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Induction Motor is known as Rotating transformer with an ______________ and ______________.
air gap & short-circuited secondary
air gap & Open-circuited secondary
Core Less & short-circuited secondary
Core Less & Open-circuited secondary
air gap & short-circuited secondary
An induction motor operates on the principle of electromagnetic induction, similar to a transformer, but with a moving secondary part. The stator acts as the primary winding, while the rotor acts as a short-circuited secondary winding separated by an air gap, allowing for relative motion.
An induction motor operates on the principle of electromagnetic induction, similar to a transformer, but with a moving secondary part. The stator acts as the primary winding, while the rotor acts as a short-circuited secondary winding separated by an air gap, allowing for relative motion.
s=NsтАЛNsтАЛтИТNrтАЛтАЛ тАФ Slip of the induction motor
frтАЛ=sтЛЕfsтАЛ тАФ Frequency of rotor induced EMF
When a 3-phase supply is fed to the stator, it produces a Rotating Magnetic Field (RMF). This RMF cuts the rotor conductors, inducing an EMF by Faraday's law of electromagnetic induction. Since the rotor circuit is short-circuited (either via end rings or external resistors), current flows through it, producing a torque that drives the rotor to follow the RMF, thus bridging the gap between stationary input and rotating output.
The air gap is essential for mechanical rotation, though it increases the magnetizing current.
The secondary (rotor) is always short-circuited to ensure circulating current flow for torque generation.
Induction motors are effectively generalized transformers where the secondary frequency varies with speed.
Rugged and simple construction
Self-starting capability (in polyphase)
Low maintenance requirements
Low starting torque compared to DC motors
Power factor drops significantly at light loads
Industrial pumps and compressors
Conveyor belts and HVAC fans
The air gap in an induction motor is kept as small as possible to minimize the magnetizing current (ImтАЛ) and improve the power factor.
Option B is incorrect because an open-circuited secondary would result in no rotor current and hence zero torque.
A is correct тАФ Induction motors require an air gap for rotation and a short-circuited secondary to allow induced currents to create electromagnetic torque.
Always remember that the rotor frequency (frтАЛ) is directly proportional to slip (s); at standstill, frтАЛ=fsтАЛ, making it a stationary transformer at the exact instant of start.