# Star-Delta Motor Starting Polarity.



## cthom (Sep 1, 2014)

Have a 6 lead motor that is intended for wye-delta starting, but there are two leads labeled T1, T2, T3, and no T4, T5, or T6. Does the polarity of the windings matter. I believe it does, but there has been some discussion about it among coworkers. If it does matter, how would I determine the polarity of the motor windings, when not labeled as six separate designations. Thanks in advance.


----------



## micromind (Aug 11, 2007)

What HP is the motor? Is it single voltage? If it's dual voltage, it needs to be supplied with the lower voltage. 

Polarity of the windings does matter. 

I may need to amend my PM to you........lol.

If it's over 100HP, it's possible that it's single voltage and not capable of Y-∆ starting. 

Big motors often have more than one lead per phase in order to cut back on wire size. 

The way to tell is to check continuity from one of the T1s to the remaining 5 leads. If there is continuity from one of the T1s to one of the T2s and one of the T3s but nothing else, then it's not Y-∆. 

If there's continuity between one of the T1s and one of the T2s, then it is Y-∆, and 3 of the leads are marked wrong. 

In this case, the T1 that has continuity to T2 is actually T1 and T4. Leave T1 and change T2 to T4. The remaining T2 will have continuity to one of the T3s. This is actually T2 and T5. Leave T2 and change T3 to T5. This leaves T3 and T1 which is actually T3 and T6. Leave T3 and change T1 to T6. 

When you're done, you'll have T1, T2, T3, T4, T5, T6. 

Remember that when connecting to the starter, it's usually T1, T2, T3 on one starter, then T6, T4, T5 on the other. 

Rob


----------



## cthom (Sep 1, 2014)

Thanks for the quick reply Rob,

It's a 300HP motor for a compressor, it just failed. It's actually been running for about a year with wye-delta starting so it's definitely capable. What interests me most is the way in which it failed and the fact that the motor leads weren't T1-T6. Someone else had wired it in about a year ago and I am curious if it's possible that correct winding polarity was not observed on one of the windings, and the motor has been fighting itself for that whole period of time? Would it be possible it could have lasted that long if wired incorrectly? When it failed, it went out on ground fault when switching to delta. Checked contactor contacts, a little rough, but not enough to warrant failure (we're going to replace anyway). Am guessing motor insulation may have grown weak from fighting itself, and the increased voltage/current when switching to delta was the last straw. All motor leads had continuity after failure, even though only about 1.5Mohm to ground. We have a replacement motor that is labelled T1-T6 which was the original, so shouldn't have an issue with it.


----------



## micromind (Aug 11, 2007)

If one of the windings was reverse polarity, it'd most likely burn up within a few minutes, or maybe seconds. Also, it would produce less that nameplate HP. 

Hard to say what made it cook. 

One bad thing about a motor connected ∆ is that if one of the leads is not energized, the current in the remaining energized leads will go into the stratosphere. But the O/Ls won't see much of it, and eventually the motor will burn up.


----------

