Torque of DC Machine

 When a dc machine is loaded either as a motor or generator, the rotor conductors carry current.

 Since the conductors lie in the magnetic field created in the air gap, they experience a force.

 Torque (turning effect) is developed on the rotor and the rotor starts rotating.

 When the machine operates as a generator at constant speed, the torque is equal and opposite that provided by the prime mover.
 When the machine operates as a motorthe torqueis transferred to the shaft of the rotor and drives the mechanical load.

Torque of DC Machine
 DEDUCTION OF THE TORQUE EQUATION
 Voltage equation of DC motor: V = E + IaRa
 multiplying both sides by Ia: VIa=EIa+Ia2Ra
where: VIa= electrical power input to the armature
I2Ra =copper losses in the armature
Eia = electrical equivalent of gross mechanical
power developed by the armature
 Let T = average electromagnetic torque developed by the armature
 At this value of torque the electro-mechanical power conversion takes place.