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Motor running hot

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Hitch

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I believe we have a resident motor specialist here..... :wink:

Used me lathe earlier, RPML300, just long enough to turn a pen, so no big loads, not hours and hours of heavy cuts. The motor seemed to be getting hotter than it perhaps should- couldn't rest my hand on it for more than about 5-10 seconds. Bearing in mind I deal with bit of hot metal day in day out, so its not my temperature sensors in me fingertips palying up, it is hot.
Its not full up with loads of dust, it all looks pretty clean in there. Free running, doesn't feel like its binding up anywhere.....

I noticed very occasionally it didn't like to start straight away, took a second or so. It has been okay recently though.

I believe its an induction motor, probably around 1/4-1/3hp....with some form of overload protection inside....capacitor start, NVR switch....

I shall probably take it apart to confirm the free running of the bearings, clean the inside thoroughly, anything else to check out, capacitors?
 

9fingers

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Did somebody call??? :lol:


My first suspicion would be that if it has a centrifugal switch as part of the starter circuit, that this has stuck shut and the starter is connected all the time. With that power rating, I suspect that it will only have a cheap & nasty NVR with no overload protection otherwise this would have tripped under such conditions.

Two options.
1) If you can disconnect one end (at least) of the capacitor, then spin start the lathe and see it if runs hot (insulate the free end of the capacitor if it is an open wire whilst you do this. If the motor now behaves then stuck starter could well be the problem

2) Strip down the motor and see if there is a switch in there and if the contacts are welded or in poor condition - the act of opening the motor may unstick welded contacts.

Another recent motor problem turned out to be bu88ered windings which is unusual but not unheard off.

If you have a multimeter and know how to use it, I can suggest other things you can try before opening the motor.

Bob
 

Hitch

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I have a multimeter, and a basic knowledge of how it works :lol:

It has some form of 'thing' in the rear end, which I can hear click about a second or so after starting, would this be a centrifugal switch?
I thought that was some kind of overload protection affair, seems not then....
The cenrifugal switch does what, cut the capacitor once the motor is spinning sufficiently?
 

9fingers

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There are two parts to the centrifugal switch.
The 'flying weights' bit nearly always works and is responsible for the click.
The other bit is the contact that it operates and this might be the problem.

Yes the function is exactly that, once the rotor approaches synchronous speed, the starter capacitor and winding are switched out.

Connect your meter permanently across both the capacitor terminals and select ac volts at least 250 v AC. ensure that the meter probes cannot short together!!!

Start the motor. you should see a relatively high voltage across the capacitor which should then slowly decay possibly over 10s of seconds.
if the voltage stays steadily high, then the starter switch is not opening.

hth
Bob
 

Hitch

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I shall have a go with that Sunday once I havn't got my 2 'helpers' then, and take it from there. Thanyou. :)
 

9fingers

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I'll be in the workshop most of tomorrow but now I have t'internetty installed down there, I should see messages pop up.

Cheers

Bob
 

Hitch

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Looks like the same motor.

I can't say I have noticed it getting this hot before though, and not only does it feel hot to the touch, it also smells hotter than perhaps it should.
I think I will have a look over it later, and test as per Bob's advice.

Or maybe i should put a sticker on it. Easy Fix! :lol: (hammer)
 

Hitch

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Tried the meter on the capacitor, started around 180v, diminished very very slowly to about 150v by which time i got bored watching it.
In bits at the moment, cleaning the switch thing. Fiddly affair that is!
 

9fingers

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Thanks for the update. That sort of suggests that the switch is working? yes they can be a bit fiddly. Made down to a price from bent tin sadly.
One top of the range makers is using an electronic switch now but they connect the starter for a fixed duration of 2 secs and I have had one member here where a heavy machine did not get up to speed before the time was up. So that is not a perfect solution either.

The best solution I have found is on fridge and freezer motors. There they measure the current in the run winding and keep the starter winding switch in until the run current drops, indicating that the motor has got up to speed. The motor is all sealed in with the refrigerant and no possibility of a centrifugal switch operating in that environment.

Good Luck

Bob
 

Hitch

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Got it all back together, and switch contacts lightly filed smooth.
Retested on the bench. Volts dropped off much much quicker, right down to 0.
Used it last night, barely any temperature change.
Looks like its fixed, thanks for the advice Bob =D>
 

Digit

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That's another mess you've got us out of Stanley!

Roy.
 
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