1️⃣ 3
2️⃣ 9
3️⃣ 7
4️⃣ 8
✅ Answer (Detailed Solution Below)
✔️ Option 1: 3
📖 Detailed Solution
🔹 A squirrel cage induction motor has rotor bars short-circuited by end rings.
🔹 These bars are placed in rotor slots.
🔹 Although there are many rotor bars (28 in this case), they do not represent phases.
🔹 Electrically, the rotor behaves like a distributed winding.
🔹 This winding is equivalent to a three-phase, star-connected winding.
🔹 The induced currents in the rotor are due to the three-phase stator field.
🔹 Hence, the rotor current system is three-phase in nature.
👉 Therefore, the number of rotor phases is always three.
⚙️ Important Concept
🔹 In squirrel cage motors:
- Rotor has bars, not coils.
- Bars are short-circuited.
- Current is induced by stator field.
- Rotor acts like a 3-phase winding.
👉 Number of slots ≠ Number of phases.
📊 Given Data
| Parameter | Value |
|---|---|
| Supply | 3-phase |
| Poles | 4 |
| Stator Slots | 36 |
| Rotor Slots | 28 |
🔹 These values affect:
✔ Torque ripple
✔ Noise
✔ Harmonics
✔ Cogging
🔹 They do not affect the number of phases.
❌ Why Other Options are Incorrect
(2) 9 ❌
🔹 Rotor phases do not depend on slot number.
(3) 7 ❌
🔹 No relation between slots and phases.
(4) 8 ❌
🔹 Incorrect assumption based on slots.
📌 Additional Information
Rotor Construction (Squirrel Cage)
🔹 Laminated steel core
🔹 Aluminium/Copper bars
🔹 End rings
🔹 Permanently short-circuited
🔹 No slip rings or brushes
Why Rotor is Considered 3-Phase
🔹 Stator produces 3-phase rotating magnetic field.
🔹 Rotor currents follow this field.
🔹 Hence, rotor system behaves as 3-phase.
📌 Key Exam Notes
⭐ Rotor phases → Always 3
⭐ Slot number → Does not decide phases
⭐ Applicable to → Squirrel cage motors
⭐ Rotor type → Short-circuited bars
📝 Final Summary
📍 In a 3-phase squirrel cage induction motor, regardless of stator or rotor slot number, the rotor is electrically equivalent to a three-phase winding.
👉 Hence, Option 1 is Correct.