The number of pole depends on design of motor and the speed at which the motor is going to operate. The relation is given by

f = PN/120,

where 'f' is frequency (Hz), 'P' is the number of poles and 'N' is the speed (rpm).

Now, if we assume that number of slots per pole per phase is 4 (generally 3-4) and it is a 3 phase induction motor then,

number of slots = 4*3(phase)*P(number of poles)

number of slots = 12*P

Torque ripple depends on the slot opening width, layout of winding, the magnet angle, rotor and stator slots. There is an optimal number of rotor slot at which the torque ripple is minimized.

Excellent dear,

but the question can appear here:

How to determine the number of slots per pole per phase of the motor?

Best Regards

Dear Sir,

The process of determining the slots per pole per phase is dependent on various parameters like the phases of motor to be used, the synchronous speed of motor, winding layout.

Firstly, we need to select a suitable phase according to the application. Most of the motors are 3 phases. Although multiples of 3 phases can be used but they have their own pros and cons.

Next step would be to determine the number of poles, which can be easily found out if frequency and synchronous speed of the application are known.

The number of stator slots should be an integral multiple of the number of phases as integral slot windings yield the highest winding factor which reduced the harmonics of output voltage and hence torque. Slot number is the ratio of stator slots to number of poles. Fractional slot numbers yield short end windings which are beneficial. There are some possible combinations for stator slots to number of poles which can be obtained from standards which are readily available.

For eg.

To design a 3-phase induction motor running at 50 Hz and 670 rpm. We shall consider distributed winding in the induction motor.

The number of poles comes out to be p = 120f/N = 9 poles

The stator slots should be an integral multiple of number of phase and hence can take the values 3, 6, 9, 12, 15, ...

There are certain possible combinations for stator slots to number of poles for obtaining a balanced distributed winding which can be obtained from standard which are readily available. Certain possible combinations for stator slots to poles for the given specifications are 6:9, 12:9, 24:9, 30:9, 42:9, 48:9, 60:9, 66:9, 78:9.

From these a suitable set can be selected according to the application and other parameters like diameter of stator and rotor.