SiteYou can smell it before you see it. The conveyor drive motor on the infeed has stopped, the overload's tripped, and the terminal box is too hot to touch. Megger it and it's a dead short to earth.
There's a spare on the shelf. Same frame, same kilowatts, same flange. Your fitter can have it in by the end of the shift.
So you swap it. The line runs. Three weeks later, the new motor goes the same way.
A motor rarely burns out on its own. Something made it work too hard, run too hot, or take in water. Swap it without looking and you've fitted a new motor into the same problem.
Get the line back first
We're not going to tell you to leave the line stood while you run a root cause study. Production needs it back, and sometimes the spare is the right answer tonight.
The trick is to do two jobs at once. One person fits the spare. While they do, someone spends twenty minutes looking at why the old one failed. Most of the evidence is gone once the motor goes in the skip.
Before it's unbolted, take photos of the terminal box and the nameplate. Mark every lead before you pull it. In one case in HSE's control systems guide, HSG238, a replacement valve went in wrong "because the connecting wires of the solenoids were not marked". A machine moved when it shouldn't have, and an operator lost a hand. A motor wired back to front is less dramatic, but it's the same mistake.
Look at the load before the motor
In our view, most burnt-out motors are killed by the machine they drive.
The HSE guidance on the Electricity at Work Regulations gives "mechanical overloading of an electric motor" as a cause of overload current (HSR25). On a shop floor, that looks like:
- a seized bearing on the driven end, or a tail pulley that won't turn by hand
- a gearbox that's low on oil or full of metal
- a chain or belt that's been tightened until something gives
- product jammed under a conveyor, or a sorter running heavier than it was built for
- a pump running off its curve because someone throttled a valve
So turn the load by hand with the motor off. If it's stiff, you've found something. Fit the new motor to that and it'll go the same way. Our guide to bearing noise and heat is a good next read.
Then look at the supply and the starter
If the load turns freely, look upstream.
A lost phase will cook a motor fast. Check all three fuses or the breaker, then each contact on the contactor. One burnt tip is enough. Look for a loose terminal with heat marks, in the motor box and in the panel.
Then check the overload. Is it set to the motor's full load current off the plate? Or did someone wind it up last year to stop it tripping? The Electricity at Work Regulations ask for efficient means of protecting from excess of current "as may be necessary to prevent danger" (regulation 11). HSE's guidance adds that the protection has to be there "in anticipation of excess current; a fault or overload need not have occurred" (HSR25). An overload wound up past the plate rating isn't protecting anything.
If the motor runs on a drive, read the fault log before anyone resets it. Overcurrent and earth fault trips in the days before the failure tell you a lot. See inverter trips: overcurrent, overvoltage and earth fault.
And the place it lives
Some motors die from where they're fitted. Check the vent cover for fluff, dust or shrink wrap. Look for signs of wash-down water in the terminal box. Feel how hot the room gets in summer. A motor in a box of its own heat will burn out, however good it is.
Like for like isn't always like for like
The spare on the shelf might be the same frame and kilowatts. It might not be the same motor.
Check the plate against the old one. Speed, voltage, full load current, duty, IP rating, mounting. A 4-pole motor in place of a 6-pole one will fit the flange and run the conveyor at the wrong speed. A motor with a lower IP rating in a wash-down area is the next burn-out waiting.
If the spare is different, the overload setting changes too. Reset it to the new plate, not the old one.
And test the spare before you fit it. A motor that's sat on a shelf in a damp store for three years can fail an insulation test straight out of the box. Our guide to insulation resistance testing on motors covers how.
Who should be doing it
Swapping a motor is a fitter's job and an electrician's job together. The mechanical side is coupling, alignment and fixings. The electrical side is isolation, termination, rotation and the overload. Under the Electricity at Work Regulations, people doing electrical work need the knowledge or experience for it, or the right supervision (regulation 16).
On nights, sites often have one or the other. A good mechanical fitter can get the motor bolted in. Terminating it, proving rotation and setting the overload may be outside what they do. So the job stalls at 3am, with the motor sat on the bed and nobody to wire it. If you're short an electrician on a shift, have one you can call before the night you need them.
Write down what you found
Here's our view. Every burnt-out motor gets a cause written next to it, even if the cause is "don't know yet".
The Electricity at Work Regulations ask for systems to be maintained so as to prevent danger, so far as is reasonably practicable (regulation 4). A motor that keeps burning out is telling you that something isn't being maintained. If it's the second one on the same drive this year, treat it as the fault that keeps coming back, and put it on the list for a proper look.
What to do on Monday
- Check the overload setting against the motor plate on your five most critical drives.
- Insulation test every motor in the spares store, and label each with the result and the date.
- Make it a rule: photos and marked leads before any motor comes out.
- Add a "cause" box to your breakdown log for every motor failure, and fill it in.