How does a consequent pole motor accomplish speed change (flux path) 1

[electricpete]

I am interested in how a 2-speed 1-winding consequent pole motor accomplishes change in speed. (not how it is wired to accomplish the various constant horsepower, constant torque etc connections).

Here is some discussion of the principle of a consequent pole motor.

http://www.uiitraining.com/b51a/100/15104ac_motor_rewind_intro7.htm

http://www.uiitraining.com/b51a/100/15104ac_motor_rewind_intro8.htm

Each type of consequent pole motor winding will have two separate connections. One is for the high-speed number of poles (4, 6, etc.) and one is for the low-speed number of poles (8, 12, etc.). A specially designed motor controller is used to supply power to the connections (speed) desired. The poles in the high-speed connection will be connected for alternate polarities. Poles in the low-speed connection will be connected for the same polarity, and an equal number of poles of opposite polarity will result as a consequence. That is, the opposing same poles flux create a virtual opposite pole between them. The name consequent pole comes from this consequence. The core and frame must be made from a special high permiability steel to provide the flux path for these virtual poles.

So it’s clear the high speed winding is typical winding construction.

For example if we look at the pole phase groups moving around the circumference for a 4-pole motor, 60hz motor (1800rpm) we might have something like:
A, B’, C, A’, B, C’ A, B’, C, A’, B, C’

Now to get to slow speed (900rpm), something strange is required. The poles in the slow speed diagram are arranged as follows:
A B C A B C A B C A B C

It makes perfect sense to me that this slow speed arrangement yields 900rpm = half the speed of the high speed connection, but not for the reasons stated in the quote above.

I don’t understand the need for any “induced poles” to accomplish this... in my mind it is simply a re-definition of what constitutes a pole (or more specifically the time interval between poles). There is no need for another pole between ppg A and ppg B to provide return path for flux, because A and B have different phase. And their discussion of induced pole implies a return path involving through the frame involving homopolar flux (“The core and frame must be made from a special high permiability steel to provide the flux path for these virtual poles”) – I don’t see the need for that: the fluxes from A+B+C sum to 0 in the same way that the fluxes from A+B’+C+A’+B+C’ sum to 0. If one were of the opinion that the flux from A must return through A’, then one would conclude that an induced pole is necessary. However imo there is no reason the flux from A cannot return thru B and C.


To my way of thinking:
#1 - the high-speed (typical) pattern A, B’, C, A’, B, C’ A, B’, C, A’, B, C’ has 60 electrical degrees between pole phase groups. So for the field to go once around the stator = 12 pole phase groups will require 12*60 degrees = 720 degrees = 2 cycles = (2/60) sec = (1/30) sec => 1800rpm.
#2 - the low-speed pattern A, B, C, A, B, C A, B, C, A, B, C has 120 electrical degrees between pole phase groups. So for the field to go once around the stator = 12 pole phase groups will require 12*120 degrees = 1440 degrees = 4 cycles = (4/60) sec = (1/15) sec => 900rpm.

What is your opinion? Did we add 4 more poles to the 4-pole motor in converting from fast to slow as they suggest above? Or did we simply move the existing 4 poles farther apart in time as described above #1, #2?

Is there a homopolar flux path required for a consequent pole motor as they suggest? Or not (as per my analysis).

Also I can see that overlap between phases would significantly degrade the winding factors in the slow speed application unless coil span were adjusted. Is coil span typically a smaller fraction of pole span for consequent pole motors than others?


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[waross]

Yes, Pete; I am not a winder, but I remember from my motor studies that the coil span is important and not all motors are suitable for consequent pole operation.

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

 

[electricpete]

Thanks Bill. I think that in order to get at my original question, we need to visualize the flux path. One piece missing in order to visualize the flux path is knowing the coil span (and therefore overlap between adjacent pole phase groups).

How is the coil span set for a consequent pole motor?

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[waross]

Visualize a horse shoe magnet with a winding on each leg. Now remove one winding.
Visualize a motor with alternating (instantaneous) north and south poles. Now remove the windings from the north poles. This is the low speed.
Now reverse the connection to every second pole so that we again have north and south pole windings. Now every pole has a winding but there are half as many. This is the high speed connection.


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

 

[electricpete]

I understand flux always requires a return path and I understand the principle of inducing a pole.

For discussion purposes a single phase motor would have logical implementation of consequent pole
Hi-speed winding: A A' A A'
Li-speed winding: A A A A

For the low speed winding, there is no place for flux to return OTHER than between poles, so we have to induce a new pole.

I don't get the same logic for 3-phase winding.
Hi-speed winding: A, B', C, A', B, C' A, B', C, A', B, C'
Lo-speed winding: A, B, C, A, B, C A, B, C, A, B, C

For low-speed winding, why should a new pole A' be created for return path for A flux... the A flux can simply flow into B and C?

I have no doubt the explanation at the link is correct and I see it posted several places, but the physical flux path does not make sense. I think again understanding the coil span may help.



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[electricpete]

I guess the overlap pattern among pole phase groups must prevent the return path thru the other phases.

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[electricpete]

Some thoughts.

Let’s make the simplification only one coil per pole phase group at hi speed.

If you had a full-span coil at high speed, then there is absolutely no path for flux return in the low-speed configuration. All of the return flux would have to be homopolar flux. More importantly there would be no rotating field in slow speed, because each phase would span the entire circumference. Wouldn’t work.

If the high speed coil span was 1/2 of pole pitch, then there would be roughly equal airgap circumference for induced poles as exists for the driven poles in slow speed, and no need for homopolar flux in the shaft.frame. But this would give a very poor winding factor for the high speed winding.

If the high speed coil span is somewhere between 50% and 100% of pole pitch (a huge range), then there is some airgap available for induced pole flux, but not as much as for driven pole flux. Some homopolar flux in the shaft/frame would occur.


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[waross]

First, Pete, don't fight the problem. Consequent pole motors exist and work just fine.
"If the high speed coil span is somewhere between 50% and 100% of pole pitch" Yes, You are on the right track here. I ran across a passage that stated that 80% was the practical maximum.
I suspect that optimum solutions are not possible for both speeds of a consequent pole motor. They are probably a best compromise between an optimized high speed winding and an optimized low speed winding. Nevertheless, even though neither speed is an optimum design, both speeds have acceptable designs. A compromise may still be better than one alternative that I saw, which was to belt two motors to a load with different ratios and energize one or the other depending on the speed desired.

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

 

[rhatcher]

electricpete,

The site you referenced is awesome. I will link it for future reference since there is little 'formal' training material to be found for motor windings. Thanks for the reference.

To try and answer your question about winding pitch, the following is paraphrased from an article in the EASA Technical Manual.
---paraphrased quote
The high speed winding will be connected as close to 1/2 pitch as possible....For example, a 4/8 pole, 48 slot motor will have a span of 1-7 resulting in a chord factor of 0.707 for the high speed and 1.0 for the low speed....
---paraphrased quote

As waross suggested, this is somewhat of a compromise. However, it does work quite well.

To try and answer your more general question of 'how does this work?'...

Your statement: " I guess the overlap pattern among pole phase groups must prevent the return path thru the other phases."

Your question: "For low-speed winding, why should a new pole A' be created for return path for A flux... the A flux can simply flow into B and C?"

When it comes to visualizing the flux path in the motor, that is not an intuitive thing to do. The adjacent poles of 'opposite' polarity actually add flux together in a cumulative way to form a single rotating magnetic field.

Consider the following example with the 'degrees' referencing A phase. Keep in mind that flux is proportional to voltage so what you are seeing is the per unit value of voltage per phase at an instant in time. I will start with A phase at maximum positive voltage using per unit values (1V>90) and follow through with 120 degrees of electrical rotation. Note that the poles marked ' are reversed in the slots so the resulting flux is reversed.

Rotation: ----->
Degrees: 90 (A=1V>90)
Phase: A B' C A' B C'
Flux: 1 0.5 -0.5 -1 -0.5 0.5
Math: sin90 -sin210 sin330 -sin90 sin210 -sin330

Degrees: 120
Phase: A B' C A' B C'
Flux: 0.866 0.866 0 -0.866 -0.866 0
Math: sin120 -sin240 sin0 -sin120 sin240 -sin0

Degrees: 150
Phase: A B' C A' B C'
Flux: 0.5 1 0.5 -0.5 -1 -0.5
Math: sin150 -sin270 sin30 -sin150 sin270 -sin30

Degrees: 180
Phase: A B' C A' B C'
Flux: 0 0.866 0.866 0 -0.866 -0.866
Math: sin180 -sin300 sin60 -sin180 sin300 -sin60

Degrees: 210
Phase: A B' C A' B C'
Flux: -0.5 0.5 1 -0.5 -0.5 -1
Math: sin210 -sin330 sin90 -sin210 sin330 -sin90

The bold numbers represent the peak of the positive pole of the moving magnetic field. Since the total flux for all phases at a moment in time must equal to zero, you can imagine the case if the poles marked ' do not physically exist.

I hope this explanation helps. If not, ask more questions.

 

[rhatcher]

I made a mistake. Please substitute the following for my previous post...

Degrees: 210
Phase: A B' C A' B C'
Flux: -0.5 0.5 1 0.5 -0.5 -1
Math: sin210 -sin330 sin90 -sin210 sin330 -sin90

 

[electricpete]

Hi Ray. Thanks for the EASA info.

With a 50% coil pitch for the high-speed winding, then there is in theory no need for homopolar flux, and I can see no reason to use the terminology "induced pole" imo. Half the airgap circumference (not necessarily consecutive) is spanned by A. The other half of the airgap provides return path for A.. and that other half contains only B and C phase coils whose sum at any time is equal/opposite to A. For me, it fits the 2nd explanation of my original post very well.

IF it were the case that during peak of A phase mmf, a single A phase coil group "pumps" flux accross the airgap twice, THEN I would say there is one pole directly associated with A and one pole induced by A. But that is not the case here with the consequent pole motor meeting the EASA description of coil span. In this case A phase pumps flux accross the airgap once and B and C phase pump it back the other direction.... I don't see any reason the term "induced" is used since we can associate coil groups with all poles, none is created from thin air. Also I see no reason to focus on homopolar flux, except that if the design objective is not achieved there may be small homopolar flux... but homopolar flux is still not an essential feature of the machine.

Maybe (?) it is just a matter of terminology. I don't see any induced poles going on, but I do see my explanation #2 describing the situation well.

Your excercize shows the rotation of the field of an A B' C A' B C' winding such as the high speed winding. I agree 100% with the conclusion you wrote in terms of rotating field, but I don't get what it is supposed to tell us about the low speed winding (which is A B C A B C). One can also describe a rotating field with A B C A B C and that is the description that logically applies to the slow speed winding (with coil span described by EASA) imo.


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[edison123]

pete - 50% coil pitch does not mean half the air-gap circumference in any machine. Only 100% coil pitch in a two pole machine means half the air-gap circumference.

Muthu

http://www.edison.co.in

 

[edison123]

And a 2-pole machine never has a 100% coil pitch, both for mechanical (coil fitment) and electrical (harmonics) reasons.

Muthu

http://www.edison.co.in

 

[electricpete]

I made an earlier simplification that we have one coil per group. (Even if it isn't the case, it still approximately represents the group.) And "half the airgap" was followed with "not necessarily consecutive" to clarify the meaning. The only reason I talked about 100% coil pitch was to provide a discussion of the range, and never talked about 2-pole machine. So no conflicts that I can see.

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[edison123]

OK pete.

Muthu

http://www.edison.co.in

 

[rhatcher]

Pete,

I think that you are understanding this with 1/2 of your brain while the other 1/2 is fighting the idea. So...

I thought that it was important to show that a three phase winding with alternating phases (resulting in alternating poles, standard configuration) creates a single magnetic field in the air gap and in the rotor. The idea is not to think of individual phases and electrical poles but to think of the way in which the flux that is created by the electrical connection is distributed around the circumference of the air gap to create the magnetic poles. This idea is the key to understanding consequent pole windings.

As a note of disclosure, I am not copying or 'paraphasing' this material from any source or sources. If so, it would have been a lot easier for me than thinking of how to make an explanation (hours of time for each post...lol). It would have also assured me that this explanation is correct (unless the person I was copying is also wrong...the danger of copying what you don't understandfrom someone that you do not know). So, feel free to disagree, especially if you have a reference or a better explanation to offer. That being said, I do feel confident in this explanation so here goes...

Standard Winding
Rotation: ----->
Degrees: 90 (A=1V>90)
Phase: A B' C A' B C'
Flux: 1 0.5 -0.5 -1 -0.5 0.5
Math: sin90 -sin210 sin330 -sin90 sin210 -sin330

Note that the magnetic pole formed at A starts at the zero crossing between B-C', peaks at A, and extend to the zero crossing between B'-C. You can see that this pole is formed from the combined, distributed, flux of three windings; C'. A. and B'. The magnetic poles are sinusiodal, 180 degrees wide, and 180 degrees apart.

For a general definition you could say: the three phase alternating pole winding creates a single sinusoidal magnetic field with magnetic poles whose width and distance apart are equal to the distance between two electric poles of the same phase. The number of magnetic poles therefore equals the number of electric poles. This is a result of the distribution of the three phase flux around the circumference of the motor's air gap when the phases are connected for opposite poles (bipolar, like my ex-wife).

Moving on...The same winding connected consequent pole results in a four pole magnetic field in the air gap circumference. This implies that four poles exist in the stator despite the fact that there are only two electrical poles per phase. It is commonly said that the extra poles are 'induced' as a 'consequence' of the connection, hence the name consequent winding. This is where most people struggle, including me the first time that I considered this. How do you make a pole where none exists, or, how do you make a pole between two poles of different phases?

The correct explanation in terms of the general definition above is: the three phase consequent pole winding creates a single sinusoidal magnetic field with magnetic poles whose width and distance apart are equal to one-half the distance between two electric poles of the same phase. The number of magnetic poles therefore equals twice the number of electric poles. This is a result of the distribution of the three phase flux around the circumference of the motor's air gap when the phases are connected for like poles (homopolar). This requires a coil span that is 1/2 of the electrical pitch to allow for the natural formation of the opposing magnetic poles.

Consequent Pole
Rotation: <------
Degrees: 90 (A=1V>90)
Phase: A B C A B C
Flux: 1 -0.5 -0.5 1 -0.5 -0.5
Math: sin90 sin210 sin330 sin90 sin210 sin330

- Note that 1/2 between the A poles (1/2 between B-C) is sin270 = -1.
- Do not think "the A flux is simply returning through the B-C poles." Instead think "the pole at A formed by the sinusiodal flux of the portion of windings from 0-180 electrical degrees is opposed by the pole formed by the sinusiodal flux of the windings from 180-0 electrical degrees which is centered between B-C (270 degrees)".
- Finally, look at the math; a sine wave with 720 degrees, 2 cycles, and 4 poles of rotation. Also, it is very interesting for me to realize that for the alternating pole connection that the region at 270 electrical degrees (sin270) is a zero crossing and for the consequent pole connection it is -1. This fact makes the consequent pole connection seem more 'natural' in a mathematical sense.

Degrees: 120
Phase: A B C A B C
Flux: 0.866 -0.866 0 0.866 -0.866 0
Math: sin120 sin240 sin0 sin120 sin240 sin0

The two positive poles are now 30 degrees left of the A phase pole groups and the two negative poles are now 30 degrees right of the B phase pole groups.

Degrees: 150
Phase: A B C A B C
Flux: 0.5 -1 0.5 0.5 -1 0.5
Math: sin150 sin270 sin30 sin150 sin270 sin30

The positive poles are between C-A at sin90. The next two sequences follow the previous ones and need no further explanation.

Degrees: 180
Phase: A B C A B C
Flux: 0 -0.866 0.866 0 -0.866 0.866
Math: sin180 sin300 sin60 sin180 sin300 sin60

Degrees: 210
Phase: A B C A B C
Flux: -0.5 -0.5 1 -0.5 -0.5 1
Math: sin210 sin330 sin90 sin210 sin330 sin90


In conlusion, there is one more thing that I will add. If you noted above, I stated that the 1/2 pitch winding allows the 'natural' formation of the opposing magnetc poles. The reason that I specified 'natural' formation is that it is possible to have a consequent pole winding with full pitch coils that reults in the 'unnatural' formation of of the opposing magnetic poles. One of the winders in my shop did this by accident. He improperly connected a four pole motor to create an eight pole consequent winding. The result was that the motor had high no load current and that it was very noisy (electrical noise) with a rhythmic sound to it. Upon closer examination, it seemed to be running too slow. When we applied a tachometer, it was seen that the speed was 1/2 (900 vs 1800). At this point the problem was apparent and we reconnected it. In retrospect, I surmise that the unsual and rhythmic electrical noise was the sound of the unbalanced (squeezed) magnetic fields rotating around the air gap and creating areas of high and low flux density in the stator and rotor iron. My opinion is that this motor would have run as a consequent pole with the full span connection but it would have had increased heating, increased noise, and reduced torque compared to a the correct 1/2 pitch winding for this connection.

Ok Pete. Sorry for the long post but it is a tough topic. Please let me know what you think.