how to check a motor

[NormanV]

I have acquired a rather nice 3 phase motor that has a built in switch that will give 2 speeds and forward and reverse.
I don't have a 3 phase supply. How can I check that the motor is OK before I invest any time or, more importantly money, into it?
Thanks, Norman

[BillTodd]

use your nose. if it smells burned, it probably is.

check the resistance of each winding  or put 12vdc  across each winding and feel the force on the rotor.

bill

[John Stevenson]

Norman,
Not going to try and explain how to check it, far more competent people than me on this board.
However how were you going to run this ?
A UK two speed motor needs 440v be it from a static converter or rotary converter.

They will not run from an inverter as these require the windings to be changed from star to delta and because of how a two speed motor achieves the two ranges this cannot be done.

Basically just a heads up before you get too deep into a project.

[NormanV]

Thanks for that info John. The motor came attached to some equipment that I bought at almost scrap price and I was hoping to use it on the milling machine that I am building. Two speeds would be handy.
Norman

[BillTodd]

actually john, norman,

you can run them from single phase  240v with a step up transformer or a specially converted step up VFD. Two speed switching can be tricky but is not impossible .

however ,  john's  caution is timely. You should seek the help of someone compitent before playing with four hundred  plus volts .

bill

[Arbalist]

It's worth pointing out that I bought a milling machine some years ago with a (single speed) three phase motor and I happily ran it on 240V Single Phase for several years. There were instructions on the net from a Capacitor Manufacturer on how to do this and it was a really cheap "fix" requiring only swapping of connections inside the motors terminal box and buying a large capacitor to go between the live and neutral. The instructions stated that you only get 80% of the motors rated power output but that was never an issue for me. I understand this does not work with all three phase motors but luckily for me it worked on my machine.

I really wish I had made a video of the mill running before I sold it as there have been many folks who said it can't be done!

As an aside, I read recently that it is not uncommon practice for equipment suppliers to fit three phase motors on machines intended for a single phase supply by making some alterations to the motors wiring. Using up old stock or availability of cheap motors were reasons quoted for this.

[John Stevenson]

Not hard to run a single speed 3 phase motor off single, especially if it's a low power application.
Coolant pump on a machine is a classic example.
You rewire the main motor from star to delta and fit an inverter and this puts that motor on single phase but you are left with the coolant pump.

So also change this one from start to delta and put single phase across two of the terminals, then add a  capacitor to live and the unconnected terminal and it will work.
A cap out of a florescent light is ideal for a small motor

[JohnHaine]

Um, John, I think fluorescent lights have ballast inductors in them, not capacitors?

[lordedmond]

Yes they do to correct the lag in cos thi introduced by the inductor
Basically they are for power factor correction

Inductors lag cap lead thus unity is achieved
Stuart

[DavidA]

...Um, John, I think fluorescent lights have ballast inductors in them, not capacitors?...


All the florescent lights  in my shed have both.

Dave.

[Arbalist]

What are these then?

http://www.bltdirect.com/capacitors

Just asking...

[awemawson]

They are for correcting the power factor.  Always used to be in parallel with the rest of the lighting circuit. Then they realised that when people cut them out the light still worked, so now they tend to put them in series with the ballast.

[Arbalist]

You missed the point ...

[awemawson]

You missed the point ...

??????????????????????

[Arbalist]

It was a rhetorical question in answer to reply #7.

[DavidA]

Getting back to the original question.

I used to have to do a lot of three phase motor winding checking when I worked in factory maintenance.

We used to make sure there were no links on the terminal block,  then use a Megger to check the earth/winding and the interwinding resistance.

Generally this gave us a go-no go decision for the motor.

Dave.

[NormanV]

 The last few posts have totally confused me. I know nothing!
I did the smell test suggested by Bill in post no. 2, it smelt fine.
But what do I do now?
DavidA, hi, I don't understand what you are suggesting. Could you explain it for an idiot please?
Norman

[JohnHaine]

Well you learn something every day...

[JohnHaine]

Well that was interesting, reading up a bit on fluorescent lights.  They don't all have PF correction caps, certainly not the cheapskate ones I bought from the local DIY shed, but obviously a good idea if you have quite a few lights.  Seems that commercial luminaires have them cos premises tend to have lots of lights and the PF loss can be significant.

[DavidA]

Norman,

All it means is that you have to make sure that non of your motor windings are connected together (by brass links on the connecting block) as you need to check each winding seperately.

A Megger is just a high voltage/low current insulation tester.

So you are checking for insulation breakdown between the seperate windings and also for breakdown between one or more windings and the frame.

Normaly there are three windings.  So check each one against the other two,  then each one between the winding and the frame.

Hope this helps.  It's easy to me as I started out as an industrial eletricians's apprentice. I do understand that not everyone knows this kind of thing.

Dave.

[awemawson]

Probably six windings on Normans motor as it is dual speed

[BillTodd]

It'll probably be the pole changing type like the top diagram attached.

[Arbalist]

Good post Bill.  I've saved that picture for future reference!

[awemawson]

I've always thought that those motors were good candidates for a 240 - 415 rotary phase converter, using the lower legs conventionally as a 240 single to 240 three phase converter, and picking up the higher voltage off the outer coil taps. Voltages aren't quite right but probably close enough for most applications.