Question

What is back emf. Derive the relation for the back emf and the supplied voltage in terms of armature reactance.

When the armature of the motor rotates due to the torque developed by the armature conductors.

The conductors also rotates and cut the flux

An emf is induced as per faradays law, direction of some given by Flemings right hand rule. This emf (Eb) is in opposite direction to the applied voltage V.

This is known as back emf.

Eb = ΦZN / 60 x P/A volts.

This back emf must always be lower than V so as to allow the motoring condition

As the back emf is produced, the armature is run to start as a passive load, but as it rotates, it accelerate until the condition of balance is obtained.

The supply voltage = back emf + armature voltage drop

V = Eb + Ia Ra volts.

V = terminal voltage

Eb = back emf

Ia = armature current

Ra = armature resistance