Overheating single phase motor

[BobL]

One of the blokes at the mens shed has a table saw he wants converted from 3 to single phase so we bought what we thought was a 2HP used single phase cast iron motor from Gumtree.

There was no name plate on the motor and not even a spot where there might have been one which was odd.
There was also no terminal box - the power cord just went straight inside the motor.
The motor Meggared OK and seemed to run Ok at the sellers place but when I got it home I noticed it was warming up faster than I expected it to and It was also drawing 7.9A just freewheeling which is way more than I would expect from a 2HP motor or even a 3HP motor.

Opening up the motor showed the vanes on the fan appeared to be missing/sheared off so I made another fan. I've done this before for an old washing machine motor and that is still running so I guess I had done it right that time.

Reassembled everything, lots of air coming out but it still ran hot and the 7.9A was still a worry so I added a run capacitor (40 MFD) from a 2HP motor which dropped the current to 3.9A which is similar to my TS 3HP SP motor when free wheeling.

Still runs just a liittle warmer than I would like,I suspect that this is a 3HP motor and if so a 60 MFD run cap may work better than a 40.

[Yanis]

Several thoughts. The run cap should be in series with one of the windings only to result in a lagging voltage to bring it into phase with the rotation. So to get the correct phase it will depend on the inductance of the winding so it all depends. You need some complex calculations and if possible an oscilloscope to check the relative phases after the cap is added. So there is no simple answer to the value of the cap I'm afraid.

As for the high current it could very well be since the windings may be working against each other.

On the other hand it may be shorted windings inside the motor. This would show up as a hot spot inside the motor with burnt insulation. This causes the motor to operate as a transformer with a shorted output and can draw very high currents and get very hot. I had this happen a few months ago on a cheap compressor motor. Very nasty.

John

[BobL]

Several thoughts. The run cap should be in series with one of the windings only to result in a lagging voltage to bring it into phase with the rotation. So to get the correct phase it will depend on the inductance of the winding so it all depends. You need some complex calculations and if possible an oscilloscope to check the relative phases after the cap is added. So there is no simple answer to the value of the cap I'm afraid.
Yep - I'm aware of that. I haven't got back to this but I recently bought a 60MFD which is more suited to a 3HP motor to see what this does,
I have all the gear to test it with just need a bit of time.


On the other hand it may be shorted windings inside the motor. This would show up as a hot spot inside the motor with burnt insulation. This causes the motor to operate as a transformer with a shorted output and can draw very high currents and get very hot. I had this happen a few months ago on a cheap compressor motor. Very nasty.

There's no evidence of a hot spot when I take the whole motor apart.

[BobL]

Almost a year on this motor has sat on the floor of the shed and finally got round to looking at it.

Thought I would run through the whole circuit from scratch as some of the impedances weren't adding up.
I ended up opening it up and found it had both redundant connections and unwanted connections so I stripped all these out and brought both the run and start coils, and centrifugal switch connections all outside the motor, added a larger terminal box and rewired everything from scratch.

I've used a 60 MFD run and 200 MFD start capacitors

Now the motor freewheels at 2.6A and runs a lot cooler.

All good.