Motor tripping on overload: what to check

It's 2pm on a hot afternoon. The mixer motor on line 2 trips its overload. The operator resets it and it runs. At 3:30 it trips again. Reset. At 4:10 it goes again, and this time someone asks the shift sparky if they can "just knock the overload up a bit".

You'll know that request. You'll also know where it ends. A month later the motor goes up in smoke, the mixer's down for two days and someone's ringing round for a rewind.

An overload that keeps tripping is doing exactly what it's there for. It's telling you the motor is drawing more current than it should. The job is to find out why.

Leave the setting alone

Start with the rule. The Electricity at Work Regulations require efficient means of protecting every part of a system from excess current, where that's needed to prevent danger (regulation 11). HSE's guidance on the regulations names the cause you're looking at. Overload currents can come from an outside influence on the equipment, and the example given is mechanical overloading of an electric motor (HSR25, paragraph 170)*.

So the overload is the protection. Turn it up past what the motor can take and you've taken the protection away. The motor still overheats. Nothing stops it any more.

What you should check is that the setting is right in the first place. Read the full-load current off the motor nameplate. Compare it with the dial on the overload. If someone's already been at it, put it back to the nameplate figure and write down that you did.

While you're there, check the motor on the plate matches the motor on the machine. Motors get swapped. A 5.5kW replacement on an overload set for a 7.5kW won't protect anything.

When does it trip?

The timing tells you a lot. Ask the operator, or look at the alarm history.

• On start, every time? Look at the start itself. A long run-up on a heavy load, a star-delta timer set too short or too long, a soft starter ramp that's wrong, or a load that's being started against a jam.
• After running a while? That's heat building up. Either the motor's working too hard, or it can't get rid of its heat.
• At random, with no pattern? Look for a loose connection, a phase dropping out, or a fault in the overload relay itself.
• Only in hot weather, or only in the afternoon? The motor and the panel are both hotter. Our guide to hot weather breakdowns covers panels and motors in a heatwave.

Put a clamp meter on it

Now measure. With the machine running normally, clamp each of the three phases in turn. You want the current on each phase, and you want to compare it with the nameplate.

Two patterns come up again and again.

All three phases high and roughly equal means the motor is working too hard. The cause is almost always mechanical. Look at the load.

One phase much higher or lower than the others means an electrical problem. It could be a bad supply, a high-resistance joint or a worn contactor pole. Or it could be a winding starting to fail. Check the supply voltage on all three phases. Then look for heat at terminals and contactor poles. A thermal camera makes short work of this, if you have one.

Low supply voltage pushes current up as well. If the whole site is running low, every motor is working harder.

Look at the load

If the current's high on all three, go and look at what the motor drives. Isolate and lock off first. Our guide to safe isolation covers the steps.

Then work outward from the motor:

1. Turn the shaft by hand, decoupled if you can. Rough or tight means bearings. Our guide to bearing noise and heat helps here.
2. Check the gearbox. Oil level, oil condition and temperature. A gearbox running hot is a load on the motor. See gearbox running hot.
3. Check belts and chains. Over-tight belts load bearings and the motor. Chains that are dry or stretched drag.
4. Look at the process. Is the mixer seeing a thicker product than it used to? Is the conveyor carrying more than it was built for? Has someone changed the recipe or the line speed?
5. Check for jams, build-up and anything rubbing. Product packed round a roller or a scraper set too hard will do it.

Let the motor breathe

A motor that can't shed heat will trip even on a normal load. Look at the fan cowl. Is it packed with dust, product or bits of shrink wrap? Are the cooling fins buried in muck? Has someone fitted a guard or a cover that blocks the airflow?

Check the overload relay's surroundings as well. A thermal overload in a hot panel trips earlier, because the relay itself is hot. If the panel fans have stopped or the filters are blocked, you'll get trips across several motors at once.

And check how often it starts. A motor that's started and stopped many times an hour gets hot from the starting current alone. If the process has changed and the motor is now cycling much more, that may be your cause.

Reset and run costs more than you think

Every reset puts the same motor back into the same fault. Each trip means more heat in the windings, more stress on the insulation. Sooner or later it fails for good, and it fails on a night shift with no spare on the shelf.

That's the true cost of reset and run. Our post on the most expensive habit on site goes into it. If the motor does burn out, read replace like for like or find out why first before the new one goes in.

Here's our view. Two trips on the same motor in a shift means the machine stops until someone has found the cause. That needs someone who can read a clamp meter, a nameplate and a gearbox, and who won't touch the overload dial. If you don't have that person on shift, get one. A rewind costs a lot more than a callout.

What to do on Monday

1. Walk your critical motors and check every overload setting against the nameplate. Correct any that have been turned up, and record it.
2. Make a rule: no one changes an overload setting without an electrical engineer signing it off.
3. Add "clean the fan cowl and fins" to the PPM sheet for motors in dusty or messy areas.
4. Log every overload trip with the time and the machine. Two on the same motor in a week gets investigated.

* Paragraph number correct as of 4 September 2026 and refers to HSR25, third edition (2015). Check HSE's HSR25 page for the latest.