01․ An induction motor has a rotor resistance of 0.002 Ω / phase. If the resistance is increased to 0.004 Ω / phase then the maximum torque will
Maximum torque the maximum torque is independent of rotor resistance and is inversely proportional to rotor reactance. So, if we increase the rotor resistance the maximum torque will be unaltered.
02․ The power factor of squirrel cage induction motor is
The operating power factor on no load or light loads is very low, because of magnetizing current. The losses on no load are only core losses which are negligible. Therefore magnetizing current is greater than core loss component current and the no load power factor is very low around 0.2 lagging. As the load increases the core loss component current will increase and the power factor will improve.
03․ Cogging of induction motor occurs at
Starting Torque ∝ V2. When the supply voltage is low, the starting torque is low and when the poles on stator and rotor are equal, due to locking tendency caused by the slot harmonics motor fails to start.
04․ Cogging and crawling are phenomena associated with
- Cogging : When the number of poles on stator and rotor are equal or integral multiple of each other and quite horizontal to each other, a strong alignment force occurs. Due to this locking tendency the rotor refuses to start which is called cogging.
- Crawling : Induction motor always run nearer to synchronous speed but this is a peculiar behaviour when the motor runs at a speed near to Ns/7 stably supplying rated torque due to harmonic currents.
05․ In a single phase induction motor, the starting torque developed is proportional to
Therefore the torque developed is proportional to V².
06․ A 8 pole, 50 Hz single phase induction motor is running at 690 rpm. Its slip with respect to forward field is
07․ Cogging in induction motor occurs when
Squirrel cage rotors are not designed for any number of poles on the rotor. As the poles are automatically formed in equal number, due to low starting if the rotor poles and stator poles are quite horizontal to each other or they are even multiple to each other, a strong alignment force occurs. Due to this locking tendency the rotor refuses to start which is called as cogging. Slip ring rotor design is quite complicated compared to squirrel cage with respect to cogging effect. The number of poles on the rotor should not be equal to stator poles. To achieve this rotor slots are always less than stator slots. The rotor poles also should not be in integral multiples of stator slots.
08․ Capacitor in split phase induction motor is used to
There is capacitor in the starting winding connected through a centrifugal switch. When stator is supplied with 1-phase supply the stator current, main winding current Im and starting winding current Is has a phase difference as near to 90o and the motor is a perfect two phase motor. This produces a rotating magnetic field and high starting torque with low starting current in the starting winding.
09․ A shaded pole single phase induction motor can be used for
In shaded pole induction motor, shaded coil will produce a very weak rotating magnetic field and also not at constant magnitude. Therefore these are small ratings starting from 20 watts. Applications: Toys, hair dryers, desk fans, cooling fans, slide projectors, advertising displays etc. of small ratings.
10․ A 4 pole 50 Hz induction motor is running at 1470 rpm. Find the speed of stator magnetic field with respect to rotor?
The relative speed of stator magnetic field with respect to rotor is = Ns - Nr