What is the direction of rotation of rotor with respect to the rotating magnetic field?
Answer: Rotating Magnetic Field is set up when 3 phase ac supply is provided to the stator windings of the induction motor. Rotating Magentic Field rotates at synchronous speed. Rotor rotates in the direction of the rotating magnetic field
What is Slip In Induction Motor?
Answer: Speed of the Induction Motor will be always less than synchronous speed and speed of the motor further reduces when load is applied on the motor. The difference between the speed of the stator (synchronous speed Ns) and the actual speed of the motor (N) is called slip of the induction motor and is designated by letter 's'. It is represented in RPM. However it is general tendency to represent as fraction or percentage of synchronous speed.
Fractional Slip s = Ns- N
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Ns
What is the speed of the rotor magnetic field with respect to rotor?
Answer: Currents flowing through the rotor windings due to induction caused by stator will have a frequency of ( f ', which is equal to sf where f is the applied frequency to stator). This current flowing through the rotor sets up a magentic field equal to s x Ns with respect to the rotor.
What is the relative speed between the fields set up by the stator and rotor?
Answer: Stationary. The speed of the stator field with respect to stator surface will be Ns and the speed of rotor with respect to the stator surface will be N. The speed of the magentic field of the rotor with respect to the rotor will be s x Ns . Therefore the speed of the rotor field with respect to stator surface is equal to rotor speed of the motor and rotor field with respect to rotor
N + s x Ns = Ns ( 1-s ) + s x Ns = Ns
Therefore the relative motion between stator field and rotor field will be stationary or zero.
What happens when Induction motor is loaded?
Answer: During no load operation the relative speed between stator magnetic field and rotor speed will be less (means slip is less). Hence small emf is generated which generates small currents and torque will be generated in such a manner sufficient to overcome the friction and windage losses.
When motor is loaded (Load applied to rotor), motor torque reduces as the load torque increases. Hence motor slows down (slip increases). As the motor slows down, the relative speed between rotor and stator magnetic field increases. This results in greater motor currents and generates greater motor torque to overcome the load torque. Thus as the load on the induction motor increases, motor slows down until sufficient torque is generated to overcome the applied load torque.
2 comments:
how the torque can be generated if stator and rotor run at same frequency
Rotor frequency=slip×stator frequency
So,if frequency of rotor and stator is same slip will be unity hence rotor will be at standstill.
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