The popular rotor used today in cage type induction motor, are 'die cast rotor' where skewed rotor slots are filled with molten aluminum along with end rings and fins for cooling, and are popular as there is no joints, economical to manufacture, robust in use. The rotor slots are skewed (not parallel to stator slot(but under the influence of the field of one/two stator slots) or the shaft) hence the magnetic field of poles, of rotating magnetic field comes in contact of rotor slot bar with, uniform rate of cutting of field and leaves, producing uniform current(rotor field), responsible to produce uniform torque, which make the motor (1) run quietly without the magnetic hum (2) reduces the locking tendency of the rotor( as rotor slot are not parallel to stator slot).
1 The intuitive idea follows from the need to eliminate "cogging" torque. This torque
arises due to saliency of the rotor and stator teeth . It can be thought of as the torque due to stator and rotor teeth which when straight and below each other attract ,
much like two magnets over each other.So if we slide one over the other we experience
a restoring force ,like a spring. Further we can resolve the force into tangential and normal . The normal force is like a periodic compressive force on the stampings , which
gives rise to vibration . Now if we skew one magnet at the center both the
problems are eliminated as on average over length of the rotor, the effects cancel .
2. The actual design calculation will depend on ratio of Nstator/ Nrotor , type of slot designs (full closed/semi closed / open) and effect of saturation in the teeth
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