With the increase in the number of poles - pole pitch and slot per pole per phase of three-phase induction mot

Question

With the increase in the number of poles - pole pitch and slot per pole per phase of three-phase induction motor will __________.

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The correct answer is Option B: decrease. With an increase in the number of poles (P) for a constant stator circumference, the pole pitch decreases because it is defined as the peripheral distance between two adjacent poles. Consequently, since the total number of slots is fixed, the number of slots per pole per phase (m) also decreases as the number of poles increases.

Step-by-Step Solution & Concepts:
With an increase in the number of poles (P) for a constant stator circumference, the pole pitch decreases because it is defined as the peripheral distance between two adjacent poles. Consequently, since the total number of slots is fixed, the number of slots per pole per phase (m) also decreases as the number of poles increases.

Working Principle: The pole pitch is defined as the ratio of total stator slots to the number of poles. As the denominator (number of poles) increases, the value of the pole pitch decreases. Similarly, $m = \frac{S}{P 	imes \phi}$, where S is the number of slots, P is the number of poles, and \phi is the number of phases; thus, m is inversely proportional to P.

Key Formulas: $Pole \, Pitch = \frac{Total \, Slots}{P}$; $m = \frac{S}{P 	imes 3}$ — where m is slots per pole per phase, S is total slots, and P is number of poles

Key Points: Pole pitch is the distance between centers of two adjacent poles measured in terms of slots.; Higher pole count reduces the physical size of each pole span.; Increasing the number of poles leads to lower synchronous speed ($N_s = \frac{120f}{P}$).

Advantages: Lower number of poles results in higher synchronous speed for a given frequency.; Higher number of poles provides higher torque capabilities at lower speeds.

Disadvantages: Increased number of poles leads to a smaller slot pitch, making winding installation more complex.; High pole counts can lead to increased magnetic leakage.

Applications: High-speed applications like turbo-alternators use 2 or 4 poles.; Low-speed applications like hydro-generators use high pole counts.

Additional Info: Standard constraints: The total number of stator slots is usually determined by the frame size and manufacturing limitations.. Option A is incorrect because pole pitch is inversely proportional to the number of poles.

Answer: B is correct — Both pole pitch and slots per pole per phase are inversely proportional to the number of poles in an induction motor.

Exam Tip: Remember the inverse relationship: $P \uparrow \implies N_s \downarrow$ and $P \uparrow \implies m \downarrow$, which helps in solving both winding and speed-related motor questions.

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Option A: increase

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Option C: remains the same

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Option D: cant predict from this data only

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