

What you'll see

Everything looks fine when you measure voltage with the compressor stopped. But during running (especially during star-delta changeover), the voltage drops and the current spikes. The overload trips intermittently -- sometimes the machine runs fine, other times it trips on startup or during heavy load.
Before you assume this is the problem

If voltage is stable and balanced during running, the problem is elsewhere -- check for mechanical binding or a worn relay.

See all causes of overload relay trips →

How to diagnose

  1. Measure voltage DURING running

    This is the key: measure voltage at the motor terminals while the compressor is running loaded. Not at rest -- during actual operation. If voltage drops more than 5% below nameplate, you have a supply problem. A bad connection or undersized cable causes voltage drop under load that's invisible at rest.
    Result: Voltage drop above 5% during load = supply problem.

  2. Check all connections for heat

    A loose connection creates resistance, which generates heat. After the compressor has been running, carefully feel all connection points (main isolator, breaker, contactor, motor terminals). A hot connection is a bad connection.
    Result: Hot connection found = tighten or replace.

How to fix it

  1. Fix loose connections

    Tighten all electrical connections. Clean corroded terminals. Replace damaged cables. For undersized cables, you may need to run new, larger cables from the distribution board.

Common mistakes

Don't rely on voltage measurements taken with the compressor stopped. A bad connection can deliver perfect voltage at zero current but collapse under the motor's 200+ amps starting current.

Parts & tools

Multimeter for voltage measurement during operation. Clamp meter for current. Thermal camera or hands for detecting hot connections (be careful).

Review safety precautions before starting →