Lesson Four: Magnetic Poles And Rotating Speed Of The Motor

 

Magnetic poles and rotating speed of the motor

This chapter will explain how the number of magnetic poles in the motor and the frequency of the current will affect the rotating speed of the motor

4.1Magnetic poles

This chapter describes how to create other pole numbers. To the left is the picture of the two pole motor that we have been using so far.


Figure 11. Motors with different amount of poles

In this motor each phase have two coils, and this resulted in one North Pole and one South Pole. So one pole pair or two poles
In the picture in the middle each phase has four coils. Double the amount of coils compared to the two pole motor. That means it has two north poles and two south poles which results in two pole pairs or four poles.
Now continue to increase the amount of poles by adding more and more coils for each phase like in the right picture. To create a six pole motor use six coils per phase that creates three north poles and three south poles and so on.
The number of stator slots is always a multiple of six. When it comes to the pole number it’s always a multiple of two cause the north and the South Pole always come in pairs. It’s not possible create a north pole without creating a south pole and vice versa. When talking about motors remember not to mix up the number of poles with the number of pole pairs. A combination of the frequency of the current and number of poles in the motor is what decides the rotating speed of the motor. (Electrical4u)

 

4.2Rotating speed of the motor

As was said earlier the AC shift direction 50 times per second if it is a 50Hz network. Looking at the diagram, all the three phases will reach their peak on both the positive and negative side fifty times per second. This means that the magnetic field will rotate fifty times per second in a two pole motor as it has only one pole pair.

Figure 12. Rotating magnetic field in a stator

As the rotating speed of a motor is given in revolutions per minute. This means that if the magnetic field rotates 50 times in one second it will rotate 3000 times in one minute. The real rotation speed of the asynchronous motor will always be a bit slower because of the slip.
Now with a four pole motor which means two pole pairs. It will take double the time for the magnetic field to travel around the stator as it has double the amount of winding coils for each phase. So with a frequency of 50Hz the magnetic field rotates with a speed of 25 revolutions per second which means it will rotate 1500 times in one minute.
It’s possible to calculate the speed of a motor with any given amount of pole pair number with the following formula.


Take 60 seconds times the frequency of the AC network used divided by the number of pole pairs in the motor equals to the rotation speed of the motor given as revolutions per minute.

AC with a frequency of 50Hz gives these magnetic field rotation speeds depending on the amount of pole pairs in the motor.

 

  1. 1 pole pair = 3000rpm
  2. 2 pole pair = 1500rpm
  3. 3 pole pair = 1000rpm

50Hz and 60Hz are the standards used by various countries. The reason why there are different amount of poles is that the poles determine the rotation speed of the magnetic field which in turn decides the rotation speed of the motor.
The more poles, the slower the motor will spin. So now it is possible to choose between a higher rotating speed or greater torque production based on what applications the motor will be used for. (Electrical4u)

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Post time: Jun-24-2019
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