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When does an induction machine work as a generator? (N= Rotor Speed & Ns= Synchronous speed)
A) N=NsтАЛ
B) N>NsтАЛ
C) N<NsтАЛ
D) When we connect prime mover with the Induction machine.
N>NsтАЛ
An induction machine operates as an induction generator when its rotor is driven above the synchronous speed (N>NsтАЛ) in the same direction as the rotating magnetic field. Under this condition, the slip becomes negative (s<0), causing the direction of induced electromagnetic torque to reverse and mechanical energy to be converted into electrical energy.
An induction machine operates as an induction generator when its rotor is driven above the synchronous speed (N>NsтАЛ) in the same direction as the rotating magnetic field. Under this condition, the slip becomes negative (s<0), causing the direction of induced electromagnetic torque to reverse and mechanical energy to be converted into electrical energy.
s=NsтАЛNsтАЛтИТNтАЛ тАФ Slip of the induction machine
NsтАЛ=P120fтАЛ тАФ Synchronous speed in RPM
When the rotor speed N exceeds the synchronous speed NsтАЛ, the relative motion between the stator rotating magnetic field and the rotor conductors reverses direction. This reverses the direction of the induced rotor electromotive force (EMF) and current, thereby reversing the electromagnetic torque to act as a opposing braking torque. Consequently, the machine receives mechanical power from an external prime mover and delivers active electrical power back to the grid.
For N>NsтАЛ, slip s<0 (negative slip), signifying generator action.
An induction generator cannot supply its own reactive power; it must draw magnetizing lagging reactive power from an external AC source or a capacitor bank.
Simple and rugged construction (no brushes or commutator).
Does not require synchronization to the AC grid.
Requires reactive power from the grid or external capacitors to establish the magnetic field.
Cannot operate independently without a self-excitation capacitor bank.
Wind power generation plants.
Small-scale hydroelectric power stations.
Option A (N=NsтАЛ): The slip is zero (s=0), no rotor current is induced, and zero electromagnetic torque is developed.
Option C (N<NsтАЛ): The slip is positive (0<sтЙд1), and the machine operates in motoring mode.
B is correct тАФ Driving the rotor at a speed greater than synchronous speed (N>NsтАЛ) results in negative slip and causes the machine to function as an induction generator.
Always remember that slip s is positive for motoring (0<sтЙд1), negative for generating (s<0), and greater than 1 for braking/plugging (s>1).