1️⃣ Presence of slip rings
2️⃣ Size of frame
3️⃣ Shaft diameter
4️⃣ Any of the above
✅ Answer (Detailed Solution Below)
✔️ Option 1: Presence of slip rings
📖 Detailed Solution
🔹 In a wound rotor induction motor, the rotor has three-phase windings.
🔹 These windings are generally star-connected.
🔹 The ends of the windings are connected to three slip rings.
🔹 Carbon brushes rest on these slip rings.
🔹 External resistance can be connected through brushes.
🔹 This improves starting torque and controls starting current.
🔹 In a squirrel-cage motor, the rotor has aluminium/copper bars.
🔹 Bars are short-circuited by end rings.
🔹 No slip rings or brushes are present.
🔹 External resistance cannot be added.
👉 Therefore, the main distinguishing feature is the presence of slip rings.
⚙️ Main Types of Rotors in 3-Phase Induction Motor
✔️ Squirrel Cage Rotor
✔️ Wound Rotor (Slip Ring Rotor)
✔️ Double Cage Rotor
🔄 Wound Rotor (Slip Ring Rotor)
🔹 Laminated cylindrical core.
🔹 Three-phase star-connected winding.
🔹 Three slip rings mounted on shaft.
🔹 Brushes connected to external resistance.
🔹 High starting torque.
🔹 Starting current is reduced.
🔹 Resistance is cut out gradually.
🔹 At normal speed, brushes are shorted.
🔹 Motor runs like squirrel cage.
🐿️ Squirrel Cage Rotor
🔹 Laminated cylindrical core.
🔹 Parallel slots on outer surface.
🔹 Copper/aluminium bars in slots.
🔹 Bars short-circuited by end rings.
🔹 Rotor is permanently shorted.
🔹 No electrical connection to supply.
🔹 Current induced by transformer action.
🔹 Simple and rugged construction.
🔹 Low starting torque.
❌ Why Other Options are Incorrect
(2) Size of Frame ❌
🔹 Frame size depends on power rating.
🔹 Not related to rotor type.
(3) Shaft Diameter ❌
🔹 Depends on mechanical load.
🔹 Not used for identification.
(4) Any of the Above ❌
🔹 Only slip rings clearly distinguish the motors.
📌 Additional Information
⭐ Wound rotor → Slip rings + Brushes
⭐ Squirrel cage → Bars + End rings
⭐ External resistance → Only in wound rotor
⭐ Starting torque → High in wound rotor
📌 Key Exam Notes
⭐ Distinguishing feature → Slip rings
⭐ Wound rotor → Speed control possible
⭐ Squirrel cage → Maintenance free
⭐ Most common → Squirrel cage motor
📝 Final Summary
📍 A wound rotor induction motor is identified by the presence of slip rings, which allow external resistance to be connected during starting.
👉 Hence, Option 1 is Correct.