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Examoogle • User • info@examoogle.com • EE-2024-8821
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When does an induction machine work as a generator? (N= Rotor Speed & Ns= Synchronous speed)
A) N=Ns
B) N>Ns
C) N<Ns
D) When we connect prime mover with the Induction machine.
N>Ns
An induction machine operates as an induction generator when its rotor is driven above the synchronous speed (N>Ns) 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>Ns) 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=NsNs−N — Slip of the induction machine
Ns=P120f — Synchronous speed in RPM
When the rotor speed N exceeds the synchronous speed Ns, 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>Ns, 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=Ns): The slip is zero (s=0), no rotor current is induced, and zero electromagnetic torque is developed.
Option C (N<Ns): The slip is positive (0<s≤1), and the machine operates in motoring mode.
B is correct — Driving the rotor at a speed greater than synchronous speed (N>Ns) results in negative slip and causes the machine to function as an induction generator.
Always remember that slip s is positive for motoring (0<s≤1), negative for generating (s<0), and greater than 1 for braking/plugging (s>1).