Contactor chatter and coil faults

It's 5am. The extract fan on the dust plant keeps dropping out. You open the panel and hear it before you see it: the main contactor is buzzing, pulling in and dropping out a few times a second. Then it stops, and the fan runs. Ten minutes later it does it again.

By the end of the shift there's a burnt smell in the panel. On days, the coil goes open circuit and the fan stops for good.

A chattering contactor is telling you something. The coil isn't getting a clean, steady supply, or the magnet can't close properly. Change the contactor without finding out which, and you'll be back.

Listen first, then measure

A healthy AC contactor makes a firm clunk and a faint hum. A loud buzz or a rattle while it's pulled in points at the magnet. A rapid in-out chatter points at the coil supply or the circuit feeding it.

So start by working out which you've got. Watch the contactor for a minute. Is it pulled in and buzzing, or actually dropping out? Is it every time it starts, or only after it's been running?

Chatter: check the coil supply

Most chatter comes from the control circuit, not the contactor. These are the usual causes, roughly in the order we'd check them:

• Low coil voltage. Measure at the coil terminals while it's trying to pull in, not with it dropped out. A control transformer that's too small, a long run to a remote stop station, or a sagging 24V supply can all drop the volts as the coil draws its inrush current. Our guide to 24V DC control supply faults covers the DC side.
• A bouncing contact upstream. A worn stop button, a pressure switch sitting on its set point, or a float switch in a tank with a wave on it. The contactor is only doing what it's told.
• A flickering output. If a PLC or interposing relay drives the coil, watch the output LED. If it's flickering, the problem is in the logic or the input that feeds it. Our PLC inputs and outputs guide shows how to trace it.
• A loose terminal. On the coil, the A2 common or the control fuse. Heat and vibration work them loose over time.

That last one is more than a nuisance. The Electricity at Work Regulations require every joint and connection to be "mechanically and electrically suitable for use" (regulation 10). A loose terminal in a control circuit is a hot terminal waiting to happen. Thermal imaging finds them before they burn.

Buzzing when pulled in: check the magnet

If the contactor pulls in and stays in but buzzes hard, look at the magnet. You'll need it isolated for this.

• Dirt, rust or debris on the pole faces stops them closing flat.
• On AC contactors, a cracked or missing shading ring makes a loud buzz.
• A bent or binding moving part stops the armature seating fully.

On an AC coil, an armature that never closes fully is how coils burn out. The coil draws much more current with the gap open. If it can't close, it keeps drawing it and cooks.

Why coils burn out

When you find a burnt coil, don't just fit a new one. Ask why. In our experience it's usually one of these:

1. Wrong coil voltage. A 110V coil on a 230V circuit, or a DC coil on AC. Check the coil marking against the drawing and against what you measure.
2. The armature not closing, as above.
3. Voltage too low for too long, so the contactor chatters for hours.
4. Heat. A packed panel with blocked filters runs hot, and so do the coils in it.

If you fit a new coil and it's warm after ten minutes, you haven't found the cause.

Welded contacts are a safety problem

Chatter does more than annoy. Every time a contactor drops out and pulls back in under load, the main contacts arc. Do that enough times and they weld.

HSE's guidance on light curtains notes that the relevant contactor standard "accepts the risk of welded contacts". It advises using suitable short circuit protection, agreed with the contactor maker, "to reduce the risk of contact welding" (HSG180, paragraph 55).

A welded contactor won't stop the motor when the coil drops out. If that contactor is part of a safety function, the e-stop or guard may not stop the machine either. So HSE expects contactors used this way to have a means of monitoring whether the main contacts are open or closed (HSG180, paragraph 56). The safety relay watches an auxiliary contact. If the main contacts weld, the relay won't reset. Our safety relay guide covers that feedback loop.

HSE gives a real example of getting this wrong. A contactor in an interlock had been chosen for about four operations a day. It was actually doing about 200. A contact welded, and the single channel interlock "failed to danger" (HSG238, Out of control). A contactor that chatters is racking up operations it was never sized for.

Testing safely

Measuring coil voltage while it pulls in is live work. HSE counts "testing of live exposed conductors using a test instrument" as live work (HSR25, paragraph 50). The coil may be at 24V. The main terminals a few centimetres away are at 400V.

Shroud the power side where you can. Use GS38 leads and set the meter before the probes touch anything. GS38 notes that accidents happen when a meter is "set on a current or resistance range when measuring voltage" (GS38). Isolate before you touch the magnet or the contacts. A stuck contactor can still be live on its load side.

Like for like isn't always right

The quick fix is to fit the same contactor again. Sometimes that's fine. But if the old one welded or burnt out, check it was the right size for the job. Look at the motor current, the duty and how often it switches. A contactor on a machine that starts and stops every few seconds has a much harder life than one that starts once a shift.

Getting this right takes an electrician who asks why before swapping the part. When that person isn't on shift, the same contactor gets changed three times in a month and nobody adds up the downtime. Our piece on the fault that keeps coming back is about breaking that loop.

What to do on Monday

1. Walk your main panels with the doors shut and listen. Note any contactor that buzzes loudly.
2. Check coil voltages on the drawings match the coils fitted, on your three most critical machines.
3. Find out which contactors in safety circuits have a monitored auxiliary contact, and which don't.
4. Hold a spare coil, as well as a spare contactor, for your critical starters.

Paragraph numbers refer to HSG180, first edition (1999), and HSR25, third edition (2015).