Driving and Retarding Torques, Back EMF

Driving and Retarding Torques, Back EMF Points : Driving and Retarding Torques, Back EMF DC Motor The operation of the dc machine as generator. A prime mover drives the generator in anticlockwise direction. It generates an emf and if an external load is connected to the terminals, a current flows through the armature conductors in the directions shown by crosses and dots. Since the conductors are situated in a magnetic field and are carrying current, an electromagnetic torque whose magnitude is developed in the clockwise direction (as per Fleming’s left hand rule). This is a retarding torque as it is opposing the rotation. To keep the generator moving, the prime mover must supply rough driving torque so as to overcome this retarding torque and also to overcome action at the bearing and brushes, windage forces and magnetic drag caused by hysteresis and eddy currents in the armature coil.

The machine is connected to a dc source and a current flows through the armature conductors. A torque is devolved and the machine rotates in the anticlockwise direction. This torque drives the devce (e.g. late, pump, etc.) which is mechanically coupled to the motor. As the conductors rotate in the magnetic field, an emf is induced in the conductors. The direction of this emf is the same. (a). This emf opposes the flow of current in the machine and is thus opposite in direction to the applied voltage. This induced emf because it opposes the applied voltage) is known as back emf or counter emf of the motor of applied voltage must be sufficient to overcome the back emf as well as to overcome the voltage drop in the armature. The back emf E_b is the voltage induced due to roation of the armature and its magnitude is E_b = K_e NФ