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No-load current in three-phase induction motor is Very┬а __________ due to presence of an _________.
Large, Core Less
Small, Air-gap
Large, Air-gap
Small, Core Less
Large, Air-gap
The no-load current in a three-phase induction motor is significantly higher than in a transformer of equivalent rating. This is primarily due to the existence of an air-gap between the stator and the rotor, which requires a high magnetizing current to establish the necessary rotating magnetic field.
The no-load current in a three-phase induction motor is significantly higher than in a transformer of equivalent rating. This is primarily due to the existence of an air-gap between the stator and the rotor, which requires a high magnetizing current to establish the necessary rotating magnetic field.
I0тАЛтЙИImтАЛ
ImтАЛ=2╧АfLmтАЛVтАЛ
In an induction motor, the magnetic circuit consists of high-permeability iron cores and a low-permeability air-gap. The reluctance of the air-gap is much higher than that of the iron core, necessitating a large magnetizing current (ImтАЛ) to maintain the flux (╬ж). Since ImтАЛ=RFтАЛ, where R is dominated by the air-gap reluctance, the total no-load current (I0тАЛ=ImтАЛ+IwтАЛ) becomes quite substantial, typically 30% to 50% of the full-load current.
The air-gap between stator and rotor acts as a high reluctance path.
Magnetizing current is inversely proportional to the reluctance of the magnetic circuit.
High no-load current leads to a low power factor at light loads in induction motors.
Induction motors typically draw 30-50% of rated current at no-load.
Robust construction
Self-starting capability (in polyphase motors)
Low power factor at no-load
High magnetizing current requirement
Industrial pumps
Conveyor systems
Compressors
In transformers, the air-gap is essentially zero, hence the no-load current is only 2-5% of full-load current.
Option B is incorrect because a small air-gap would actually reduce the magnetizing current, but the air-gap is physically necessary for the rotor to rotate.
C is correct тАФ The no-load current is large because the high reluctance of the air-gap requires a significant magnetizing current to establish the rotating magnetic field.
Always remember: higher air-gap reluctance means higher magnetizing current, which directly impacts the motor's power factor at no-load.