# DC Motor

1. Jun 5, 2007

### OutCell

Hi,

What is the effect of "back e.m.f." on the torque output of a permanent magnet d.c. motor? I googled it but didn't get any results.. I just need a brief description to understand the concept..

Thanks

2. Jun 5, 2007

### ank_gl

when armature of dc motor rotates, it also acts as a generator also n generates a counter emf(lenz law) which is called back emf. now this back emf is proportional to the speed at which the armature is rotating. at a particular speed it becomes equal to the applied voltage. this is the maximum speed of the dc motor.
at the start, when back emf is zero, voltage across armature(applied voltage-back emf) is maximum, current is maximum, therefore torque is maximum(torque is proportional to current in the armature, T = I*L*B*r, I current, L length of conductor,B magnetic field, r = radius, school physics). as the speed builds up, back emf increases, voltage across armature drops, current drops and therefore torque starts to decrease.
finally when speed is maximum, ie applied voltage= back emf, no current, no torque. therefore at no load n maximum speed, motor has no torque

3. Jun 5, 2007

Thanks mate!

4. Jun 5, 2007

### ank_gl

no probs dude. pleasure to help u