The power factor & Overload capacity of the machine decrease with ____________

Question

Examoogle · Accepted Answer

Option A: increase in magnetizing current

Explanation & Concepts:
The power factor and overload capacity of an induction motor are inversely related to the magnetizing current ($I_m$). An increase in magnetizing current implies a weaker magnetic circuit design or a larger air gap, which necessitates higher reactive power consumption, thereby lowering the power factor and reducing the torque capability margin.

Working Principle: The total stator current is $I_s = I_1 + jI_m$. An increase in $I_m$ increases the reactive component of the current, reducing the displacement power factor $\cos \phi$. Since the maximum torque (overload capacity) of an induction motor is inversely proportional to the leakage reactance and influenced by the excitation requirements, a higher $I_m$ effectively reduces the breakdown torque margin.

Key Formulas: $P.F. = \cos(\phi) = \frac{I_p}{I_s}$ — where $I_p$ is active current and $I_s$ is stator current; $T_{max} \propto \frac{V^2}{2\omega_s(R_2 + \sqrt{R_2^2 + X_{eq}^2})}$ — showing inverse sensitivity to reactance influenced by $I_m$

Key Points: Magnetizing current is determined by the air gap length and magnetic circuit reluctance.; A larger air gap (to improve mechanical robustness) increases $I_m$, leading to lower efficiency and power factor.; Induction motors are typically designed with the smallest air gap possible to minimize $I_m$.; Overload capacity (Pull-out torque) is directly linked to the machine's flux capability; excessive $I_m$ consumes flux linkage capacity.

Advantages: Smaller air gap improves Power Factor; Optimized magnetic path increases peak torque

Disadvantages: Very small air gaps increase mechanical maintenance requirements; High magnetizing current leads to increased $I^2R$ losses in stator windings

Applications: Industrial motor design; Power system stability analysis

Additional Info: Option B is incorrect because a decrease in magnetizing current would actually improve the power factor by reducing reactive power demand.. Option D is incorrect as smaller machines often have relatively higher magnetizing current requirements due to manufacturing tolerances and air gap ratios.

Answer: A is correct — An increase in magnetizing current consumes more reactive power and reduces the magnetic flux margin, directly degrading both power factor and overload capability.

Exam Tip: Always remember: Anything that increases the reactive component of the stator current (like large air gaps or low frequency) will negatively impact the power factor.

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Option B: decrease in magnetizing current

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Option C: increase in machine size

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Option D: decrease in machine size

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