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Answered 2009-02-05 02:09:08

The cogging effect is the attraction between the magnets and the stator poles when at rest; this attraction and cogging is easily felt when trying to turn an electric motor by hand. A common misconception is that cogging is caused by the copper coils; however, this is not the case. An unequal attraction between the magnets and the stator results in the rotors reluctance to turn. The magnets sit comfortably at rest over a pole, and when the rotor is turned, they must move over an air gap before reaching the next pole. The rotor resists the turning, the magnets attempting to stay bonded to the current poles, until just over halfway through the air gap, where the attraction to the next pole is stronger than that of the previous pole, resulting in a "clunking" or jumping effect at low RPM (such as when trying to turn by hand). This is the cogging effect, resulting in a large torque required to begin rotation. The cogging effect is the attraction between the magnets and the stator poles when at rest; this attraction and cogging is easily felt when trying to turn an electric motor by hand. A common misconception is that cogging is caused by the copper coils; however, this is not the case. An unequal attraction between the magnets and the stator results in the rotors reluctance to turn. The magnets sit comfortably at rest over a pole, and when the rotor is turned, they must move over an air gap before reaching the next pole. The rotor resists the turning, the magnets attempting to stay bonded to the current poles, until just over halfway through the air gap, where the attraction to the next pole is stronger than that of the previous pole, resulting in a "clunking" or jumping effect at low RPM (such as when trying to turn by hand). This is the cogging effect, resulting in a large torque required to begin rotation.

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How do you prevent cogging in an induction motor?

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The cogging effect is the attraction between the magnets and the stator poles when at rest; this attraction and cogging is easily felt when trying to turn an electric motor by hand. The rotor resists the turning, the magnets attempting to stay bonded to the current poles, until just over halfway through the air gap, where the attraction to the next pole is stronger than that of the previous pole, resulting in a "clunking" or jumping effect at low Speed (such as when trying to turn by hand). This is the cogging effect, resulting in a large torque required to begin rotation.


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When the number of rotor and stator poles are equal in the motor and if these rotor poles have opposite polarity with respect to the stator poles magnetic locking occurs. This magnetic locking is know as "Cogging" in 3 phase squirrel cage induction motor. In order to avoid this problem, the rotor slots housing the conductors are skewed by an angle of 5 degrees over the surface.


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When rotor bars are made to run parallel with stator , the torque rises & falls correspondingly causing more pulsations. This is termed as cogging in other qords magnatic locking. This is reduced by making the rotor bars run at an angle to the stator ie crawling in order to make the torque uniform. Crawling on the other hand signifies running of motor at almost one seventh of the rated speed due to interference of seventh harmonics.


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