ABSTRACT
A direct current (DC) motor is used to drive a mechanical load. Variable DC drives have been used to control DC motors longer than variable frequency drives have been used to control Alternating Current (AC) motors. Most industrial DC motors will operate reliably over a speed range of about 20:1 down to about 5%–7% of base speed. In a brushed DC motor, torque control is also simple since output torque is proportional to current. The electromagnetic forces or torque developed by the driving motor tend to propagate motion of the drive system. This motion may be uniform translational or rotational motion or non-uniform as in case of starting, braking, or changing the load. To change the load speed, there are mechanical and electrical methods as well as their combination. Different motors have a different electrical method of speed variation; therefore DC and AC motors will take each alone with their possible speed variation methods.
