2-Pole vs 4-Pole Motors

[Johnster]

Can anyone tell me other than speed where I would want to use a 2-pole motor instead of a 4-pole motor and vice-versa?



Thanks
Johnster

 

[stardelta]

2 pole motors are available in smaller frame sizes than 4 pole motors of the same power.

 

[ScottyUK]

2-pole machines typically have (slightly) lower magnetising current than the 4-pole equivalent. This effect becomes more pronounced as the number of poles increases.


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[SteveKW]

Four poles have a longer bearing life, more torque and are typically cheaper to purchase.

 

[spargher]

2 poles rotor are more expensive than a 4 pole, expecialli in mid-power application (from 200 to 4000kW).
Vibrations on this kind of 2 poles is a big problem, either for the balancing stability (i.e. termal bow) either for the twice line frequence component)

 

[electricpete]

I agree with everything said above, except that 2-pole in my experience are less expensive (initial cost), then 4-pole of the same horsepower.

4-pole does a little worse on power factor and a little worse in core losses. 2-pole does a little worse in friction and windage losses.

If you start looking at efficiency, don't ever overlook the driven load which usually is where most of the losses are. (motors typically above 95% efficiency, pumps often 60% efficiency). The losses in the pump are tied to how it's sized and speed is a big part of that whole process.

I agree with the last two posters on the subject that 2-pole motors by and large cause more maintenance costs over their lifetime than 4-pole motors. 20% of our machines cause 80% of our maintenance. Of those 20%, 80% are 2-poles wherease only around 50% or less of all our machines are 2-poles. In other words, a disproportionate share of our problem-machines are 2-poles.

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[UKpete]

If you disregard speed then there is little to choose between 2-pole and 4-pole.

But in what circumstances wouldn't the speed matter? If you had a variable speed drive application you might think that the number of poles is a variable but you would probably still go for the machine most closely matched to the maximum required speed. For example, if you needed a maximum speed of say 1500-2000rpm, the best choice is still a 4-pole machine because you would need a larger size (more expensive) 2-pole motor as it would be running at below its rated voltage.

 

[electricpete]

Yup. Usually it would be driven load considerations that play a lot bigger role in determining speed than motor considerations. The motor is generally picked to match the load.

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