(a) only 6-pole
(b) 6 or 12 pole
(c) any number of pair poles in an induction motor
(d) none of the above
Answer: (c) any number of pair poles in an induction motor
Explanation:
Detailed Solution
🔹 In a squirrel-cage induction motor, rotor poles are not physically constructed.
🔹 They are produced due to electromagnetic induction by the stator’s rotating magnetic field.
🔹 The rotor automatically develops poles corresponding to the stator poles.
🔹 Hence, the number of rotor poles always matches the stator poles.
🔹 This property allows the motor to operate with different pole configurations.
🔹 It makes squirrel-cage motors suitable for pole-changing and speed control methods.
👉 Therefore, the squirrel-cage rotor is highly versatile.
⚙️ Various Main Parts Related to Pole Formation are:
✔️ Stator Winding
✔️ Rotor Bars
✔️ Air Gap
✔️ Laminated Core
✔️ Magnetic Field
⚡ Stator (Pole Creation):
🔹 Stator windings are designed for a specific number of poles.
🔹 When AC supply is given, a rotating magnetic field is produced.
🔹 The number of poles depends on winding arrangement.
🔹 This magnetic field induces corresponding poles in the rotor.
🔄 Rotor (Pole Induction):
🔹 Rotor has no fixed poles.
🔹 Poles are formed by induced currents.
🔹 Always follows stator field.
🔹 Adjusts automatically for different pole numbers.
🔹 Works for 2, 4, 6, 8, 12 poles, etc.
❌ Why Other Options are Incorrect
(a) Only 6-pole ❌
🔹 Incorrect because the rotor is not restricted to a single pole number.
🔹 It adapts to the stator design.
(b) 6 or 12 pole ❌
🔹 Too restrictive.
🔹 The rotor can work with any number of stator poles.
(d) None of the above ❌
🔹 Incorrect because option (c) is correct.
📌 Key Exam Notes
⭐ Rotor Poles → Not Fixed
⭐ Pole Formation → By Induction
⭐ Depends On → Stator Winding
⭐ Speed Control → Possible by Pole Changing
⭐ Flexibility → High
📝 Final Summary
📍 In a squirrel-cage induction motor:
✔️ Rotor poles are not physically built
✔️ They are induced by stator field
✔️ Rotor matches stator poles automatically
✔️ Enables flexible operation
👉 Hence, Option C is Correct.