Induction motor problem (help wanted)

[jonny boy]

Thanks for the info Dave.
Bob, which capacitor is the starting one, or can I check both while running?
Is it quite possible that the tiny overload dial in the NVR swithch IS set too sensitive?
I've had the saw for about four months now but only recently started to use it for longer periods.
jon.

 

[jonny boy]

I think we can definitly say that it's overheating first of all.
Now, there's nothing obstructing the motor fan and air can flow around the motor, so can we say that it has to be the starter winding not cutting out?
jon.

 

[9fingers]

It is most likely that the larger value capacitor will be the starting one.

The value will be a number followed by MFD or greek letter Mu and capital F
there will be also a voltage rating.

The starting one might be marked 'intermittent rating'.

If they are not in parallel then one capacitor should have volts across it all the time the motor is running and the starting one should not.

If you are measuring volts with a modern digital voltmeter, there are possible mechanisms that will still give you a voltage reading when there it not a problem. to be sure use the light bulb test. - easy to do with a desk lamp or similar. Take off the plug and connect it to the meter terminals then you can read the meter and look at the bulb.

If you decide to tweak the overload trip, then read the current off the motor plate and set the trip to say 20% up on that figure


In answer to your next post. I think that it is highly likely that the starting circuit is faulty unless the trip was never set properly in the fisrt place.

Depending on the design of the motor, it is not too unusual for a motor working reasonably hard to be too hot to hold your hand on.

Finally, if the starting winding is enrgised all the time, then I would expect more buzzing noise from the motor than normal. If it running quite quietly then it could be an incorrectly set trip.



Good luck

Bob

 

[jonny boy]

On a final note Bob, the motor runs very smoothly with no buzzing at all and the motor is rated at 12.4 amps with the NVR tiny dial set at bang on 12!!, should it be a little higher?

jon.

 

[9fingers]

I think we have the solution.


12.4 sounds quite low anyway I think my 3hp saw is 19amps!

yes set the overload trip to about 15amp and see how you get on.

Bob

PS at least you will know what to do if the starter ever sticks in the future :lol:

 

[misterfish]

Something I came across when looking at the possibility of using US machinery in the UK was that although rated for 220v the fact that we have 50Hz as opposed to the US 60Hz would mean that motors would run a bit slower and somewhat hotter.

Anyway, this was enough of a dis-incentive for me to not bid on a nice Delta thicknesser on the 'Bay some time ago.

MisterFish

 

[ivan]

If the motor/saw is an import, check the motor label which almost certainly says 60 cycles, not the UK's 50. 60 cycle motors (and transformers too for that matter) contain less iron (to be magnetised by the windings) than 50 cycle motor, and are thus a bit cheatper to make. If rated as 230V (I thought the USA was 110/220V?) it is safe to run, but will be prone to overheating. If rated at only 220V the overheating will be worse. This still might not be a problem if you can switch off between cuts....

 

[herdsman]

Only just read this thread and misterfish is on the right track.

I experienced the same problem about twenty years ago with a batch of approximately ten single phase capacitor start motors.

The problem was due to two things;

1 the centrifugal switch

2 Supply frequency

The centrifugal switch would not operate and allow the motor to utilise the 'run' winding, as the motor develops speed the switch weights are thrown out operating the contact to switch from 'start' to 'run' winding. The weights were checked and were fine for a motor connected to a supply with a frequency of 60 Hz (USA) but no good when connected to a supply with a frequency of 50 Hz (UK)

motor speed = supply frequency x 60 / the number of pairs of poles

for example a motor used in the UK with two pairs of poles will have a nominal speed of 50 x 60 / 2 = 1500 rpm
in the USA the same type of motor will have a nominal speed of 60 x 60 / 2 = 1800 rpm. Therefore, the imported motor is not achieving speed to allow the centrifugal switch to operate...but it is worth checking all the other points mentioned.

Maybe talking to a local rewind company may help?

 

[jonny boy]

Hello again,
First of all, the manual say's the motor is made to run on 220-240v which should mean it's ok with our electrics, now the hz on the motor is 60 and ours over here is 50. I have just looked inside the nvr starter and the overload amp dial is actually set at just under 12 amps and this is a three horse motor. The motor on my extractor though is only two horse and that dial has been set at 17amps, as it came from Axminster. That motor must only be about 8-9 amps. I've reset the saw switch from under 12 to just under 13 amp and i'll see how it goes. I'm sure there's loads of unisaw owners over here without this problem.

cheers,
jon.

 

[misterfish]

I'm sure there's loads of unisaw owners over here without this problem.

jon.

I think they are a rare species this side of the 'Pond' - I can only recall seing one on the 'Bay recently. Also, I believe it is (or was) possible to specify a 50Hz 220/240 volt motor if buying new from Delta. I looked around for a while for a Unisaw but eventually plumped for a good old Wadkin which hasn't disappointed me.

MisterFish

 

[DaveL]

plumped for a good old Wadkin which hasn't disappointed me.

Good bits of kit, I love mine.

 

[jonny boy]

Hello, I have had another close look and listen to the motor when running and there is definitely the distinctive click sound from the motor when you press the stop button and the motor starts to slow down, does anyone know if this is the sound of the centrifugal switch turning off?
cheers,
jon.

 

[9fingers]

Yes this is fine. Stop worrying now and make some sawdust!! :lol:

Bob

 

[jonny boy]

I would do Bob If it stopped cutting out after 10 mins use. (still no joy)
Going to speak to a motor engineer tommorrow.
jon.

 

[herdsman]

Perhaps you could test and see how much current the motor is taking.

Is the blade driven directly by the motor or belt driven?

 

[9fingers]

Ah! I thought you had progressed past that problem. See if you can borrow a clamp ammeter from him. He will tell you how to use it. You can then get a good measure of the current being drawn. It might have a peak hold feature- mine has and then you can measure both the start and run current.

Have you set the trip to 15amps yet? worth a try.
you could measure time to cut out against current setting in say 1 amp steps

Would be useful to your motor engineer.

Bob

 

[jonny boy]

The blade is driven by a three belt pulley configuration. I'm absolutely certain that the overload is cutting in due to overheating, now why it's overheating is something else!

jon.

 

[jonny boy]

Bob, I have tried it when the starter is set on 12 just over. It say's inside the starter cover, do not exeed the FL amperage of the motor or motor burnout could occur. I dont want to risk that.

jon.

 

[9fingers]

Please re-read my mails.

The overload trip does not measure temperature or overheating of the motor ever.

It simply measures current and integrates to reduce the effect of short term peak currents.

Bob

 

[herdsman]

Surely the overload relay reacts to temperature thta's why they are known as 'thermal overload relays'? The bi-metal strips inside heat up as the current increases and bend due to the temperature increase causing the trip mechanism to operate.