(a) aluminum
(b) silicon steel
(c) closed grained cast iron
(d) stainless steel
(e) bronze
Answer: (c) closed grained cast iron
Explanation:
CONCEPT: Frame of Induction Motor
The frame (also called housing or casing) is the outer body of the induction motor.
It provides:
✔ Mechanical support
✔ Protection to internal parts
✔ Mounting base
✔ Vibration damping
Why Closed Grained Cast Iron Is Used
Closed grained cast iron is used because it provides:
- High mechanical strength
- Good rigidity
- Excellent vibration damping
- Resistance to deformation
- Long service life
So, it is ideal for industrial motors.
Note:
For small motors, aluminium frames may be used, but for heavy-duty motors, cast iron is preferred.
MATERIALS USED IN MAIN PARTS OF INDUCTION MOTOR
MAIN PARTS OF INDUCTION MOTOR
An induction motor mainly consists of:
- Stator
- Rotor
- Fan
- Bearings
- Shaft
1. STATOR
Construction
- Stationary part of motor
- Contains laminated core and windings
- Three-phase windings placed 120° apart
Functions
✔ Produces rotating magnetic field
✔ Provides low reluctance path for flux
✔ Maintains uniform air gap
Features
- Laminated silicon steel core (reduces losses)
- Uniform air gap
- Insulated copper windings
2. ROTOR
The rotating part of the motor.
Types of Rotor
(a) Squirrel Cage Rotor (Most Common)
- Aluminium/Copper bars
- Short-circuited by end rings
- No brushes or slip rings
Hence called:
👉 Short-circuited rotor
Features
✔ Simple construction
✔ Low maintenance
✔ High reliability
✔ Cheap
Important Point
External resistance cannot be added in squirrel cage rotor.
(b) Slip Ring Rotor (Special Cases)
- Has slip rings and brushes
- Allows external resistance
Rotor Core
- Laminated
- Reduces eddy current loss
- Conductors are skewed
Why Skewing Is Done
✔ Prevents cogging
✔ Reduces noise
✔ Improves torque
✔ Better flux distribution
3. FAN
Function
- Attached to rotor
- Provides cooling
- Removes heat
Maintains motor temperature within safe limit.
4. BEARINGS AND SHAFT ASSEMBLY
Bearings
Functions:
✔ Support rotor
✔ Maintain air gap
✔ Reduce friction
✔ Allow smooth rotation
Bearings require maximum maintenance.
Shaft
Functions:
✔ Supports rotor
✔ Transfers mechanical power to load
✔ Made of mild/carbon steel
5. BEARING AND SHAFT IMPORTANCE
Maintain alignment
Prevent vibration
Improve efficiency
Increase motor life
Faulty bearings cause:
❌ Noise
❌ Heating
❌ Shaft misalignment
❌ Failure
IMPORTANT EXAM POINTS ⭐
One-Line Facts
✔ Frame material → Closed grained cast iron
✔ Shaft material → Mild steel
✔ Stator core → Silicon steel
✔ Rotor bars → Aluminium / Copper
✔ Cooling → Fan
✔ Support → Bearings
MCQ Tips
Q: Frame material of induction motor?
👉 Closed grained cast iron
Q: Why laminations?
👉 Reduce eddy current loss
Q: Why skewed rotor bars?
👉 Prevent cogging
FINAL CONCLUSION ⭐
The frame of an induction motor is made of closed grained cast iron because it provides high mechanical strength, rigidity, and vibration damping. It protects internal parts and supports the stator and rotor assembly. Other major parts such as stator, rotor, shaft, bearings, and fan work together to ensure smooth, efficient, and reliable motor operation.