If all the stator coils of an induction motor are connected for the same magnetic polarity, there will be formed an equal number of

(a) rotor poles with same polarity
(b) rotor poles with opposite polarity
(c) consequent poles with opposite polarity
(d) consequent poles with same polarity

Answer: (c) consequent poles with opposite polarity

Explanation:

CONCEPT: Magnetic Poles in Induction Motors

• When current flows through stator windings, it produces a magnetic field.
• This magnetic field creates North (N) and South (S) poles.
• Normally, stator windings are arranged to produce alternate N and S poles.

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WHAT ARE CONSEQUENT POLES?

👉 Consequent poles are poles that are not directly produced by a winding, but are formed due to magnetic flux distribution.

They appear automatically because magnetic flux must complete a closed path.

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PRINCIPLE OF CONSEQUENT POLES

Magnetic flux always flows in closed loops.

So:

• If all coils produce the same polarity,
• The flux must return through adjacent regions,
• This creates opposite polarity poles automatically.

These automatically formed poles are called consequent poles.

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WORKING EXPLANATION

Case: All Coils Produce Same Polarity (Say North)

1. Suppose all stator coils are connected to produce North poles.
2. Flux emerges from all these North poles.
3. Flux must return to complete its path.
4. Therefore, in between these North poles:

• South poles are formed automatically.

1. These South poles are not wound directly.
2. They are formed by magnetic action.

👉 These are called consequent poles.

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IMPORTANT RESULT ⭐

If all coils are connected for the same polarity, then:

✔ An equal number of opposite polarity poles are formed.

So:

Number of North poles = Number of South poles

Hence:

👉 Equal number of consequent poles with opposite polarity are formed.

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APPLICATION: Pole-Changing Motors ⭐

This principle is used in:

Pole-Changing Induction Motors

1. Used for speed control.
2. By changing coil connections:

• Number of poles can be changed.
• Speed changes automatically.

Formula:

$${\mathrm{<span\; style="font-size:\; medium;"><span\; style="font-family:\; Source\; Serif\; Pro;">N</span></span>}}_{\mathrm{<span\; style="font-size:\; medium;"><span\; style="font-family:\; Source\; Serif\; Pro;">s</span></span>}}<span\; style="font-size:\; medium;"><span\; style="font-family:\; Source\; Serif\; Pro;">=</span></span>\frac{\mathrm{<span\; style="font-size:\; medium;"><span\; style="font-family:\; Source\; Serif\; Pro;">120</span></span>}\mathrm{<span\; style="font-size:\; medium;"><span\; style="font-family:\; Source\; Serif\; Pro;">f</span></span>}}{\mathrm{<span\; style="font-size:\; medium;"><span\; style="font-family:\; Source\; Serif\; Pro;">P</span></span>}}$$

Where:

• Ns = Synchronous speed
• f = Supply frequency
• P = Number of poles

Changing poles → Changes speed.

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EFFECT ON MOTOR PERFORMANCE

Using consequent poles:

✔ Changes number of poles
✔ Changes synchronous speed
✔ Enables multi-speed operation
✔ Improves flexibility of motor design

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WHY OTHER OPTIONS ARE INCORRECT

(a) Same Polarity Only ❌

Magnetic flux cannot exist without return path. Opposite poles must form.

(b) No Poles ❌

Poles must exist for magnetic field.

(d) Random Poles ❌

Poles form in a systematic pattern, not randomly.

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FINAL CONCLUSION ⭐

When all stator coils are connected for the same magnetic polarity, an equal number of consequent poles with opposite polarity are formed automatically.

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KEY ONE-LINE EXAM STATEMENT ⭐

When all stator coils produce the same polarity, an equal number of consequent poles of opposite polarity are formed.

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