Motor Problem - Sanity Check!

[puffernutter]

I have a 3HP, single phase, twin capacitor electric motor. When you turn it on, it buzzes. Give the pulley a spin and it will speed up, then slow down, then speed up ad nauseum!

I have three pairs of wires I can see at the terminal block. One pair show a low resistance (4 or 5 ohms), one pair show no resistance, and one pair show infinite resistance.

Looking at the wiring and making an educated guess! The o/c pair are the main winding. The s/c pair are the centrifugal switch and the low resistance is the starter winding.

My theory, which is mine and belongs to me (showing my age!) - On turning power on, without the main winding, it wont move. Give it some help and the starter winding will get it turning, it gets to the speed where the centrifugal switch opens and it slows down until the centrifugal switch closes and the cycle repeats.

Does that make sense? It's supposedly a new motor (but second-hand to me), so there's not a lot I can do. I may pull the pully off and take of the end case and have a look and see if there is anything obvious, but I suspect a new motor is required!

Cheers

Peter

 

[Rrobson]

On single phase motors, the centrifugal switch is closed to power the start winding. Once it spins up to a certain speed the switch opens and power is no longer supplied to the start winding. Run winding is always powered.

The fact that you have to spin it to get it going most likely suggests you have a problem with either your start capacitor or centrifugal switch. If the start winding is continuously powered it will be interfering with the run windings and rotation of the rotor. Check for open circuit or on the start winding or start capacitor to eliminate these. You could even remove the start capacitor and that will tell you whether the rest of the motor is working if you aren’t sure how to test the switch or windings.
It could also be an earth fault in the start windings if they’ve been overheated but less likely unless the thing it’s turning is really tough to get going.

 

[Dapph2]

Don't leave it plugged in and switched on for very long either ....till it is repaired.

I've lost count of the EXPERTS who have done that and cooked the windings .

There are a couple of excellent shops on eBay that do a massive range of capacitors and with numerous different can types .

 

[puffernutter]

All comments noted. I have been careful with the measurement of the three pairs of wires and made sure that one end was totally disconnected when I made the measurement. So I am assuming the resistance I see is one of the windings, the short circuit I see is the centrifugal switch and the open circuit is the other winding.

When I look at the connections on the terminal block, the pair of wires that are open circuit are directly across the live and neutral. Looking at a typical circuit, that indicates to me it is the run winding that is open circuit.

I will remove the start capacitor, but I am not sure how I can have mis-measured that the run winding is open circuit. The only other thing I could do is find a way of spinning the motor externally and watching to see if what I think is the centrifugal switch goes from s/c to o/c.

 

[puffernutter]

When I removed the start capacitor and applied power, a slight hum and nothing. It made no attempt to continue to turn after I gave it a spin.

I have pulled the front cover off slightly and confirmed my theory with the wires. I haven't spun the motor externally, but I am certain the centrifugal switch is operating correctly.

 

[Brad93]

Right so you're sure about which pair are the wires from the centrifugal switch? You've visibly looked at them and traced the route/colours to the switch.

In that case one of your windings is burnt out.

If you've got six pairs of wires coming from the inside of the motor and you've only got 4-5 Ohms on one pair and not at least 8+ Ohms on another pair you've got a winding that's O/C and dead.

Test continuity between each winding and the frame of the motor and then winding to winding.

You won't always find insulation breakdown with a multimeter but you will find if the winding is shorted to earth.

An insulation tester (megger) will tell you more about the health of the motor.

 

[angellonewolf]

@puffernutter ive got a megger if you want to borrow it ?

 

[puffernutter]

@angellonewolf - thanks for the offer, I have one. @Brad93 Thinking this through, as the winding is electrically connected but not blowing fuses or my RCBOs, I suspect it is simply open circuit rather than anything to earth. I will check tonight though.

The bigger problem is finding a replacement motor at a reasonable price, new seem to be north of £200! Any second-hand ones available? 3kW, single phase, 3000rpm foot mount.

 

[Pete.]

Symptoms certainly suggest you have only the stay winding connected.

I'd have the cover off and check the flying leads. My mill didn't work on high speed and it was only a break where the lead was soldered to the winding that caused it.

 

[Jack_K]

@angellonewolf - thanks for the offer, I have one. @Brad93 Thinking this through, as the winding is electrically connected but not blowing fuses or my RCBOs, I suspect it is simply open circuit rather than anything to earth. I will check tonight though.

The bigger problem is finding a replacement motor at a reasonable price, new seem to be north of £200! Any second-hand ones available? 3kW, single phase, 3000rpm foot mount.

Buy an old air compressor and use the motor. I see them all the time for free or cheap. Assuming you mean 3hp like the original post. 3kw is an unusual size that you won't find used.

 

[Guineafowl]

It does sound like a popped main winding, but if you’re measuring from the terminal box, you might be trying ohm the start winding through the capacitors. I’d be digging a bit deeper before looking for a new motor.

 

[puffernutter]

Buy an old air compressor and use the motor. I see them all the time for free or cheap. Assuming you mean 3hp like the original post. 3kw is an unusual size that you won't find used.

You are correct, teach me to type late at night! 3HP, 2.2kW, but also 3000 rpm.

 

[puffernutter]

It does sound like a popped main winding, but if you’re measuring from the terminal box, you might be trying ohm the start winding through the capacitors. I’d be digging a bit deeper before looking for a new motor.

Every measurement was taken with a leg know to be disconnected, so I am sure I am only seeing the windings.

 

[puffernutter]

UPDATE - I dismantled the motor tonight. On inspecting the stator, on one side, the high temperature insulation was slightly darker than the other. I removed the insulation (a challenge with twine and varnish to deal with!) and the wire from the terminal block was crimped to two winding wires, or at least it may have been at one time. I scraped the insulation off the winding wires, opened out the crimp, then re-crimped the connecting wire with the coil wires. I then measured 1.2 Ohms, rather than an O/C. Put it all back together, applied power and I have a working motor!

Thanks for all the help and advice.

Peter

 

[Dapph2]

UPDATE - I dismantled the motor tonight. On inspecting the stator, on one side, the high temperature insulation was slightly darker than the other. I removed the insulation (a challenge with twine and varnish to deal with!) and the wire from the terminal block was crimped to two winding wires, or at least it may have been at one time. I scraped the insulation off the winding wires, opened out the crimp, then re-crimped the connecting wire with the coil wires. I then measured 1.2 Ohms, rather than an O/C. Put it all back together, applied power and I have a working motor!

Thanks for all the help and advice.

Peter

Peter may I suggest that you get some high temp repair varnish on the re-crimped joint and adjacent wires to hold everything tight & in place and have it insulated again .
You used to be able to buy a small tin of such varnish on eBay for £7 or so 12 yrs ago

This was searched a few seconds ago and is from Amazon .

MG Chemicals 4226A Clear Insulating Varnish, 55 ml Liquid Bottle ( £9 in todays money )​

 

[Brad93]

UPDATE - I dismantled the motor tonight. On inspecting the stator, on one side, the high temperature insulation was slightly darker than the other. I removed the insulation (a challenge with twine and varnish to deal with!) and the wire from the terminal block was crimped to two winding wires, or at least it may have been at one time. I scraped the insulation off the winding wires, opened out the crimp, then re-crimped the connecting wire with the coil wires. I then measured 1.2 Ohms, rather than an O/C. Put it all back together, applied power and I have a working motor!

Thanks for all the help and advice.

Peter

Great result

 

[Brad93]

Peter may I suggest that you get some high temp repair varnish on the re-crimped joint and adjacent wires to hold everything tight & in place and have it insulated again .
You used to be able to buy a small tin of such varnish on eBay for £7 or so 12 yrs ago

This was searched a few seconds ago and is from Amazon .

MG Chemicals 4226A Clear Insulating Varnish, 55 ml Liquid Bottle ( £9 in todays money )​

I would put insulation over the joint then spray the whole lot with varnish.

 

[puffernutter]

I would put insulation over the joint then spray the whole lot with varnish.

From a previous project (bolt induction heater), I had some high temperature insulation, which I used. Whilst having to cut the lacing cord, there were some nylon cable ties as well, that I have replaced. I am not sure if you need high temperature cable ties (or can even get them). Whilst it will be a pain to take is apart again, I see the sense in applying the varnish, so I will certainly do that.

 

[Brad93]

This is the gear we used.
I can't remember the spray now. Blue and Yellow aerosol.

I'll try and find it.

 

[Pete.]

I bought a tin of red goop from brocott and some cotton cord.

Definitely, secure it all and varnish.