Question

The desired properties of magnetic materials used for electrical machines are :
A. Narrow hysteresis loop to give small hysteresis loss
B. Increased resistivity to give small eddy current loss
C. Free from ageing problem
D. High ductility and free from brittleness
E. High permeability and High saturation flux density Choose the correct answer from the options given below :

• A. A and B only
• B. A, C and E only
• C. A, C and D only
• D. A, B, C, D, E

Hint:

Good magnetic materials should have:
• low hysteresis loss,
• low eddy current loss,
• high permeability,
• high saturation flux density,
• good mechanical strength.

Answer 1

The Correct Option is D

Concept: Magnetic materials used in electrical machines should possess excellent magnetic, electrical, thermal, and mechanical properties. Good magnetic materials improve:
• efficiency,
• magnetic performance,
• machine life,
• mechanical strength. Hence several desirable properties are required simultaneously.

Step 1: Understanding statement \(A\). Statement \(A\): \[ \text{Narrow hysteresis loop to give small hysteresis loss} \] This is correct. Hysteresis loss is proportional to hysteresis loop area. A narrow loop means:
• smaller energy loss,
• better efficiency. Hence statement \(A\) is correct.

Step 2: Understanding statement \(B\). Statement \(B\): \[ \text{Increased resistivity to give small eddy current loss} \] Eddy current loss decreases when electrical resistivity increases. Therefore high resistivity materials are preferred. Hence statement \(B\) is correct.

Step 3: Understanding statement \(C\). Statement \(C\): \[ \text{Free from ageing problem} \] Magnetic ageing causes deterioration of magnetic properties over time. Electrical machines require stable long-term performance. Thus freedom from ageing is desirable. Hence statement \(C\) is correct.

Step 4: Understanding statement \(D\). Statement \(D\): \[ \text{High ductility and free from brittleness} \] Machine cores are manufactured into sheets and laminations. Therefore materials should:
• withstand mechanical stress,
• be easy to fabricate,
• avoid cracking. Hence good ductility is important.

Step 5: Understanding statement \(E\). Statement \(E\): \[ \text{High permeability and high saturation flux density} \] High permeability ensures easy magnetization. High saturation flux density allows stronger magnetic flux without saturation. Both properties are essential in electrical machines. Hence statement \(E\) is correct.

Step 6: Selecting the final answer. All statements: \[ A,\;B,\;C,\;D,\;E \] are correct. Hence the correct option is: \[ \boxed{(4)} \]