Do more poles = more MAXIMUM torque?

[geneiusxie]

Hi guys,

I've had this question bother me for a few months now, but do more motor poles mean more MAXIMUM torque - not the torque created from the same current, but assuming that you have enough current to saturate the iron cores, would increasing the # of poles increase the maximum torque you could get from a particular motor, assuming everything else i.e., weight and size? And if so, would there be a limit to this torque increase? So, for example, if I put a million poles in a motor, would increasing the pole count still increase the max. torque? If so, could I, theoretically (assuming I could design a motor controller that could drive such a motor) create a motor with (almost) infinite torque?

Of course, increasing pole count also means the current must switch faster, demanding faster motor controllers and increasing proximity effect losses, etc.

-Gene

 

[circuit]

If in short, no.

 

[Miles]

Increasing the number of poles reduces Kv in the ratio of RPM:eRPM. Energy has to be conserved - Kt increases proportionately. So, you get a greater electro-magnetic field for a given current flow. Saturation depends on the field strength though, not the current. Right? You will get a motor that is more efficient at greater torque levels but given the presumption that you can extract all the heat that's generated, this won't help when you have completely saturated the stator core.

Does this make sense? I'm not sure it does, to me The electro-magnetic field strength should only depend on the current flow and the number of turns. Right?

The downside to increasing the number of poles is that eddy current losses will go up approx. as the square. Also, unless you reduce the magnet area, you will have greater flux leakage between adjacent poles.

 

[Spacey]

Technically ...no. But does it feel like it has more torque....yep.

 

[Miles]

Why, "technically no"?

 

[johnrobholmes]

A higher pole count motor may be able to get into an efficient speed sooner, but unless there are other design changes you can't add torque by adding poles or slots. Adding slots reduces iron or copper fill per tooth, reducing current ability or saturation level per tooth. Adding poles decreases the distance that the tooth needs to pull on the magnet per cycle, but also decreases the distance moved per cycle. For a given frame size and RPM, the standard pole/ slot counts are relatively optimized.


Some design considerations must be made however. Take the NeuMotor outrunner. It is a much higher slot count than typical outrunner of the same size. Why is this? The main reason is because of the very short length of the motor. For practical KVs it would have been a very high wind count and coil resistance, so the poles and magnets were doubled and coils terminated in parallel. High quality laminations ensure switching losses are low.

 

[Miles]

A higher pole count motor may be able to get into an efficient speed sooner, but unless there are other design changes you can't add torque by adding poles or slots. Adding slots reduces iron or copper fill per tooth, reducing current ability or saturation level per tooth. Adding poles decreases the distance that the tooth needs to pull on the magnet per cycle, but also decreases the distance moved per cycle. For a given frame size and RPM, the standard pole/ slot counts are relatively optimized.

Strictly, the question is "can one get more torque given a saturated core by increasing the pole count?"

Suppose one had an 18 slot core and the choice of either 16 poles or 20 poles. The winding factor for either would be exactly the same.

 

[Miles]

I already started the comparison:

[pre]Comparison of 16 pole to 20 pole

16p 20p

Fundamental winding factor | 0.945 | 0.945 |
Slots/pole/phase | 0.375 | 0.300 |
Cogging steps per rev. | 144 | 180 |
Torque ripple | - | + |
Km | - | + |
Iron losses | + | - |
Active material economy | - | + |
Magnet area | + | - |[/pre]

NB "+" always represents a beneficial factor

Any more factors to add?