Crawling and Cogging

Effect of harmonics
  • The speed of harmonic torque is 1/n th speed of the fundamental torque ( where n is order of harmonic ).
  • As the third harmonic remain absent under balance load condition, it does not produce any torque.
  • The fifth harmonic rotates at speed of Ns / 5 and rotates in the anticlockwise direction ( 120 × 5 = 600o = 600o – 720o = – 120o ). The 5th harmonic produces small torque in the reverse direction but it is neglected.
  • The seventh harmonic rotates at speed of Ns / 7 and rotates in the clockwise direction ( 120 × 7 = 840o = 840o – 720o = 120o ). 
  • If we neglect all higher order harmonic, total torque is sum of fundamental torque and seven harmonic torque. 
  • The seventh harmonic torque reaches its maximum positive value just before 1 / 7th synchronous speed beyond which it becomes negative. 
  • If the total load torque assume remain constant, the resultant torque may fall below load torque. 
  • The motor in this condition run at normal speed, but runs at 1/7th synchronous speed.
  • Sometimes the squirrel cage induction motor refuses to start particular at low voltage, it is known as crawling effect. 
  • When the number of stator teeth is equal to number of rotor teeth, there is possible locking tendency of the stator and rotor teeth.  
  • This is happens due to fact that when stator teeth and rotor teeth face to other, the magnetic path becomes minimum. 
  • The rotor tends to remain fixed in this condition and causes serious problem in starting.
Overcome Effect of Cogging
  • The problem of Cogging is overcome by making the number of rotor slots prime to the number of stator slots.
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