Three Phase Rotation
Three Phase Rotation
(OP)
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
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





RE: Three Phase Rotation
Different motors will rotate different directions with the same power hookup.
Different loads may require different directions.
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)' ?
RE: Three Phase Rotation
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"
RE: Three Phase Rotation
RE: Three Phase Rotation
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.
RE: Three Phase Rotation
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.
RE: Three Phase Rotation
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)' ?
RE: Three Phase Rotation
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(2B)+(2B)' ?
RE: Three Phase Rotation
RE: Three Phase Rotation
Bill
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"Why not the best?"
Jimmy Carter
RE: Three Phase Rotation
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
RE: Three Phase Rotation
1. It doesn't.
2. Do not listen to teachers that confuse reality with their misinterpretation of Lenz's law.
Gunnar Englund
www.gke.org
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Half full - Half empty? I don't mind. It's what in it that counts.
RE: Three Phase Rotation
RE: Three Phase Rotation
No.
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)' ?
RE: Three Phase Rotation
RE: Three Phase Rotation
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)' ?