Basic principle Induction motor:
In induction motor ac current is fed in to the stator directly but no current is supplied to the rotor externally. As the stator is fed by ac, varying magnetic flux generated by the stator induce current in the rotor.Now by Lenz’s law a current carrying conductor will be subjected to a force in a magnetic field. As in induction motor rotor carries current and subject to the magnetic field of the stator, a force is exerted on the rotor. By some means this force is harness as rotational force.
[From Fitzgerald] In induction machines, alternating currents are applied directly to the stator winding. Rotor currents are then produced by induction, i.e., transformer action. The induction machine may be regarded as a generalized transformer in which electric power is transformed between rotor and stator together with a change of frequency and a flow of mechanical power.
How rotor of induction motor rotates:
[Lenz’s law] A current carrying conductor will experience a force when placed in a magnetic field. The force will such that, it will try to cancel out its cause of creation.
In induction motor the stator is standstill, but the varying ac current fed in to the special phase displaced stator coil creates a rotating magnetic field in the stator. (How rotating magnetic field is created in induction motor stator)
Slip, rotor current frequency of induction motor:
The difference between stator magnetic field synchronous speed and the rotor speed is commonly referred to as the slip of the rotor; in this case the rotor slip is (ns – n) r/min.Slip is more usually expressed [alvaa]as a fraction of synchronous speed. s=(ns-n)/n. From it n=(1 – s)ns. The relative motion of the stator flux and the rotor conductors induces voltages of frequency fr,fr = Sfe,
fr= frequency of rotor induced current (Slip frequency), fe= line frequency, s=slip.
Frequency of rotor current of induction motor:
Q.The frequency of the rotor current in a 3 phase 50 Hz, 4 pole induction motor at full
load speed is about
(A) 50 Hz. (B) 20 Hz.(C) 2 Hz. (D) Zero.
Ans: C, But Why?
This is the reason –
We first calculate the slip. For low sized motor (ns-n) is assumed to be 2. With s=(ns-n)/n; s=1/25. Then with fr=sfe, fr=(1/25)*50=2