Three Phase Rotation

[HughRob]

Why do leads need to be changed to get correct rotation on a 3 phase motor?

I understand about that sine waves, etc.

But if all things are equal so the power system from the main transmission lines entering the building are pulled the same way so the relationship between L1, L2 and L3 are in the same order shouldn't all motors be correct?

Is there an issue that due to the distances the cables run or something L1, L2 and L3 can end up having their peak voltages occurring at different times between two locations?

Thanks in Advance

 

[electricpete]

But if all things are equal so the power system from the main transmission lines entering the building are pulled the same way so the relationship between L1, L2 and L3 are in the same order shouldn't all motors be correct?

"Correct" requires more than just that.
Different motors will rotate different directions with the same power hookup.
Different loads may require different directions.

Is there an issue that due to the distances the cables run or something L1, L2 and L3 can end up having their peak voltages occurring at different times between two locations?

No.
It is a matter of phase rotation. Of course you are aware, if you swap two leads of a 3-phase machine, the direction will reverse.


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(2B)+(2B)' ?

 

[jraef]

Think about it is way. When you wind a motor and it comes time to label the leads, which one is A? Then is B the one to the right, or the one to the left of it? You should then ask yourself, Right or Left relative to what? And why is that what it is relative to? Then how does that relate to the supply source rotation? Not all utilities are the same in case you didn't know. For example here in Northern Cailfornia where I live, PG&E is the largest power utility and their rotation is CBA.

Basically, I'm trying to point out that rotation direction is somewhat arbitrary, so it really only matters at the final point of use, where a simple test determines what you need.

"Will work for (the memory of) salami"

 

[OperaHouse]

When I first read this I had a completely different answer. Reading it again I believe that for many electricians, L1 L2 L3 on the power side are a totally random assignment.

 

[cranky108]

How do you know which lead the power company gives you is A, B, or C? Do they tell you? Or does the electriction who connects up the three leads even care?
Three black wires are hot, blike wire with white stripe is neutral.

However the electrictions always seem to know which is the wild leg in those applications.

But consitering GE large generators are ABC rotation, and Westinghouse large generators are CBA rotation, we figure it out on the generator side.

 

[OperaHouse]

A company I uses to work for designed and sold a three phase rotation tester. It would work from 10mv to 600V. Spin the motor by hand and residual magnetism would be enough to determine ABC then check power. If everything matched up it would un in correct direction first time.

I was at a business and an outside electrician asked me a strange question about the wiring of a three phase compressor. Next day I wondered if he knew what he was doing. The compressor had a gear pump for lubrication. The following day I stopped in to check. Going in the back way I could see an oil mist coating on cardboard boxes and a NEW compressor.

 

[electricpete]

Good point. The thread was stuck on why it isn't enough to hook all motors up the same,but deenergized checks of motor can be an important part of the strategy.

For small motors, there are dedicated rotation testers. The ones we have require you to accelerate the rotor rapidly to get a good first initial-deflection direction, which doesn't work well for motors.

Gunnar had suggested simple test with two channel oscillscope and I posted procedure here

http://www.eng-tips.com/viewthread.cfm?qid=265331

Actually the procedure attachment disappeared from that thread (weird.... I guess engineering.com is only a temporary repository?)
I have re-attached here.
This type of procedure we now use at our plant for changeouts when timing is critical. Even if we use the procedure, we still confirm direction of rotation during uncoupled run.


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(2B)+(2B)' ?

 

[electricpete]

I left out a word, corrected below

The ones we have require you to accelerate the rotor rapidly to get a good first initial-deflection direction, which doesn't work well for large motors.

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(2B)+(2B)' ?

 

[Gyo]

What if you have just the stator only, but need to verify if the phase sequence is correct? What are the test equipment typically used to do such test, whether it is for small or large motors?

 

[waross]

A steel bearing ball.

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

 

[DRWeig]

Bill,

I never got a definitive answer to the steel ball issue. A professor long ago in school said that the ball acts like a car wheel and revolves opposite the field. It's not intuitive to me.

A more clear (I hope) phrasing of my question:

Is the ball pulled by the rotating field to revolve around in the direction of the magnetic field, which is the ultimate rotor direction?

Or, does the ball rotate in that direction and drive itself by friction to revolve the other way?

Best to you,

Goober Dave

Haven't see the forum policies? Do so now: Forum Policies

 

[Skogsgurra]

That is the weirdest thing I ever heard! Why should the bearing rotate opposite the field?

1. It doesn't.
2. Do not listen to teachers that confuse reality with their misinterpretation of Lenz's law.



Gunnar Englund

http://www.gke.org

--------------------------------------
Half full - Half empty? I don't mind. It's what in it that counts.

 

[Gyo]

Electricpete has attached a procedure on how to carry out the test with an oscilloscope. Can this test be done on a squirrel cage induction motor? Do I need to connect to an external source? How could there be residual of magnetism when there are no excitation source to the motor? I would thought that when a conductor carrying current cuts the flux, then voltage can be induced. Could you please enlighten me on this?

 

[electricpete]

Electricpete has attached a procedure on how to carry out the test with an oscilloscope. Can this test be done on a squirrel cage induction motor?

Yes.

Do I need to connect to an external source?

No.

How could there be residual of magnetism when there are no excitation source to the motor?

With the machine deenergized, the rotor iron still has residual magnetism leftover from the last shutdown.

It is enough to give a few hundred millivolts when manually rotating.

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(2B)+(2B)' ?

 

[Gyo]

Thanks, electricpete. I have a scenario here. In the next 3 days or so, a 10kV 650kW 1450rpm motor will reach our repair shop for bearings replacement after more than 15 years of operation. The terminal markings, which are just some colored taped, have faded. I would assume that any residual magnetism would have fully decayed by the time. So, instead of re-energising from a 6kV or 10kV source, there is a 415V 3phase source readily accessible that I could use for this test.

 

[electricpete]

I don't know the timeframe for losing residual magnetism, but I've never seen a more that didn't have it. Many new small motors pulled from the warehouse after years in storage are tested by "rotation tester" using similar principle and it works fine.

So Of the purpose is soley to determine rotation, I would definitely go ahead and hook up and rotate the motor to see if you get any induced voltage before energizing anything.

If you want to induce residual magnetism in the rotor, maybe apply dc to one pair of leads using DLRO or ductor resistance measurement instrument. There may be other dc methods as well,just be give some though to how you interrupt the current.

I'm not sure what you are planning to do with the 415v three phase. You might be able to get the rotor spinning but I'd be careful. Setting up a large motor test stand requires some planning (never done it myself... talk to a motor shop guy)

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(2B)+(2B)' ?