Troubleshooting guide

Why Does a Contactor Chatter or Drop Out?

A contactor chatters when the coil lacks steady voltage. Covers control-voltage dips, coil rating, intermittent commands and wear.

Electrical & Power · Last reviewed October 17, 2026

A contactor pulls in with a strong inrush current, and holds with much less. If the voltage across the coil falls below its pick-up value, even briefly, the armature opens, the current falls and the cycle repeats, which is the buzzing or chatter. Heat from the chatter soon damages the contacts and the coil.

Control circuits and the power terminals of a contactor can both be live, and the power circuit can carry large fault energy. Measurements and repairs belong with qualified personnel following lockout/tagout. Never hold a contactor closed by hand.

What you observe	Likely cause	What to check
Chatters when it closes, then stays quiet	Control voltage drops during the inrush, from a weak control transformer or long wiring	Coil voltage during pull-in, measured at the coil terminals
Steady buzz while closed	Dirty or worn pole faces, a broken shading ring (AC coil), or low voltage	Mechanical condition of the magnet and coil voltage
Drops out when another large load starts	Control or supply voltage dips below the drop-out value	Control voltage recording during the event, and the source of the control power
Chatters from the start on a new installation	Wrong coil rating, or the wrong voltage or frequency	Coil rating against the control voltage. AC coils on DC or the reverse
Closes and opens at random	Intermittent command from a loose wire, a bouncing contact or a failing sensor	Pilot devices, terminals, interlocks and PLC outputs
Chatters when hot or at high load	Temperature-sensitive coil, or an undersized control transformer	Transformer rating against the total coil inrush

A coil-voltage drop example

A contactor has a 24 V AC coil with an inrush current of 2.5 A. It is wired 150 ft from the control transformer with 18 AWG copper (6.385 Ω per 1,000 ft). The round trip is 300 ft:

R = 6.385 × 0.300 = 1.92 Ω     Drop = 2.5 × 1.92 = 4.8 V     Coil voltage = 24 − 4.8 = 19.2 V

That is 80% of the rated coil voltage. Contactors are typically specified to pick up at about 85% of rated coil voltage or more (check the datasheet), so the contactor may fail to close or chatter. Once the armature closes, the current falls to a few hundred milliamps and the voltage recovers, which is why the problem often shows only at the instant of closing. Heavier wire, a higher control voltage or a local control transformer fixes it.

Separate electrical from mechanical

Measure the coil voltage at the coil terminals, during the closing, with a meter that can capture a dip.
If the voltage stays near rated and the contactor still chatters, suspect the magnet face, shading ring or mechanical binding.
If it chatters only when other equipment starts, look at the supply of the control circuit.

Write a useful problem statement

Contactor make, size and coil rating, and the control voltage and source.
When it chatters (closing, steady, when other loads start) and how long it has been happening.
Coil voltage during closing and while closed, and the wiring length and size.
Condition of the contacts and any burn marks.

Related tools and labs

Calculators, labs and simulators are part of the All-Access Pass.

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Questions

Can I replace just the coil?

Often yes, using the exact coil for the contactor. Replace the contacts too if they are burnt, and fix the cause of the chatter.

Why does an AC coil buzz?

The magnetic force on an AC coil pulses at twice the line frequency. A shading ring keeps it pulled in between pulses. A dirty or broken part makes the buzz audible.

Does a PLC output need a relay?

Often yes. The output may not be able to supply the coil inrush, so an interposing relay is used.

Educational material. Examples use simplified, hypothetical inputs and are not a design, a construction document or professional engineering advice. Confirm requirements with the codes and standards adopted where the work is built and with a licensed engineer.

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