How to check polarity of each winding in 3-phase motor?

[moonshineJ]

There is a 3-phase motor with 6 leads; U1, V1, W1, U2, V2, W2. It can be connected either for low voltage (delta), or for high voltage, 450 VAC (wye).
None of leads has any identification other than colors (black, brown, red, yellow, blue, and green)...no labels with U1, W2, etc. No numbers, like 1, 2, 3, etc.

The easy part is that one can easily identify 3 pairs of windings with an ohmmeter. How can 1s and 2s be identified?
I saw that some Yahoo-types advise to connect U, V, W arbitrarily together (for wye) and hook up L1, L2, L3 to the remaining ends of U, V, W and then bump test it, while observing if motor will run. Obviously, it's a bad idea, as polarity of one winding can be different from other two...best case scenario - this will blow fuses.

I did some more research and found some more scientific approaches:

a) "I've heard once that you have to connect a DC voltage (battery 12VDC) to one coil. You then have to release the voltage while measuring on another winding. The counter EMF will say you how the other coils is connected. Repeat this 3 times".

b) "Ring out one winding.
Attach a 9V battery to the ends. NOTE the polarity of the battery. Using an analog meter ring out the other 2 windings, the meter deflection should be positive if not switch meter leads over. Now label the 2 leads you have rung out with the red or positive meter lead A1 and B1. Now connect the battery to the A1 and negative to A2 Then put your meter across the winding the battery WAS connected to and check for positive deflection of the meter. Then all you do is switch ends on the last winding so the wire that was connected to the the negative of the battery is now C1
So now we know which group is the beginning and end of each winding without running the risk of getting the ends mixed up".

I did some research on color coding - if it's possible to determine U1, V1, W1, U2, V2, W2 by color of motor leads. It turned out that these colors can be anything possible - which I know from my own experience (even if there were some solid standard for it, I would not take it for granted).

Is there some good reference for this procedure?
Are there other ways to do this check?
And last but not the least - what is the proper name for this check?

 

[Jraef]

3 phase = AC
Polarity = DC

There is no "polarity" on an AC system, or really, polarity swaps back and forth every 16.67ms (at least over here) anyway so it doesn't make any difference. Explain to me how you might "blow the fuses" in the drawing below if, for instance, 1 and 4 were swapped? Where is the short circuit?


You've identified the 3 sets of coils, that was the hardest part. You can now arbitrarily assign numbers to them for your connections. Don't know what you read on Yahoo, but it was bogus, or it might have had to do with something different.

 

[milldrone]

Are there other ways to do this check?

Check this thread @ PLC's.net http://www.plctalk.net/qanda/showthread.php?t=434

Pay attention to posts #7 and #20.

 

[ActionDave]

We have a transformer set up for stepping down our three phase voltage to 120V. With the pairs identified one can run the motor unloaded and find out if one of the coils is reverse wired by the way it runs without letting the smoke out. Easiest way I know to do it.

The battery thing works also, but not the way it is described in the OP. I don't know what is meant by "releasing the battery and testing for counter EMF". You just pull the rotor out, hook up a pair of leads and check inside the stator with a compass and see if the needle points North or South. Each pole of the motor should alternate as you switch pairs and work your way around.

 

[moonshineJ]

3 phase = AC
Polarity = DC

There is no "polarity" on an AC system, or really, polarity swaps back and forth every 16.67ms (at least over here) anyway so it doesn't make any difference. Explain to me how you might "blow the fuses" in the drawing below if, for instance, 1 and 4 were swapped? Where is the short circuit?

View attachment 12491
You've identified the 3 sets of coils, that was the hardest part. You can now arbitrarily assign numbers to them for your connections. Don't know what you read on Yahoo, but it was bogus, or it might have had to do with something different.

Instantaneous polarity = AC.

Fuses will blow for obvious reason. If all three windings for the wye are connected the right way, there will be three voltages 120 degrees apart. Their vector sum will equal zero.
If one of windings is connected backwards (1 and 4 swapped), the vector sum will be far more than zero and the motor will smoke within seconds > fuse(s) blow. It's why it's smarter to conduct such test (which would normally be done on occasion at motor rewinding shop) instead of stinking a motor.

 

[moonshineJ]

Check this thread @ PLC's.net http://www.plctalk.net/qanda/showthread.php?t=434

Pay attention to posts #7 and #20.

Good stuff; it's what I was looking for! Thanks!

 

[moonshineJ]

We have a transformer set up for stepping down our three phase voltage to 120V. With the pairs identified one can run the motor unloaded and find out if one of the coils is reverse wired by the way it runs without letting the smoke out. Easiest way I know to do it.

It's smart! Honestly, for quite some time I was under impression that induction motors cannot run on low voltage. Until I've got in motor repair shop and saw how they test motors after repair/refurbishing. If I remember right, they would start from 120, than twice more, and finally at nominal voltage of 450. The test you mentioned is probably the best, the fastest, yet safe way to determine which lead is which. Unfortunately, the guys who deal with that unlabeled motor don't have this type of panel, but I will let them know, may be they will be able to find some shop around.

The battery thing works also, but not the way it is described in the OP. I don't know what is meant by "releasing the battery and testing for counter EMF". You just pull the rotor out, hook up a pair of leads and check inside the stator with a compass and see if the needle points North or South. Each pole of the motor should alternate as you switch pairs and work your way around.

Is this what you meant?
http://www.vintagemotorcycleworks.com/field_coils.htm
I recalled that one time I saw a guy running a small steel ball from a bearing inside the energized 3-phase induction motor (with rotor removed). He explained that if there were a short between turns of one winding, the ball would stick there. Normally this would refer to as "false rotor', but it's rather to determine the rotation of field?

 

[kwired]

Is this what you meant?
http://www.vintagemotorcycleworks.com/field_coils.htm
I recalled that one time I saw a guy running a small steel ball from a bearing inside the energized 3-phase induction motor (with rotor removed). He explained that if there were a short between turns of one winding, the ball would stick there. Normally this would refer to as "false rotor', but it's rather to determine the rotation of field?

I have always wanted to try the ball bearing inside the stator thing, never wanted to burn up an otherwise good motor doing so though so I never do this. Often wondered if it is ok to apply full voltage to the stator without a rotor in place or if that changes total impedance enough that you will have high current flow and burn out the winding if you don't reduce voltage or at least put other resistance in series with it anyway.

 

[kwired]

Instantaneous polarity = AC.

Fuses will blow for obvious reason. If all three windings for the wye are connected the right way, there will be three voltages 120 degrees apart. Their vector sum will equal zero.
If one of windings is connected backwards (1 and 4 swapped), the vector sum will be far more than zero and the motor will smoke within seconds > fuse(s) blow. It's why it's smarter to conduct such test (which would normally be done on occasion at motor rewinding shop) instead of stinking a motor.

You still have same voltage at same phase angle no matter which end of the winding it is connected to.

Where you have a problem is with a 9 or 12 lead dual voltage motor where you are putting two coils per phase in series or parallel with one another depending on what voltage is applied. Then you need each paired up coil to be same physical polarity with each other or else their magnetic fields will cancel each other out and effect torque output of the motor.

One single coil per phase shouldn't matter which end is connected to where just that the coils themselves are in proper wye or delta configuration.

 

[winnie]

You still have same voltage at same phase angle no matter which end of the winding it is connected to.
[...]
One single coil per phase shouldn't matter which end is connected to where just that the coils themselves are in proper wye or delta configuration.

The part that I bolded above is the key issue. If you have 3 coils with 2 terminals available each, and you swap the connection of one of those coils, then you _don't_ have a proper wye or delta configuration.

Consider a delta connection and simply remove one of the coils. You now have an 'open delta' configuration with a voltage across the 'open' terminals. What happens if you _reverse_ the coil being connected to the 'open' terminals?

-Jon

 

[kwired]

The part that I bolded above is the key issue. If you have 3 coils with 2 terminals available each, and you swap the connection of one of those coils, then you _don't_ have a proper wye or delta configuration.

Consider a delta connection and simply remove one of the coils. You now have an 'open delta' configuration with a voltage across the 'open' terminals. What happens if you _reverse_ the coil being connected to the 'open' terminals?

-Jon

Delta connection - remove one coil - remove one coil from circuit - the only thing open is the fact you removed that coil - input conductors are still connected to the other two coils, you have no power being input to the disconnected coil - but may have induced voltage on those leads. You also have no torque because you removed that coil, or at least no starting torque - an already running motor may continue to run with reduced torque.

Reconnect that coil - the coil itself and the magnetic field it contributes to the operation of the motor are still in same physical location around the stator. Details in how the stator is wound may make some difference in how much torque is produced should you reverse polarity of this coil - IDK all those details as I am not a motor builder, but I don't see reversing polarity as having at totally destructive effect and if one were to do some experimenting with swapping leads around on a lightly loaded motor - whatever configuration draws least current is probably where you should leave it connected.

 

[winnie]

kwired, I think that you are describing 'single phasing' a three phase motor, when you remove one of its supply connections. If you have a motor that is wye connected internally and you remove one of the phases, then you will have 'single phased' the other two phases, with similar results.

However if you have a delta connected motor and you 'remove' one of the internal phases, then you still have the full 3 phases connected to the remaining windings, and you still have a 120 degree phase difference in the currents flowing through the remaining winding. The motor will still develop a rotating field and still start.

There 'disconnected' winding is sitting in the rotating field of the motor, and there is voltage induced in that winding. Connect the winding one way and you have a normal delta connection. Connect it the other way and you have huge circulating currents.

Think about it this way: in the simplest possible three phase motor, there are _6_ regions on the stator, one carrying the phase A, then the 'back side' of phase C, then phase B, then the back side of A, then phase C, then the back side of B, and then you are back at the beginning. In terms of phase angle you have 0, 60, 120, 180, 240, 300. If you reverse the connection of phase C, then the phase angle patten would change to 0, 240,120,180,60,300.

-Jon

 

[moonshineJ]

I have always wanted to try the ball bearing inside the stator thing, never wanted to burn up an otherwise good motor doing so though so I never do this. Often wondered if it is ok to apply full voltage to the stator without a rotor in place or if that changes total impedance enough that you will have high current flow and burn out the winding if you don't reduce voltage or at least put other resistance in series with it anyway.

It was not a 1-2 sec bump-test. Definitely longer than that, as we stood around and looked at it for a while. And it was full voltage. But I see what you mean. Probably back then I should have toched the stator after the test - just to see if it warmed up.

 

[ActionDave]

I have always wanted to try the ball bearing inside the stator thing, never wanted to burn up an otherwise good motor doing so though so I never do this. Often wondered if it is ok to apply full voltage to the stator without a rotor in place....

No it's not. Not for more than a few seconds anyway.

 

[kwired]

kwired, I think that you are describing 'single phasing' a three phase motor, when you remove one of its supply connections. If you have a motor that is wye connected internally and you remove one of the phases, then you will have 'single phased' the other two phases, with similar results.

However if you have a delta connected motor and you 'remove' one of the internal phases, then you still have the full 3 phases connected to the remaining windings, and you still have a 120 degree phase difference in the currents flowing through the remaining winding. The motor will still develop a rotating field and still start.

There 'disconnected' winding is sitting in the rotating field of the motor, and there is voltage induced in that winding. Connect the winding one way and you have a normal delta connection. Connect it the other way and you have huge circulating currents.

Think about it this way: in the simplest possible three phase motor, there are _6_ regions on the stator, one carrying the phase A, then the 'back side' of phase C, then phase B, then the back side of A, then phase C, then the back side of B, and then you are back at the beginning. In terms of phase angle you have 0, 60, 120, 180, 240, 300. If you reverse the connection of phase C, then the phase angle patten would change to 0, 240,120,180,60,300.

-Jon

You may be partially right on your assumption -

I was not talking about removing/opening one of the three supply conductors - though that can still leave you with some torque in some cases - you definitely will not get full rated horsepower from that motor without high current in remaining two supply leads.

What I am most accustomed to is nine lead wye connected motors or 12 lead delta connected motors (both dual voltage) - say you get the 5-8 pair reverse connected - that motor still operates - and maybe even without noticing any problem if the motor is driving minimal load, but you have that one section in the windings with peak current at the wrong timing in relation to the other windings, and it will essentially be a loss of efficiency because it is working against all the other coils.

I have to give it more thought on how it may effect a six lead motor like in the OP that is connected wye or delta depending on input voltage. I'm thinking if it is a 4 pole motor it may be a bigger problem then it is for a 2 pole motor, unless I am not seeing things quite right worst case it may just end up reversing shaft rotation on a two pole motor but no impact on output torque.

 

[winnie]

In any case, back to the original question: figuring out the correct phasing of the coils in the motor.

While a motor is described as a 'three phase' machine, the phases are not independent. The stator windings for all of the phases share the same core, and the 'phase belts' overlap, meaning that slots will contain wires from different phases.

With _any_ transformer or inductor, when you apply voltage to a coil you will get a changing magnetic field which will induce 'back emf' in that coil. Voltage will also be induced in any other coils which are magnetically coupled to the driven coil. By applying a changing voltage to one coil in a motor, and measuring the voltage induced in the other coils, you can figure out the proper phasing of those coils.

When you use a battery and voltmeter(s) to phase the coils, you are basically using the _connection and disconnection_ of the battery as a low frequency current limited AC source. When the battery is connected, the induced 'back emf' has to match the battery voltage. Pretty quickly the current builds up until it is limited by the internal resistance of the battery, and the induced voltage decays to zero. Then when you disconnect the battery you have the current dropping and induced voltage of the opposite polarity. Hook a voltmeter to other coils in the motor and you can observe the induced voltage and its polarity, and from this you can figure out the phase relations of the coils.

Here is a link to the technique as applied to 9 lead motors on a manufacturer website:
http://www.usmotors.com/Service-Support/FAQ.aspx#{5A6BC3BD-BE48-4D67-915C-F7E5E8A07B7E}

A search for 'checking the polarity of a 6 lead induction motor' will pull up similar descriptions.

If you have the necessary equipment, I like the test already proposed: simply pick your best guess at the connection, apply 3 phase _reduced voltage_, and measure the currents for proper balance. If you consider all 8 possible wye connections of a 6 lead motor, two will lead to balanced rotation and six will appear to have one 'inverted' phase drawing excessive current.

-Jon