Which of the following statements associated with polyphase induction motors is/are correct?

(a) It can run in one direction only
(b) It is simply a transformer whose magnetic circuit is separated by an air gap into two relatively movable portions, one carrying the primary and other the secondary winding
(c) It needs more maintenance than that required by a dc shunt motor
(d) It is more costly than a dc shunt motor of the same output rating

Answer: (b) It is simply a transformer whose magnetic circuit is separated by an air gap into two relatively movable portions, one carrying the primary and other the secondary winding

Explanation:

CONCEPT: Working Principle of Polyphase Induction Motor

• A polyphase induction motor works on the principle of electromagnetic induction.

• It operates similar to a transformer:

• Stator → Primary winding (stationary)

• Rotor → Secondary winding (rotating and short-circuited)

• The stator is supplied with polyphase AC (usually three-phase).

• This produces a rotating magnetic field (RMF).

• The rotating field cuts rotor conductors.

• According to Faraday’s Law:

• EMF is induced in rotor.

• Rotor current flows.

• Interaction of stator flux and rotor current produces torque.

• Rotor starts rotating in the same direction as RMF.

Thus:

👉 Induction motor works like a transformer with a rotating secondary.

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ROLE OF AIR GAP

• An air gap exists between stator and rotor.
• It allows free rotation of the rotor.
• It forms part of the magnetic circuit.

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WHY OTHER OPTIONS ARE INCORRECT

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(a) Direction Cannot Be Reversed ❌

• Direction depends on phase sequence.
• By interchanging any two supply lines:

1. Phase sequence changes.
2. Direction reverses.

Hence, this statement is wrong.

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(c) Requires More Maintenance ❌

Induction motors (especially squirrel cage):

• Have no brushes.
• Have no commutator.

Therefore, maintenance is minimum.

But DC shunt motors need:

• Brush replacement
• Commutator maintenance

So this option is incorrect.

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(d) Higher Initial Cost ❌

Induction motors have:

• Simple construction
• No commutator
• Fewer parts

Hence, cost is low.

DC motors are costlier due to:

• Commutator
• Brushes
• Complex construction

So this option is incorrect.

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IMPORTANT FEATURES OF POLYPHASE INDUCTION MOTOR ⭐

Construction

Stator

• Contains polyphase windings.
• Supplied with AC.
• Produces rotating magnetic field.

Rotor

Two types:

• Squirrel cage rotor
• Wound rotor

Rotor is not electrically connected to supply.

Current is induced.

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TYPES OF ROTORS

1. Squirrel Cage Rotor

• Bars short-circuited by end rings.
• No slip rings or brushes.
• Very rugged.
• Maintenance-free.

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2. Wound Rotor

• Has rotor windings.
• Connected to slip rings.
• External resistance can be added.
• Used for high starting torque.

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TORQUE PRODUCTION

• Stator RMF induces rotor current.
• Rotor current produces magnetic field.
• Interaction of two fields creates torque.
• Rotor follows RMF but never reaches synchronous speed.

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IMPORTANT FORMULAS ⭐

Synchronous Speed

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

Where:
Ns = synchronous speed (rpm)
f = frequency (Hz)
P = number of poles

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Slip

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

Where:
Nr = rotor speed

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WHY IT IS CALLED ASYNCHRONOUS MOTOR

• Rotor speed is always less than synchronous speed.
• Rotor never runs at Ns.
• Therefore, it is called asynchronous motor.

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ADVANTAGES ⭐

✔ Self-starting
✔ Rugged construction
✔ Low maintenance
✔ Low cost
✔ High efficiency
✔ Long life

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APPLICATIONS ⭐

Used in:

✔ Pumps
✔ Fans
✔ Conveyors
✔ Compressors
✔ Lifts
✔ Machine tools

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SPEED CONTROL

• Speed is controlled using VFD (Variable Frequency Drive).
• VFD changes supply frequency.
• Maintains proper slip.

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FINAL CONCLUSION ⭐

A polyphase induction motor works on the principle of electromagnetic induction, similar to a transformer, in which the stator acts as the primary and the rotor acts as the rotating secondary. Hence, statement (b) is correct.

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KEY ONE-LINE EXAM STATEMENT ⭐

A polyphase induction motor works on the principle of electromagnetic induction, with the stator as primary and the rotor as rotating secondary.