Question

For a Three phase alternator, the effect of slot harmonics can be reduced by :
A. Skewing the slots
B. Fractional slot winding
C. Short-chorded winding
D. Distributed winding
E. Increasing the number of poles Choose the correct answer from the options given below :

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

Hint:

Methods commonly used to reduce harmonics in alternators:
• Fractional slot winding
• Short-pitched winding
• Distributed winding These improve the sinusoidal nature of generated EMF.

Answer 1

The Correct Option is B

Concept: Slot harmonics in alternators arise because armature conductors are placed in slots instead of being uniformly distributed. These harmonics distort generated EMF waveform and may produce:
• additional losses,
• noise,
• vibration,
• waveform distortion. Several winding techniques are used to reduce harmonic content.

Step 1: Understanding fractional slot winding. Fractional slot winding distributes conductors more effectively. It helps reduce harmonic components in generated EMF. Thus: \[ B \] is correct.

Step 2: Understanding short-chorded winding. Short-pitch or chorded winding reduces harmonic EMFs. Pitch factor for harmonics becomes smaller: \[ k_p=\cos\left(\frac{\alpha}{2}\right) \] Thus unwanted harmonics are suppressed. Hence: \[ C \] is correct.

Step 3: Understanding distributed winding. Distributed winding spreads conductors across several slots. This reduces harmonic components because:
• harmonic EMFs cancel partially,
• waveform becomes more sinusoidal. Hence: \[ D \] is correct.

Step 4: Checking skewing of slots. Skewing is mainly used to reduce:
• magnetic locking,
• cogging,
• noise. It is not the primary method listed here for reducing slot harmonics in alternator windings. Hence statement \(A\) is not taken as correct in this question.

Step 5: Checking increase in number of poles. Increasing number of poles does not directly reduce slot harmonics. Thus: \[ E \] is incorrect.

Step 6: Selecting the final answer. Correct methods are: \[ B,\;C,\;D \] Hence the correct option is: \[ \boxed{(2)\; B,C,D\text{ only}} \]