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