6-Lead Induction Motor - Swap Leads 1 and 4? 3

[Laplacian]

What would be the consequence of inadvertently rolling lead 1 and 4 for example on a 6 lead, wye connected induction motor assuming 1, 2, 3 were the phase leads and 4, 5, 6 were the neutral point.

The new configuration would be 4, 2, 3 phase leads and 1, 5, 6 neutral. This is probably a simple question, but we just received a fairly large motor (8000HP) from the shop that was re-leaded. They didn't mark the leads themselves, only wrote the lead layout on the side of the motor frame where the leads exit.

 

[edison123]

It won't turn. It would just hum.

Muthu

http://www.edison.co.in

 

[electricpete]

What Muthu said makes sense. Effectively the power supply now looks like 3 vectors 60 degrees apart instead of 3 vectors 120 degrees apart.

The good news... you can certainly figure out the correct leads with a 3-channel oscilloscope.

Channel 1: + = T1, - = T4
Channel 2: + = T2, - = T5
Channel 3: + = T3, - = T6

Now rotate the rotor in the direction of normal rotation and you will see 3 sin waves. If the waves are 60 degrees apart, one of the pairs is flipped.

If the waves are 120 apart and follow the same sequence as your power system, then the rotation is correct. For example if you have A/B/C sequence and you plan to hook T1-A, T2-B, T3=C, then you should see positive peaks of channel 1 followed by 2, followed by 3, followed by 1 etc. If you saw 1, 3, 2, 1, 3, 2 etc then the planned hookup would result in reverse rotation (and you know how to fix that).

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[electricpete]

It's probably obvious but I should mention that the overview that you would be manually rotating the rotor and monitoring the voltage induced in the stator winding as a result of the rotor residual magnetism.

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[waross]

If the motor did start, the back EMF of one phase will be opposite to the applied voltage. Expect heavy currents.

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[Skogsgurra]

An illustration is given in attached pdf.

Gunnar Englund

http://www.gke.org

--------------------------------------
100 % recycled posting: Electrons, ideas, finger-tips have been used over and over again...

 

[waross]

Very nice, Gunnar. LPS

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[electricpete]

That is of course exactly what I described.

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[Laplacian]

Thanks for the information. The leads were correct, but I would like to purchase a 3 channel scope for future verification. Any recommendations? I found a 4 channel handheld unit, but am not familiar with the manufacturer. Also, how much/fast does the rotor need to turn? On motors this size, it takes a 6' cheater bar to turn the rotor in 45 degree increments. It's not possible to spin it more than this and keep it going. Thanks again for sharing your experience.

 

[electricpete]

We have done similar procedure (for rotation check, not polarity swap) on a 9000hp vertical with oil lift pump and a 3500hp 324 rpm vertical with no lift pump. In both cases people could get inside the motor stool with a strap wrench and walk around 360 degrees.... no problem to keep it moving as long as desired.

Is yours a horizontal sleeve bearing motor? I can see that could be more of a challenge.

I'm going to think awhile about what useful you can get out of 45 degree rotation or whether there might be some alternate test for that scenario.

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[electricpete]

I have no 3-phase motors sitting around my garage. Here's a request for gunnar (or anyone who wants to try it).

Take a motor with known correct lead markings.

Apply a voltage and measure current in the following two configurations:
V+ == T1------ T4 ==T2------T5==V-
V+ == T1------ T4 ==T5------T2==V-

(note that === represents external/test connections and --- represents internal wiring identified by end-terminals).

Is there a difference in current measured with same voltage applied?


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[electricpete]

Correction in bold:

(note that === represents external/test connections and --- represents internal winding identified by end-terminals).

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[electricpete]

Also I should've mentioned it should be an ac voltage.

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[electricpete]

I believe that the first test configuration (V+ == T1------ T4 ==T2------T5==V-) should give a higher current for a given voltage than 2nd configuration (V+ == T1------ T4 ==T5------T2==V-).

The reason is that the three phase windings are arranged something like a b' c a' b c' ... with overlap among adjacent groups. With 3-phase power supply, the phase difference among adjacent groups is 60 degrees.

When we hook it up per the first configration, the phase difference between adjacent groups will be 180 degrees... the fluxes will tend to cancel... in ideal case of complete cancelation of fluxes (not achieved) the magnetics are irrelevant and the current limited only by resistance (high current).

When we hook it up per the 2nd configration, the phase difference between adjacent groups will be 0 degrees... the fluxes will tend to add, the inductance will increase and current decrease.

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[electricpete]

Induced current in rotor and return path for flux tend to confuse the picture a little bit, but I believe the main effect will be as described in my preceding post.

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[electricpete]

Even without knowing the answer to the above experiment, there is another test we can do by symmetry.

Hook up all three windings in series (ac supply):
V+==T1---T4==T2--T5===T3--T6==V-

A = test as is.
B = test with T1/T4 reversed from above
C = test with T2/T5 reversed from above
D = test with T3/T6 reversed from above

One of the four tests will be significantly different from the other three:
* If test A is significantly different than the other three tests, the the labeling of leads is correct as-is.
* If test B is significantly different than the other three tests, then T1/T4 are labeled in reverse.
* If test C different, then labeling of T2/T5 is backwards.
* If test D is different than the other 3, then labeling of T3/T6 is backwards

All of this assumes we have started by doing a continuity test to verify we at least have the pairs correctly.

None of this will tell you the rotation.

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[purplepete]

Guys,
I used this test (file attached) a few years ago on repaired submersible pumps to identify the lead ends. I'd really appreciate comments thanks.