(a) mild steel/Carbon Steel
(b) cast iron
(c) high speed steel
(d) stainless steel
(e) aluminium
Answer: (a) mild steel/Carbon Steel
📘 Explanation
The shaft of an induction motor is generally made of mild steel (carbon steel) because it provides the best combination of:
- High mechanical strength
- Good toughness
- High rigidity
- Resistance to torsional and bending stresses
- Cost-effectiveness
During motor operation, the shaft is subjected to:
- High torque
- Torsional stresses
- Dynamic and centrifugal forces
- Load variations
Mild steel can safely withstand these forces without cracking or excessive deformation, making it ideal for motor shafts.
❌ Why Other Options Are Incorrect
(a) Cast Iron ❌
- Brittle in nature
- Low tensile strength
- Easily cracks under shock loads
- Mainly used for motor frames, not shafts
(b) High-Speed Steel ❌
- Used mainly for cutting tools
- Very expensive
- Unnecessary hardness for motor shafts
(c) Stainless Steel ❌
- High corrosion resistance
- Costly
- Not required for normal industrial motors
(d) Aluminium ❌
- Lightweight but low strength
- Poor fatigue resistance
- Not suitable for high-stress shafts
- Mostly used in rotor bars or small motor frames
📌 Additional Information
Material Used in Induction Motor Parts
| Part | Material Used |
|---|---|
| Frame | Closed Grained Cast Iron |
| Shaft | Mild Steel / Carbon Steel |
| Stator Core | Silicon Steel Laminations |
| Rotor Bars | Copper / Aluminium |
| Bearings | Alloy Steel |
⚙️ Main Parts of an Induction Motor
1. Stator
- Stationary part
- Carries three-phase windings
- Produces rotating magnetic field
- Made of laminated silicon steel
2. Rotor
- Rotating part
- Contains shaft and conductors
- Squirrel-cage type uses copper/aluminium bars
- Laminated to reduce losses
3. Shaft and Bearings
- Shaft supports rotor
- Transmits mechanical power
- Bearings maintain air gap
- Reduce friction
4. Fan
- Mounted on rotor
- Provides cooling
- Maintains temperature
⭐ Advantages of Mild Steel Shaft
✔ High strength
✔ Good fatigue resistance
✔ Maintains uniform air gap
✔ Long service life
✔ Economical
✔ Easy machining
📌 Final Conclusion
Mild steel (carbon steel) is used for induction motor shafts because it offers high strength, rigidity, durability, and low cost, making it suitable for transmitting high torque safely.