Help Wanted Please. Single Phase Induction Motor Turned Noisy After Thermal Trip

[Preso]

For all you electrical gurus out there.... I have a Colchester Student metal lathe with a 1 1/2HP single phase induction motor which is around 8 years old. Yesterday I was taking some heavy cuts in steel and I could hear that the motor was labouring under the load. However I only had a few more cuts to go when the motor suddenly stopped during the cut. I suspected the motor's thermal protection had kicked the motor off line so I waited for an hour or so but it still wouldn't start. I reluctantly squeezed down the back of the lathe and exposed the motor to find that the thermal overload was a manual reset so I pressed the switch in and heard it click and the motor restarted. My problem is that now the motor is quite noisy. It buzzes quite loudly when running but the noise disappears as soon as the power is cut. The bearings are fine and the centrifugal switch can be heard to click as the shaft speed drops. The motor runs with normal power and it starts easily. The buzz sounds electrical and the frequency is around 50Hz. I have not removed the motor yet. I know the motor is undersize. The original was a 7HP three phase. If anyone tells me the motor is badly damaged I will probably not replace it with another single phase. I will probably get a larger 3 phase motor and run it from a VFD as I don't have 3 phase power in the shop. If someone thinks there is a simple fix I will wrestle the motor out and work on it. Oh, I also removed the belt and ran the motor with no load and it is still noisy so it is not the lathe. I can feel a slight vibration but nothing major. The drive pulley is still mounted solidly on the shaft and nothing looks bent.
If anyone has some guidance I would appreciate it.
Regards,
Preso

[Noitoen]

Let it run for a few minutes without load and see if the temperature changes. A short in the windings will make noise and heat and smell after a few minutes.

[J Tiers]

One thing that makes motors buzz is a stuck start switch. You do not seem to have that... you hear the switch, and with the testing you did, the start capacitor should have exploded by now.

I suppose the heat might have loosened a wire, so it can vibrate. That WOULD make a line frequency buzz. See if you can find a loose wire in the windings. You can "glue" it back in place with a glyptal varnish, and should, because a vibrating wire will likely not last long, it is likely to break.

Other than that, the only thing I can think of is a possible cracked rotor bar.... heating could credibly cause that, maybe. But that would tend to make more of a vibratory sort of noise, and I am not sure it would be easily mistaken for line frequency. Maybe it could be.

Most other problems would lead to relatively rapid heating, such as a shorted turn due to insulation damage from heating. I'd expect that to overheat in relatively short time, and begin to smell, even of it did not blacken the winding. You do not mention any such.

[Preso]

I let the motor run with no load for a few minutes and it didn't appear to be getting hot or running poorly. It just makes this louder than usual hum. It is definitely related to the thermal switch as I noticed the increased noise as soon as I reset the switch. I think I will have to remove the motor tomorrow and delve around in it's guts. I am not looking forward to it. I have been reading a rather long post about the advantages of 3 phase motors over single phase in relation to vibration and surface finish. If this motor is dead or dying I think that is how I will proceed.
Regards,
Preso

[PStechPaul]

Thermal overloads don't always provide total protection, and it is possible that a winding has become damaged and partially shorted or leaky. Another possibility might be the heat caused oil to boil off the bearings, causing poor lubrication, noise, and friction. In either case it would be worthwhile to pull the end bells off and check the bearings and windings. If there is a blackened area on the windings and smell of burnt insulation, it may be damaged beyond economical repair, but you could try coating it with insulating varnish to get a bit more use out of it. Another thing to try might be measuring the current under no load to see if it draws more than it should.

[Preso]

The funny thing was I couldn't smell any burning at the time the motor stopped and it doesn't have that hot electrical smell now. I know what a burnt out motor smells like and I have worked around motors that are getting hot enough to damage them and I was not getting any warning signs when things went haywire. I guess I won't know any more until I get the motor out. I'm hoping it's something simple!
Regards,
Preso

[Noitoen]

In that case, let it run. If the problem is serious, it will manifest soon enough.

[Richard P Wilson]

For all you electrical gurus out there.... I have a Colchester Student metal lathe with a 1 1/2HP single phase induction motor which is around 8 years old. Yesterday I was taking some heavy cuts in steel and I could hear that the motor was labouring under the load. However I only had a few more cuts to go when the motor suddenly stopped during the cut. I suspected the motor's thermal protection had kicked the motor off line so I waited for an hour or so but it still wouldn't start. I reluctantly squeezed down the back of the lathe and exposed the motor to find that the thermal overload was a manual reset so I pressed the switch in and heard it click and the motor restarted. My problem is that now the motor is quite noisy. It buzzes quite loudly when running but the noise disappears as soon as the power is cut. The bearings are fine and the centrifugal switch can be heard to click as the shaft speed drops. The motor runs with normal power and it starts easily. The buzz sounds electrical and the frequency is around 50Hz. I have not removed the motor yet. I know the motor is undersize. The original was a 7HP three phase. If anyone tells me the motor is badly damaged I will probably not replace it with another single phase. I will probably get a larger 3 phase motor and run it from a VFD as I don't have 3 phase power in the shop. If someone thinks there is a simple fix I will wrestle the motor out and work on it. Oh, I also removed the belt and ran the motor with no load and it is still noisy so it is not the lathe. I can feel a slight vibration but nothing major. The drive pulley is still mounted solidly on the shaft and nothing looks bent.
If anyone has some guidance I would appreciate it.
Regards,
Preso

7HP sounds an awfully big motor for a Colchester Student. www.lathes.co.uk website quotes the standard 3phase motor as 3hp, with an option for 1.5 hp for schools and training establishments. If you do decide to change to 3 phase, I don't think you need go bigger than 3 hp, after all you've been managing with 1.5hp for the last 8 years.

[Preso]

Ahhh, the plot thickens. I removed the motor from the mounting plate inside the lathe housing and set it on the floor. I removed the terminal housing cover and pulled all the wires looking for one that was loose but couldn't find anything amiss. I also cycled the thermal overload switch and started the motor. Guess what? It seems to be perfectly OK. It's getting late now but tomorrow I will put it back on the lathe mounting plate and try it again. Could it be that I inadvertently reconnected a loose connection? I will tighten everything just to be sure however I still cannot see how the motor overheating has caused something to suddenly come loose after 8 years of problem free running. Maybe it was the thermal overload switch? I will try to track down the problem and report back. With regard to the motor. The lathe originally came from a TAFE college (a trade training college). It could be that they fitted extra large motors because they were being used by inexperienced operators. Having said that, you would think they would do the opposite so that if some muppet did something stupid, the motor would stall and protect the spindle bearings and other vulnerable parts.
Regards,
Preso

[Glug]

Another guess is the motor had a bad connection and that contributed to the thermal overload. The poor connection also further degraded. So when you reset and resumed, you had a buzz.

I looked at some colchester students last year and they seemed to be over-motored. They were 5hp as I recall.

Spindle drive gear failures are apparently not uncommon in those lathes. It is a very expensive repair. I would not push it too hard and would always have good oil, etc.

[Preso]

RESOLVED!
This morning I checked all the connections by tightening them with a screwdriver. I found one that seemed a little loose. It was a terminal block which joined the start winding to the reversing switch. I was able to get another turn on the screw. I put the motor back on the lathe base plate and turned it on and it was as quiet as it should be. The problem is I still have no real idea as to what was causing the noise. Still, at least I don't need to replace the motor or pay for some costly repairs. My very uneducated guess is that there was some resistance at that terminal block and it may have been reducing voltage to the run windings. In any case, the long term plan is to replace the single phase motor with a three phase of around 2.2kw which should give me double the grunt and a much smoother drive. Maybe less startup current draw as well.
Thanks to those of you who gave suggestions. It helps to be able to share one's grief in times of crisis
Regards,
Preso

[J Tiers]

Sometimes all it takes is pulling out the offending item and putting it back correctly. And you never come up with a real answer.

I like to know WHY there was a problem, but it just is not possible to determine that sometimes. Usually it is something that you "fix" just by getting into the unit and disturbing stuff.

One often hears the possibility of resistance in a connection. And that is certainly possible. But in general, any small to medium amount of resistance will lead to power dissipation, heating up, etc, and the results of that are generally observable. Melted insulation, charred insulation, etc.

I'd suspect a loose connection of simply vibrating, arcing , and creating a problem just by the intermittent connection. But if you did not see evidence of arcing, that probably was not going on for very long. Such things have a tendency to weld themselves together, as well.

See if it happens again.

[Mr Fixit]

Glad you got it fixed with the easy route. You said the thermal overload switch seemed iffy, I would say since it is part of the winding's circuit if it didn't make good closure, when pushed it may have been the trouble by not fully closing and when you took it out and pressed it again it was good to go.
Just one more opinion.

TX
Mr fixit for the family
Chris