MOTOR TESTING IN THE UK

[lor]

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HI ALL,
My job (IN THE UK) requires me to test 3 phase motors . I have a clamp meter and have taken readings but am unsure what to look for . On some motors all 3phases read the same ,others the readings are different between the 3 phases .

the motors are turning fans fitted to industrial boilers .

any advice gratefully recieved

thanks
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[bhm1712]

if your testing motors as a profession, why do you not know this??

there will be slight differences in current readings across the windings and this is normal as it depends on the phase voltage and resistance of the windings. some motors are perfectly balanced some are not..

for example, i tested a motor at work today in which all 3 windings were 0.6 ohms, but the current drawn on the 3 phases was slightly different. our phases are not bang on 415 volts, usually they are between 408 - 417 volts.

the load on the motor will also have an effect on the current.

 

[OilLecky]

Presumably these motors are all of the same type doing the same job? As bhm said there can be differences in the phases that a straight forward (dc) resistance reading will not show up. The impedance (ac resistance if you will) of each winding depends on many things and the way the windings actually lay can affect this. Also are you sure that the phase voltages are the same and are transmitted over equal impedance cables. A phase unbalance ( elsewhere in the supply circuit) can change phase voltages and there may even be an unven inductive of cpacitive load elsewhere.
Also a clamp meter is not a very accurate instrument. The reading can depend on the method and speed of closing the clamp!

But to answer your basic question. On a "clean" power supply and with perfectly equal windings a 3 phase motor should have an equal current flow in each winding.

 

[RF Lighting]

(IN THE UK)

Has the UK ceased power of Northern Mariana Islands then?

 

[conny]

I repair motors as a profession so test motors on a daily basis.
As stated, there are a number of factors which can influence different readings between phases. We know for a fact that our workshop 230v supply is taken from what we call our 'blue' phase while the office supply is spread across the other 2 phases. The office supply across both of these phases is balanced and much less than the workshop load so we always get one phase, (blue), with a different reading.
Are your readings only marginally different, i.e. 1 or 2 amps?
Have you taken readings of the same motor at different times of the day and if so do the readings differ or remain the same? If they differ this could be due to differing loads at various times of the day.
What I would suggest is to take a reading of 2 or 3 motors at say 10.00am and note them down.

Motor 'x',
Time - 10.00am
Red Phase - xx Amps
Yellow Phase - yy Amps
Blue Phase - zz Amps

Do the same for motor 'y' and the same for motor 'z'

Then run the tests again at lunch time when the canteen will be in full flow with kettles/toasters/microwaves etc running and note down the results as above.

Finally run the tests again at mid afternoon but not during a time when people are likely to be having a tea break.

Compare your readings and note any differences in them. If they fluctuate at these times but still have a marginal difference then everything is probably ok.
Another test you can do if you are able to switch them off and isolate them is to move each phase cable along one, (i.e. Red is moved to the Yellow terminal, Yellow is moved to the Blue terminal and Blue is moved to the Red terminal). This will ensure the direction of rotation is maintained and by taking readings now and comparing them with your previous tests you will be able to distinguish if the lower reading follows the phase cable or stays on the same motor lead. If it stays on the motor lead it may simply be because of the way the stator is wound, (basket wound, rope wound, concentric wound, layer wound etc). So as you see there are a whole variety of reasons why they may be different but still be perfectly normal.
Finally, if you are still concerned you could always call in a motor repair company to do some 'on-site non destructive testing' procedures using specialist equipment which will tell you what state the windings are in and can give early indication of impending problems. from this you can then plan a maintenance schedule to have the motor removed and taken away for service/repair.

If you would like to know more you can PM me to save filling up the forum with something that nobody else would want to read.

Good luck and let us know how you get on.

 

[ericmark]

I will admit I had similar problem. Working on the Falklands I had to look at a crushing plant. Resistance continuity and insulation both showed as OK and current was high but even.

Question was if the high current was caused by one or two shorted windings or something mechanical.

The only method to be certain is to measure torque.

At this point the chief engineer arrived and I related problem and was considering how to measure torque. Lucky he had an alternative solution and instructed the fitter to remove main bearings and remove hand full of grease and this did prove to be problem. I have never thought of over greasing.

However fact remains unless one can load a motor one can't be sure. When testing DC motors (Starters) my test rig had a built in dynamometer I have also used dynamometers with car and stationary engines but it is not a standard bit of kit for electricians and the norm is use your nose, ammeter, and ohmmeter and if still having problems send it to rewind specialist for testing. I would assume they would have a dynamometer.

I can still remember the formula had to do it in collage before excel was invented and we would measure at all different speeds and have to draw graph as homework. 2πNT/33000 suppose now metric its changed?

 

[conny]

I would assume they would have a dynamometer

Good God, haven't used one in years!

Nowadays we use a thing commonly called a 'Baker' test. (Baker being the name of the manufacturer really). Depending on the model you can do basic insulation test to earth, surge test for shorts between the windings, resistance tests for each phase, polarisation index tests to see how the insulation stands up to increased voltage application over a measured time, core-loss testing to see if there are 'hot-spots' in the core laminations and magnetic flux tests on squirrel cage rotors to see if there are any cracks in the rotor bars invisible to the naked eye.
We only tend to do the full range of tests on larger motors due to the increased time/cost of doing them.
Oh and you can also do 'drop-testing' on armatures using an auxillary unit.
All test results are automatically recorded and can be printed out at the touch of a button. (Its so hard testing motors these days, ).

 

[ericmark]

Ups am I showing my age? Used Electric, Rope, and Hydraulic dynamometers the latter was similar to this

The rope one was built into a National engine foot bore and stroke around 10 foot flywheel water cooled. Trick was to switch off when some unsuspecting guy was standing next to it. As when it stopped all the water fell out.

 

[conny]

Born late 50's, thats all I'm saying!