MMF Wave, Pole Pitch & Rotating Magnetic Field

1. Pole Pitch

Pole pitch is the distance between two adjacent poles (N to S).

• 1 pole pitch = 180° electrical
• 1 MMF cycle = 2 pole pitches

2. MMF Wave

The MMF wave represents how magnetomotive force is distributed in space.

• Half cycle = 1 pole pitch
• Full cycle = 2 pole pitches
• Total cycles = P / 2

3. Rotating Magnetic Field (RMF)

In a 3-phase system:

• Three currents are 120° apart
• Each produces an MMF wave
• Combined effect → Rotating Magnetic Field

The wave rotates but keeps the same shape.

4. Synchronous Speed

Ns = (120 × f) / P

• Ns = speed in RPM
• f = frequency (Hz)
• P = number of poles

5. Concept Behind Formula

• Frequency gives cycles per second
• 1 electrical cycle = 1 spatial cycle
• Total spatial cycles = P / 2
• Speed depends on how fast wave completes rotation

6. Examples

4-pole, 50 Hz:

Ns = (120 × 50) / 4 = 1500 RPM

2-pole, 50 Hz:

Ns = (120 × 50) / 2 = 3000 RPM

7. Summary

• Pole pitch = half cycle
• 1 cycle = 2 pole pitches
• Number of cycles = P / 2
• Speed = 120f / P