Three Phase Motor Wiring Identification

[Graham WhartonParticipant @grahamwharton]

Hi all,

Ive just converted a three phase motor from 440V single voltage to 220/440 dual voltage. The original motor, being single voltage, was only 3 wire, therefore I had to dig out the star point and split into three additional leads. I fitted a new 6 post standard terminal block to aid switching in the future.

Well, during this process I managed to rather carelessly find myself with a motor with 6 black wires coming out of it with no labelling whatsoever.

I learn't something new today sorting it out, so I thought I would share the knowledge.

The first step to resolve is to work out which wire belongs to which of the 3 coils. To do this, use a multimeter to measure the resistance between the wires. Start with wire 1 and measure resistance to each other wire in turn until you find a wire with non infinite resistance. It will probably be sub 100 ohms depending on the type of motor. Mark the first "U1" and the second "U2"

Repeat with the 4 remaining wires until you find another pair. Tag these with "V1" and "V2".

Check the final pair and label these as "W1" and "W2"

The second step is to verify the polarity of the windings is correct. To do this, get a 12V DC power supply, or car battery and an analogue DC Volt meter (10VDC scale). The analogue meter is important as you are looking for small deflections in voltage, which you cant normally see on a digital multimeter.

• Connect Multimeter – to V2
• Connect Multimeter + to V1
• Connect Battery – to U1

Briefly touch Battery + to U2. If the voltage reading on the meter deflects +ve then V1 and V2 are correct. If the meter deflects -ve then swap the labels on V1 and V2.

• Connect Multimeter – to W2
• Connect Multimeter + to W1

Again briefly touch Battery + to U2. If the voltage reading on the meter deflects +ve then W1 and W2 are correct. If the meter deflects -ve then swap the labels on W1 and W2.

The next step is to verify the rotation. For this I configured the motor for delta operation, bridging U1-W2, V1-U2 and W1-V2, and then connected L1 of the inverter to U1, L2 to U1, and L3 to W1 and set the inverter to clockwise rotation. If the motor rotates anti clockwise, then swap the labels between U1 and V1, and then swap the labels between U2 and V2. (This last step is optional as you can easily swap rotation of a motor when hooking it up to the power source)

That should pretty much be it.

Hope this is of use to someone.

Graham

[Graham WhartonParticipant @grahamwharton]

[Roger Provins 2Participant @rogerprovins2]

Thank you!

Most useful – filed away for future use.

[John HaineParticipant @johnhaine32865]

There is absolutely no reason to check the polarity! As long as the windings are connected in a delta the motor will rotate one way or the other. If it's wrong just swap two of the wires round.

[Graham WhartonParticipant @grahamwharton]

Hi John,

I dont fully know the theory behind it, as I would assume that the magnetic field generated by an AC driven coil would produce the same magnetic field regardless of which way round the coil is wired, however it certainly has an effect on my motor.

With one of the phases wired back to front on my motor, it span at about 50 rpm, making some strange noises and was drawing full load current all of the time.

I ran the above test which showed one of my windings was inverted, swapped one of the windings round and now the motor runs fine.

I just assumed that it was the way that the coils were wired in relation to each other that was causing the effect, essentially having two windings fighting against each other.

I do see what you mean though, swapping terminals U1 with U2, in delta mode, stil results in a delta.

In my case the three points on delta were U1W1 V1U2 W2V2 instead of the usual U1W2 V1U2 W1V2.

Anyone explain the theory?

Graham

[Graham WhartonParticipant @grahamwharton]

Having read up a bit on the theory, it would appear that having a winding inverted absolutely matters resulting in the magnetic field produced by that winding being 180 degrees out from where it should be. In simplified terms, instead of having fields at 0, 120 and 240, you get fields at 0, 60 and 120.

Swapping L1 and L2 on the supply lines inverts the rotation direction, but swapping U1 and U2 on the terminal block breaks the motor regardless of L1,L2 and L3 configuration.

Graham

[Brian OldfordParticipant @brianoldford70365]

Posted by John Haine on 31/05/2015 18:42:15:

There is absolutely no reason to check the polarity! As long as the windings are connected in a delta the motor will rotate one way or the other. If it's wrong just swap two of the wires round.

But also importantly identifying the winding ends correctly as described gives the flexibility to connect star if required at a later date,

Edited By Brian Oldford on 31/05/2015 20:27:27

[Graham WhartonParticipant @grahamwharton]

Brian, Are you suggesting that inverting the polarity of one of the windings would have no effect on delta configured motors and is only relevant for star configurations.

My recently gained understanding that by reversing a winding, you are effectively reversing the polarity of the inductor, which would result in an unbalanced delta. This surely would have a bad effect on both wiring types?

Graham

[John HaineParticipant @johnhaine32865]

Well this has been a very interesting learning experience! It's 45-odd years since I did courses in electrical machines in my degree and even then I regarded them as a bit of an irrelevance to electronics. So I did a bit of digging and analysis and found that the reason the polarity matters is because the windings in a 3 phase motor are magnetically coupled (as the test method described indicates). If you reverse one winding then it turns out that the resulting 3-phase impedance is serious imbalanced, and at least one impedance seen by the supply is much lower than normal which explains the high phase current.

Edited By John Haine on 01/06/2015 21:34:33

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