Why Are Rotor Bars Skewed in An Induction Motor?

skewing of rotor

What is Skewing of Rotor Slots?

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As shown in the above picture, the rotor bars of an induction motor are not parallel with the shaft and the bars are skewed. The rotor bars are skewed purposefully to get the following performance advantages.

Advantages of Skewing of Rotor of Induction Motor

There are following advantages of skewing of rotor slots.

  1. Increased Starting Torque
  2. Prevents Cogging
  3. Prevents Crawling
  4. Reduce Magnetic Hum

Now, we will discuss all the above points to understand the importance of skewing of rotor.

Increased Starting Torque:

The torque of the induction motor is proportional to the product of the rotor current and the rotor power factor. The length of the rotor bars gets increased with skewing.

skewed rotor of induction motor

The length of the rotor conductor is more if the conductor is put in the skewed rotor slots.The resistance of the conductor depends on the length and the cross-sectional area of the conductor. With skewing, the rotor conductor length increases and the cross-sectional area gets reduced.

Resistance(R)=p*L/A

The increased resistance improves the power factor of the rotor circuit and improves the starting torque delivering capacity of the motor. The higher the rotor resistance(R2), the higher the starting torque.

The speed-torque curve of the induction motor is as given below.

Torque Slip Characteristic of an induction motor

Prevents Cogging:

If the number of stator slots is equal to the rotor slots or equal to the integral multiples of the rotor slots and if the rotor slots are parallel to the stator slots the motor may refuse to start because of the magnetic locking between the rotor and stator teeth. The phenomenon is called cogging.

If the rotor and the stator conductors are parallel to each other, there is a strong possibility of the magnetic locking between the rotor and the stator. Therefore, the rotor slots are skewed.

Prevents Crawling:

The Phenomenon of the running of the induction motor at very low speed is known as the crawling. The harmonics are responsible for the crawling phenomenon. The motor may crawl at the Ns/7 or Ns/13 the speed. The skewing of the rotor reduces the tooth harmonics, and thus prevents the crawling effect.

Reduces Magnetic Hum:

The skewing helps in reducing the magnetic hum, and the motor runs quieter.

Skewing Angle in Induction Motor:

The rotor skew angle formula is as given below.

= 720/n(P/2)  Degree electrical

Where,
P= Number of poles
n= Order of Harmonics

Related Posts

  1. Cogging and Crawling of Induction Motor
  2. Impact Of Harmonics On Induction Motor
  3. Why Induction Motor Takes High Starting Current?
  4. Why does an Induction Motor has Poor Power Factor at NO Load
  5. Torque Slip Characteristics Of Induction Motor
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