Dynamo Formula for EMF Rotating Magnetic Field

Industrial Electric Motors and Generators Menu

A rotating magnetic field in a electric motor is a magnetic field that has a moving polarities in which its opposite poles rotate about a central point or axis. Ideally the rotation changes direction at a constant angular rate. This is a key principle in the operation of the alternating-current motor .

A symmetric rotating magnetic field can be produced with as few as two polar wound coils driven at 90 degrees phasing. However, 3 sets of coils are nearly always used because it is compatible with a symmetric 3-phase AC sine current system . The three coils are driven with each set driven 120 degrees in phase from the others. For the purpose of this example, the magnetic field is taken to be the linear function of the coil's current.

Rotating magnetic fields are often utilized for electromechanical applications such as induction motors and electric generators , however they are also used in purely electrical applications such as induction regulators .

Dynamo Formulae for EMF

Average e.m.f. generated in each conductor =


Z = total number of armature conductors
c = number of parallel paths through winding between positive and negative brushes
Where c = 2 (wave winding), c = 2p (lap winding)
Φ = useful flux per pole (webers), entering or leaving the armature
p = number of pairs of poles
N = speed (revolutions per minute)

Generator Terminal volts = EG – IaRa
Motor Terminal volts = EB + IaRa


EG = generated e.m.f.
EB = generated back e.m.f.
Ia = armature current
Ra = armature resistance

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