Understanding electric motor connections

[BobL]

A friend of mine sent me this link on understanding Electric motor connections.
http://www.ecollege.ie/site/liu/electrical_notes/LL241.pdf

It's one of the clearest explanation of a number of basic 3 phase motor concepts that I have come across.
There is a also a useful section on basic tests for these and single phase motors.

[Bob Willson]

Thanks for that Bob.

I am only up to page 10 but I have a question that I haven't seen answered yet.

How do you differentiate between the 'start' of a winding and the 'finish' of a winding? (P9 just beneath fig.8) It seems to me that they are one and the same thing.

[Skew ChiDAMN!!]

I believe it simply means to ensure that all 3 windings are oriented the same physical way in the stator. eg. consider all the [U/V/W]1's as the 'starts' and all [U/V/W]2's as the 'ends'... or vice-versa.

If the motor runs in reverse, then one of the windings is 'backwards' to t'other two.

[Bob Willson]

So, if we were looking at this from a clock point of view, The 'U' would start at 12 and finish at 4 as U1. Then, V would start at 4 and finish at 8 as V1. Then, W would start at 8 and finish at 12 as W1, so that everything goes in a clockwise direction, but it would be equally true to say that the opposite is also correct so that we could start with U1 at 12 etc.

Ahah, I see now.

Thank Skew

[Malibu]

Hi guys,

I think what they're referring to in the pdf is that magnetically, all windings need to be aligned in the stator. As you say Bob, (S1-E1)(S2-E2)(S3-E3) in your clock example is correct.
If the windings are physically installed to (S1-E1)(S2-E2)(E3-S3) one of the windings creates a parasitic magnetic field to the other two (according to Fleming's right hand rule). It won't necessarily run in reverse - at least no more than normal with a 3ph motor, but the motor will growl and run badly... Usually running way over current as well - A lot like a single phasing 3 phase motor would.

The direction of the windings is normally sorted out by the armature winder/manufacturer of the motor, but can sometimes be a problem in the field if the terminal block is burned off or missing and you only have 6 wires coming out of the stator and not labeled. It's easy to identify the coil pairs with a multimeter, but impossible to determine which is the start and which is the end, so it becomes a guessing game. The only way to identify them is to wire up the motor and run it... If it runs badly, one of them is reversed, so the first step is to swap the S1-E1 pair and run it again. If the motor runs good, that was the correct pair to change, if not, swap them back and change S2-E2 around and so on until the motor runs good.

In 30-odd years as an electrician, I've had to do this only a couple of times so it's not a common problem.

Hope this helps :)