(a) rotor conductors are short-circuited through end rings
(b) rotor conductor ends are short-circuited through slip rings
(c) rotor conductors are kept open
(d) none of the above
Answer: (a) rotor conductors are short-circuited through end rings
Explanation:
Detailed Solution
🔹 The rotor consists of copper or aluminium bars embedded in a laminated steel core.
🔹 These bars are permanently short-circuited at both ends by heavy conducting end rings.
🔹 This arrangement forms a closed electrical circuit.
🔹 Due to this cage-like appearance, it is called a squirrel cage rotor.
🔹 When the stator produces a rotating magnetic field, EMF is induced in the rotor bars.
🔹 This EMF causes current to flow through the closed rotor circuit.
🔹 Interaction between stator flux and rotor current produces electromagnetic torque.
🔹 The rotor rotates and drives the output shaft.
👉 Thus, motion is produced by the principle of electromagnetic induction.
⚙️ Various Main Parts of an Induction Motor are:
✔️ Stator
✔️ Rotor
✔️ End Rings
✔️ Shaft
✔️ Bearings
✔️ Cooling Fan
⚡ Stator:
🔹 The stator is the stationary part of the motor.
🔹 It consists of laminated steel core and three-phase windings.
🔹 Windings are placed in slots and connected to AC supply.
🔹 It produces a rotating magnetic field.
🔹 The outer housing is made of laminated metal sheets with wire coiling.
🔄 Rotor (Squirrel Cage Type):
🔹 Made of laminated steel core.
🔹 Contains aluminium/copper bars.
🔹 Bars are short-circuited by end rings.
🔹 Forms a closed circuit.
🔹 Has no slip rings or brushes.
🔹 Connected to the output shaft.
🔹 Looks like a cage structure.
🚫 Why External Resistance Cannot Be Added?
🔹 Rotor bars are permanently short-circuited.
🔹 No external terminals are available.
🔹 No slip rings are provided.
🔹 Hence, external resistance cannot be connected.
🔹 Speed control is limited in squirrel cage motors.
👉 Therefore, speed control is difficult in squirrel cage induction motors.
📌 Additional Information
⭐ In slip ring (wound rotor) induction motors:
🔹 External resistance is added using a star-connected rheostat.
🔹 This increases starting torque.
🔹 As speed increases, resistance is gradually reduced.
🔹 This provides better speed and torque control.
⭐ In squirrel cage motors:
🔹 No external resistance
🔹 Simple construction
🔹 Low maintenance
🔹 High reliability
📝 Final Summary
📍 A squirrel cage induction motor is called so because:
✔️ Rotor bars are short-circuited by end rings
✔️ Structure looks like a cage
✔️ Works on electromagnetic induction
✔️ No external resistance can be added
👉 Hence, Option a is Correct.