Speed of multi-pole 3 phase motors

I think you maybe a little confused with how you have your question lined up.. At least you're confusing me..;)

Lets assume for the moment Synchronous speed to make the math simpler..

All Examples at 60Hz. Hz*2/Poles*Seconds = RPM's 2 pole motor

60*2/2*60 = 3600 RPM's 4 pole motor 60*2/4*60 = 1800 RPM's 6 Pole motor 60*2/6*60 = 1200 RPM's 8 Pole Motor 60*2/8*60 = 900 RPM's Ett.. You may have noticed that i've taken the Hz*2 ? This is because you get a motion per half cycle change.

Now because these types of motors are not synchronous types, They have slip, so actually speed is ~ SynchronizeSpeed*.965. This varies depending on the rotor design and application.

So a 1800 RPM motor is ~ 1750 RPM's and when fully loaded drops back a little more on top of that.

With your numbers that you have displayed here, it looks like you are depicting a 9 wire motor? Which is design to allow you to operate the motor in 2 different voltages. The pole count does not matter for this fact. I have seen some motors where you could bundle the poles to select 2 different speeds..

If you do have a motor with 3 sets of stators all evenly offset, you need to know the number of poles per stator winding and sum them. Each pole should be a even number.

In a standard 9 wire 3 phase motor, the coils are not offset, but in the same place, together.. You need to parallel them or series them, depending on the voltage you are applying.. In some applications, motors have for example, an electric brake that releases when energized with 230 volts how ever, the motor is wired for 460 volts, in whic case you simply connect the brake in the middle of the series tap, which will give you 230 volts and if the motor was wired to operate at 230 volts, then of course, the brake would be connected to the main source coming in.

Jamie

Sorry if the way I posed my question was confusing. The motor analogy was actually intended to make it simpler :-)

It is basically a physics question. Forgetting standard motor configurations ... if I have a ring of nine coils and feed them with a 60Hz sinewave in the following phase order, is the field rotation

60Hz or 60Hz / 3 = 20Hz?

1: 0

2:: 120 3: 240

4: 0

5: 120 6: 240

7: 0

8: 120 9: 240

Thanks,

Frank Gardener

Are the spatial distributions of the above coild still as stated in your original post? If so, then the field is rotating at 60 Hz (3600 RPM). A two pole motor, as described by Jamie.

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Paul Hovnanian     mailto:Paul@Hovnanian.com
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Don't get even -- get odd! :¬þ

I see three phase three pole. Where are you getting two poles from?

If the each pole is only progressing a third of the circumference 60 times a second, then why is the combined rotational speed of the three poles 20 not times a second?

Frank

This is a 6-pole motor. there can be no motor with an odd number of poles, as there has to be a continuing sequence of N-S-N-S. So, a 3-phase motor with 1 coil per phase would be a 2-pole motor. It gets more complicated in 3-phase, as both other phases act as the opposite pole for each phase. So, it is 6-pole, and would operate at somewhat less than 1/3 of 60 revs/second. At 60 Hz, it would run at somewhat less than 1200 RPM.

Jon

this point for me?

No, it's entirely suitable to have a nine-pole motor driven by three phases; It just means that the rotor has three N and three S poles, and that one phase at any given time is at a near-zero field.

Thus, motor catalogs refer to 3600/1800/1200 (-epsilon) RPM speeds for AC motors at 60 Hz . Typically, the minor precession of the field inside the rotor gives true speeds 3475/1725/1140 RPM..

this point for me?

You can't have an odd number of poles. If it has 3 N and 3 S, that sounds like six to me.

Jon

The three phase windings distributed around the stator produce a flux distribution in the air gap that has an 'N' pole and an 'S' pole (2).

At the instant that one phase coil reaches its peak current (in a three stator pole motor), the other two sum to produce the opposite pole between them.

6 stator windings arranged 60 degrees apart and connected A-B-C-A-B-C will produce a flux Distribution in the air gap with peaks 90 degrees apart N-S-N-S. That would be a 4 pole motor with a synchronous speed (at 60 Hz) of 1800 RPM.

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Paul Hovnanian  paul@hovnanian.com
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Have gnu, will travel.

[where the 'it' that has 3N and 3 S poles is the rotor]

Yes, that's true, but a mechanical tear-down will reveal the rotor to be a lump of soft iron with no particular magnetization, and the stator to be wound with nine pole pieces evident. I'd call it a nine-pole motor, The only-while-powered polarization of the rotor is something not evident to the observer while the motor is at rest...

Jon is correct when talking about the flux distribution in the air gap. Not to be confused with the number of stator windings (assumed to be lumped for easier visualization) per phase.

--
Paul Hovnanian  paul@hovnanian.com
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Have gnu, will travel.