Torque and speed of a DC motor

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Torque and speed of a DC Motor

Torque of a motor is independent of speed.It is rather a function of flux and armature current.

               N=K.(V-IR)/flux

N=number of turns, K=proportional constant, R=resistance of armature. Also,Torque is proportional to flux and armature current.

Increase in flux decrease the speed but increases the torque.If torque is decreased by decreasing the field current the following sequences are found:

1.Back EMF drops instantly remaining the speed constant because of the inertia of armature which is heavy. 2.Due to decrease of EMF armature current I is increased because of I=(V-E)/R. 3.A small decrease of flux is more than counterbalanced by a large increase of I which means net increase of torque. 4.If torque increases the speed also increase.

If applied volatge is kept constant motor speed has inverse relation with flux.

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