A 3-phase, 6 pole, 120 slots alternator has a coil span of 18 slots. The short pitch angle is:

Explanation:

Short Pitch Angle in Alternator

Definition: The short pitch angle, also known as the short pitch or coil pitch angle, is the angle by which the coil span is less than the pole pitch in electrical degrees in an alternator. The use of a short pitch coil helps to reduce harmonics in the generated voltage and improves the waveform quality.

Given Data:

• Number of poles (P) = 6
• Number of slots = 120
• Coil span = 18 slots

Step-by-Step Solution:

To calculate the short pitch angle, we must follow these steps:

Pole pitch is the number of slots per pole. It represents the slot distance for one complete pole.

Pole pitch = Total number of slots ÷ Number of poles
Pole pitch = 120 ÷ 6 = 20 slots

The short pitch is the difference between the pole pitch and the coil span.

Short pitch = Pole pitch - Coil span
Short pitch = 20 - 18 = 2 slots

One pole corresponds to 180° electrical. Hence, 1 slot in terms of electrical degrees can be calculated as:

Electrical degrees per slot = 180° ÷ Pole pitch
Electrical degrees per slot = 180° ÷ 20 = 9° per slot

Now, multiply the short pitch (in slots) by the electrical degrees per slot to find the short pitch angle:

Short pitch angle = Short pitch × Electrical degrees per slot
Short pitch angle = 2 × 9° = 18°

1. Calculate the Pole Pitch:
2. Determine the Short Pitch:
3. Convert the Short Pitch to Electrical Degrees:

Correct Option:

The correct option is:

Option 2: 18°

This result indicates that the short pitch angle of the given alternator is 18°, which aligns with the given data and calculations.

Additional Information

To further understand the analysis, let’s evaluate the other options:

This option is incorrect because it assumes a short pitch angle that does not match the given coil span and pole pitch data. Based on the calculations, the short pitch angle is 18°, not 15°.

This option is incorrect as it suggests a short pitch angle greater than the calculated value. A short pitch angle of 20° would imply a short pitch of more than 2 slots, which is inconsistent with the given data.

This option is incorrect because a short pitch angle of 0° would imply a full-pitch coil where the coil span equals the pole pitch. In the given data, the coil span is less than the pole pitch, resulting in a non-zero short pitch angle.

• Option 1: 15°
• Option 3: 20°
• Option 4: 0°

Conclusion:

Understanding the concept of short pitch angle is essential for analyzing alternator designs. The short pitch angle of 18° (Option 2) is calculated based on the given data of pole pitch and coil span. This angle is critical for reducing harmonics and improving the quality of the alternator's output voltage. Analyzing other options reveals that they do not align with the given data, further confirming the correctness of Option 2.