1️⃣ It has no commutator and no slip rings
2️⃣ It is provided with cage rotor which is simple, robust and almost indestructible in construction
3️⃣ It has high starting torque
4️⃣ Almost 90 percent of induction motors are of this type
✅ Answer (Detailed Solution Below)
✔️ Option 3: It has high starting torque
📖 Detailed Solution
🔹 A squirrel-cage induction motor works on electromagnetic induction.
🔹 It does not use slip rings or brushes.
🔹 The rotor consists of aluminium/copper bars short-circuited by end rings.
🔹 This forms a cage-like structure.
🔹 Due to permanent short-circuiting, external resistance cannot be added.
🔹 Hence, starting current is high.
🔹 But starting torque is comparatively low.
🔹 Motors requiring high starting torque use slip-ring motors.
👉 Therefore, the statement “It has high starting torque” is incorrect.
⚙️ Main Features of Squirrel-Cage Induction Motor
✔️ No commutator
✔️ No slip rings
✔️ Simple construction
✔️ Rugged design
✔️ Low maintenance
✔️ High reliability
✔️ Self-starting
✔️ Long life
🐿️ Why It Is Called Squirrel Cage Motor
🔹 Rotor looks like a squirrel cage.
🔹 Made of steel laminations.
🔹 Embedded aluminium/copper bars.
🔹 Short-circuited by end rings.
🔹 Hence called squirrel cage rotor.
🏗️ Construction of Squirrel Cage Induction Motor
1️⃣ Stator
🔹 Stationary part.
🔹 Contains 3-phase winding.
🔹 Windings placed 120° apart.
🔹 Laminated iron core.
🔹 Produces rotating magnetic field.
🔹 Uniform and symmetrical air gap.
2️⃣ Rotor
🔹 Rotating part.
🔹 Consists of:
Shaft
Copper/Aluminium bars
Laminated core
🔹 Bars short-circuited by end rings.
🔹 No insulation on bars.
🔹 No brushes or slip rings.
🔹 Conductors are skewed to:
✔ Prevent cogging
✔ Reduce noise
✔ Improve performance
3️⃣ Fan
🔹 Mounted on rotor.
🔹 Provides cooling.
🔹 Removes heat.
🔹 Maintains temperature.
4️⃣ Bearings
🔹 Support rotor.
🔹 Reduce friction.
🔹 Ensure smooth rotation.
⚡ Working Principle
🔹 AC supply is given to stator.
🔹 Rotating magnetic field is produced.
🔹 EMF is induced in rotor bars.
🔹 Rotor current flows.
🔹 Rotor produces its own magnetic field.
🔹 Interaction produces torque.
🔹 Rotor starts rotating.
🚫 Limitation
🔹 External resistance cannot be added.
🔹 Starting torque is low.
🔹 Speed control is difficult.
⚖️ Comparison: Slip Ring vs Squirrel Cage Motor
| Feature | Slip Ring Motor | Squirrel Cage Motor |
|---|---|---|
| Rotor Type | Slip ring | Squirrel cage |
| Construction | Complicated | Simple |
| External Resistance | Possible | Not possible |
| Starting Torque | High | Low |
| Brushes | Present | Absent |
| Maintenance | High | Low |
| Air Gap | Non-uniform | Uniform |
| Power Factor | Low | High |
| Speed Control | Possible | Not possible |
📌 Important Points
⭐ About 90% induction motors are squirrel cage type
⭐ Most widely used in industry
⭐ Reliable and durable
⭐ Easy maintenance
⭐ Economical
🏭 Applications
✔️ Pumps
✔️ Fans
✔️ Compressors
✔️ Conveyors
✔️ Machine tools
✔️ Mills
✔️ Elevators
📝 Final Summary
📍 A squirrel-cage induction motor is simple, rugged, maintenance-free, and widely used. However, it does not have high starting torque.
👉 Hence, Option 3 is Correct.