The rotor slots, in an induction motor, are usually not quite parallel to the shaft because
Right Answer is:
Reducing the tendency of the rotor teeth to remain under the stator teeth
The rotor bars are not exactly parallel to the shaft but are given a slight skew. This skew helps to make the motor run quietly by reducing the magnetic hum. Another advantage is that this skew reduces the locking tendency of the rotor.
Rotor conductors are skewed because of the following main reasons
- Cogging:- Cogging is magnetic locking. When an induction motor refuses to start even if the full voltage is applied to it, this is called as cogging. This happens when the rotor slots and stator slots are the same in number or they are integer multiples of each other. due to this the opposite poles of the stator and rotor come in front of each other and get locked. Skewing the rotor bars prevents the locking thus preventing Cogging.
- Starting torque of an induction motor depends on the product of the magnitude of stator and rotor current and sine of the angle between both.
- If the conductors remain linear, the angle between stator and rotor current will be 180 degrees. As sin(180) = 0, the starting resultant torque will be zero and thus motor will fail to start. This phenomenon is called cogging.
- Skewing helps the motor run more quietly because the magnetic fields are slightly skewed to offset alignment with the rotor field coils. This feature tends to reduce vibration or magnetic hum as the rotor speed changes slightly every time the conductor bars align with the rotor magnetic field. As the speed is the function of frequency so the induction motor is unable to attend the resonance frequency, therefore, the magnetic vibration is reduced.
- Skewing makes the rotor conductor longer with the reduced cross area. This increases the rotor conductor resistance hence starting performance and the torque of an induction motor are improved.
Crawling:– Crawling is a phenomenon where harmonic components introduce oscillations in torque. With the bar skewed, the amount of the bar cutting the field line grows continuously and the next bar starts cutting the field lines as the first finishes. Due to this, we get Uniform Torque.