Motor Poles count

[MiketheLec]

Can somebody help a novice here?

I'm trying to understand motor theory of induction motors and relate to what I see when I open one up. Formulae allows calculation of speed from "number of poles" and frequency but what does the use of language "poles" actually mean. I see in the text book an illustration of a three phase motor with six coils and it says it is 2 pole three phase so do i interpret "poles" as coils per phase? If a single phase induction motor has two coils does that make it a 2 pole motor? Some texts use language of "pole pair" which seems to further confuse. Is there an expert in terminology that can help me here, Much appreciate.

 

[electricpete]

A pole would be a grouping of physically adjacent coils wired in series so they carry the same current. They act sort of like one big coils with a number of turns equal to the number of coils multiplied by turns/coil (with a few extra fudge factors... pitch factor and distribution factor). A pole acts like the pole of a magnet. Just like magnets, electrical poles never come in odd numbers, they always come in pairs. Each pole pair contains two poles of opposite polarity. The number of pole pairs is one half the number of poles.

In a four pole three phase motor, there would be twelve pole phase groups. You could imagine that each pole consists of three adjacent pole phase groups... one per phase. The pattern would be a b' c a' b c' a b' c a' b c'


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[unclebob]

I don't work with motors very often, but I remember that a 2 poles motor runs at 3600 rpm, 4 poles 1800 rpm, 6 poles 1200 rpm. Now there are 2 formulas:

RPM = (60 * frequency)/ P P = number of pole pairs

or,

RPM = 2 * 60 * frequency / n n = number of poles

 

[electricpete]

A pole would be a grouping of physically adjacent coils wired in series so they carry the same current

Correction - that would be a pole-phase group. Combination of pole phase groups to form a pole was discussed above.

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[MiketheLec]

Thanks electricpete, I think I understand what you are saying but still have some difficulty in relating to what you physically see. I understand of course that a magmet has two poles and that is a pole pair but in the case of a coil the core or centre of the coil will have one polatity in one half of the ac cycle and the reverse polarity in the other half cycle. When I look at the centre, or the core at the centre, of an individual coil, does that count as a pole or a pole pair. I think this is the knub that I can not really get clear in my mind.
One other point that I am struggling with is whether the number of poles on the stator needs to match the number of poles on the rotor; if the ratio is not the same does that have to be taken into account with the formulae for calculating speed?
Many thanks from a slow learner!

 

[ozmosis]

MiketheLec
Have a look on this:

http://automation.usa.siemens.com/step/templates/lesson.mason?motors:2:6:1

and follow some of the steps. It shows in diagrams what electricpete describes

 

[waross]

For a single phase motor, The least number of poles is one North pole and one South pole. This is a two pole motor.
For a single phase motor the least number of poles is six.
This would be one North pole and one South pole for each phase. N-A, N-B, N-C, S-A, S-B and S-C. This is still considered a two pole motor for the purpose of determining speed.

There is an exception for three phase, two speed motors.
This is the consequent pole motor. A 4 pole motor will have half the coil groups reversed so that instead of a coil for each North pole and a coil for each South pole, each coil produces a North pole. South poles form in the spaces between the windings. The motor must be constructed in such a way that there is space between the coils for these poles to form.
In this way, a 4 pole motor may be converted into an 8 pole motor without increasing the number of coils.
respectfully

 

[sreid]

The number of poles on the rotor is the same as the number of poles on the stator. The poles on the rotor, however, are induced (hence induction motor) by the stator currents. There are no explicit poles on the rotor squirel cage.

 

[aolalde]

One way to identify the number of poles is the coil’s pitch or span. Pitch = 360°/Poles.
Full pitch for two poles is 180 geometric grades, 90° for four and 45° for 8 poles.
The most basic winding has at least one coil per pole per phase.
2 poles single phase need at least one coil.
3 phase 2 poles need at least 3 coils.
Polyphase windings space evenly each phase coil. (0, 120 and 240 electric grades for 3 phases).
For two poles geometric an electric grades are the same.
For more than 2 poles the circumference (360°) is divided by the pairs of poles and the result is the equivalent to 360 electric degrees. ( for 8 poles, 360/(8/2)= 90 geometric degrees are equivalent to 360 electric degrees.
One concentrated coil per phase per pole is not practical, instead of one, several coils are series connected (distributed winding).
Two sides per slot make a practical nesting in a slotted core; that gives the 6 coils (or groups of coils) for 2 poles 3 phases.
To eliminate harmonics generated, and some times to make the coil assembly accessible, the full pitch is reduced (fractional pitch winding). 5/6 of full pitch is very popular.
These are only the basic concepts; each motor could have different solutions or arrays.