What is the reason for the rotor of an induction motor having a different number of poles than the stator?

The number of rotor poles in Induction motors is usually the same as the number of stator poles, because the working principle of the Motor depends on the rotating magnetic field generated by the interaction between the stator and the rotor. The following explains in detail why the number of rotor poles is usually the same as the number of stator poles, and explores whether reversing the number of poles can improve the performance of the motor.

Why is the number of rotor poles the same as the number of stator poles?

Synchronous magnetic field

• Stator winding: The rotating magnetic field generated by the stator winding is a fixed number of poles, usually an even number of pole pairs (such as 2-pole pairs, 4-pole equivalents).

• Rotor winding: In order for the rotor to be able to rotate with the stator magnetic field, the rotor must also have the same number of poles so that it can synchronize with the stator magnetic field to produce a constant electromagnetic torque.

Torque generation

• Induced current: When the stator generates a rotating magnetic field, a current is induced in the rotor, and the magnetic field formed by these currents in the rotor interacts with the stator magnetic field to produce torque.

• Pole matching: Only when the number of rotor poles is the same as the number of stator poles can the rotor magnetic field be synchronized with the stator magnetic field, thereby effectively generating torque.

Slip rate

• Synchronous speed: The synchronous speed ns of the motor is proportional to the pole number p and the power supply frequency f, that is, ns= 120f/ p

• Actual speed: the actual speed n of the rotor is always less than the synchronous speed, and the ratio of the difference to the synchronous speed is called the slip rate s.That is s= (ns−n)/ns.

Does reversing poles improve performance?

The effect of polar inversion

• Magnetic field asymmetry: If the number of rotor poles does not match the number of stator poles, it will lead to magnetic field asymmetry, which will affect the normal operation of the motor.

• Torque fluctuation: Pole mismatch will cause torque fluctuation to increase, the motor operation is unstable, and even can not start or run normally.

Performance impact

• Reduced efficiency: Pole mismatches can lead to reduced motor efficiency because of reduced energy conversion efficiency.

• Noise and vibration: Asymmetrical magnetic fields can cause the motor to produce additional noise and vibration, affecting the service life of the equipment.

Other considerations

• Design flexibility: In some special designs, such as two-speed motors, the number of poles can be changed by changing the connection of the stator windings to achieve different speeds. But this is still pre-set at design time, rather than randomly changing the number of poles.

• Types of motors: Different types of motors (such as permanent magnet synchronous motors) may have different combinations of poles, but these are designed for specific application purposes.

Sum up

The number of rotor poles in an induction motor is usually the same as the number of stator poles, which is to ensure that the rotor can rotate synchronously with the stator magnetic field, resulting in a stable electromagnetic torque. If the number of poles is reversed (that is, the number of poles is changed), the magnetic field will be asymmetrical, the torque fluctuation will be increased, the motor efficiency will be reduced, and additional noise and vibration will be generated. Therefore, reversing the number of poles will not improve the performance of the motor, but will make the motor not work properly. In practical applications, any modification of the motor pole number should be carried out under the guidance of professionals and ensure that it meets the design requirements of the motor.