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What is the permissible limit of tooth flux density (Wb/m┬▓) in the design of three-phase induction motor?
1.4
1.8
1.7
7
7
In the design of a three-phase induction motor, the flux density in the stator and rotor teeth is limited to approximately 1.7 Wb/m┬▓. This limit is imposed to prevent excessive magnetic saturation in the iron core, which would otherwise result in high magnetizing current and increased core losses.
In the design of a three-phase induction motor, the flux density in the stator and rotor teeth is limited to approximately 1.7 Wb/m┬▓. This limit is imposed to prevent excessive magnetic saturation in the iron core, which would otherwise result in high magnetizing current and increased core losses.
BtтАЛ=AtтАЛ╬жtтАЛтАЛ тАФ where BtтАЛ is the tooth flux density, ╬жtтАЛ is the flux per tooth, and AtтАЛ is the net cross-sectional area of the tooth.
ImтАЛтИЭ╬╝rтАЛ1тАЛ тАФ where the magnetizing current ImтАЛ increases as relative permeability ╬╝rтАЛ drops due to saturation.
The flux density in the teeth depends on the slot dimensions and the total flux per pole. If the flux density exceeds 1.7-1.8 Wb/m┬▓, the iron enters the knee of the magnetization curve, leading to a non-linear increase in the reluctance of the magnetic circuit. This causes the magnetizing current (ImтАЛ) to increase significantly, reducing the power factor and increasing the iron losses (hysteresis and eddy current).
High flux density in teeth leads to magnetic saturation.
Saturation causes the magnetizing current to increase, lowering the power factor.
The permissible limit is a compromise between motor size and electromagnetic performance.
Operating at 1.7 Wb/m┬▓ ensures the motor remains in the near-linear region of the B-H curve.
Improved power factor
Reduced iron losses
Lower magnetizing current requirements
Increased motor size if flux density is kept too low
Increased cost due to more magnetic material requirement
Industrial Induction Motor Design
Transformer core design optimization
For silicon steel laminations used in induction motors, 1.6 to 1.8 Wb/m┬▓ is the standard industry design range for teeth.
Option A (1.4) is generally considered too low (results in a bulky motor), while Option B (1.8) is near the upper thermal/magnetic limit but 1.7 is the widely accepted design standard for conservative sizing.
C is correct тАФ The standard permissible limit for tooth flux density in a three-phase induction motor is maintained up to 1.7 Wb/m┬▓ to balance performance and size.
In machine design, always relate flux density limits to the B-H curve saturation point; exceeding these values drastically increases the required Ampere-turns.