Electrical & Logic | Module 219

Line Voltage Magnetic Contactor Audit

Field Status: A magnetic contactor is not a solid wire; it is a mechanical bridge. Over thousands of start-stop cycles, high-voltage arcing creates carbon deposits (pitting) on the silver-alloy contact points. As these points pit, their resistance increases, creating a localized “hot spot” that induces a voltage drop across the contactor, eventually leading to phase imbalance or “single-phasing” on 3-phase motors.

In a 3-phase compressor, if one contactor pole has 1.0 Ohm of resistance due to pitting while the others have 0.01 Ohms, the voltage arriving at the motor will be unbalanced. This imbalance causes the motor to run hot, draw uneven amperage, and eventually trigger thermal overloads or winding failure.

SYS AUDIT: Pole Resistance Matrix
CONTACT STATUS: —
—

The Voltage Drop Test

Never rely on Ohms-mode alone for a contactor audit. The ultimate test is a Loaded Voltage Drop Test. With the compressor running, measure the voltage drop *across* the closed contact (Line to Load). If you read more than 0.5V AC drop while the motor is under load, the contacts are bad.

Resistance (Ω)StateAction
0.000 – 0.020Clean / Like NewServiceable.
0.050 – 0.150Slight PittingClean with contact burnisher (only if silver).
0.200+Failed / CarbonizedCondemn. Replace contactor.

Diagnostic Gateway Challenge

You test a contactor on a 3-phase motor. The motor draws 10 Amps on Pole 1, 10 Amps on Pole 2, and 18 Amps on Pole 3. What is the most likely cause of the abnormal current on Pole 3?

END OF MODULE 219

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