The formula
E = (P × Φ × Z × N) / (60 × A), with A = P for lap and A = 2 for wave
- E
- generated EMF (volts)
- P
- number of poles
- Φ
- flux per pole (webers)
- Z
- total armature conductors
- N
- speed (rpm)
- A
- parallel paths — P for lap winding, 2 for wave
Worked example
A 4-pole lap-wound machine: Φ = 0.02 Wb, Z = 500 conductors, N = 1500 rpm.
- Lap winding, so A = P = 4
- E = (4 × 0.02 × 500 × 1500) / (60 × 4)
- E = 60000 / 240
E = 250 V. The same machine wave-wound (A = 2) would give 500 V at half the current rating.
Where you'll use it
DC generator and motor problems throughout machines papers. The back-EMF form explains starting current: at standstill E is zero, so only armature resistance limits the current — hence the starter.