Question

If the air gap of an induction motor is increased, then,

• A. Efficiency will increase
• B. Magnetizing current will increase
• C. Power factor will improve
• D. Speed will increase

Hint:

A small air gap is crucial for maintaining a high power factor in induction motors.
In contrast, synchronous motors can tolerate larger air gaps because they have a separate DC excitation source on the rotor to supply the magnetizing MMF.

Answer 1

The Correct Option is B

Step 1: Understanding the Question:
This question focuses on the design parameters of an induction motor, specifically the magnetic path length through the air gap between the stator and the rotor, and its electromagnetic consequences.

Step 2: Key Formula or Approach:
The magnetic reluctance ($R$) of the magnetic circuit is dominated by the air gap, as its permeability is much lower than that of the iron core:
\[ R = \frac{l_g}{\mu_0 A} \]
where $l_g$ is the air gap length.
The magnetizing current ($I_m$) required to establish the necessary flux ($\Phi$) is:
\[ I_m = \frac{\Phi R}{N} \]

Step 3: Detailed Explanation:

• The air gap of an induction motor is kept as small as mechanically possible to minimize the reluctance of the magnetic path.

• If the air gap ($l_g$) is increased, the overall reluctance ($R$) of the magnetic circuit increases significantly because the magnetic permeability of air ($\mu_0$) is very small compared to the magnetic permeability of the steel core.

• To establish the same amount of magnetic flux in the core to maintain the induced voltage, a higher magnetizing force (MMF) is required.

• Consequently, the magnetizing current ($I_m$) drawn from the stator supply must increase.

• A higher magnetizing current increases the reactive component of the stator current, which leads to a decrease in the operating power factor ($\cos\phi$) of the motor, especially at low-load or no-load conditions.

• Therefore, increasing the air gap leads to an increase in the magnetizing current, which degrades the power factor.

Step 4: Final Answer
Thus, increasing the air gap of an induction motor results in an increased magnetizing current.