The motor speed can be controlled by controlling armature
voltage and armature current. These method have some
demerit because some power is wasted in control resistance.
PWM method also can be used to control dc motor speed.
PWM circuit work by making a square wave with a variable
on to off ratio. The ratio can vary between 0 % - 100%. In this
manner variable power is transferred to load. Main advantage
PWM circuit over a resistive power control is the
efficiency[2]. On PWM 50% amount off pulse and on pulse is
equal.