Everywhere DC motors are used in large applications, the use of
drives are very necessary for the smooth running and operating of
these motors. The DC motor drives are used mainly for good speed
regulation, braking and reversing.
We know that, normally the rotor of a DC motor is energized by the
commutation process through brushes. So the maximum allowable
starting current is determined by the current which can be safely
carried out by the brushes without sparking. In general, the motors
are such designed that they can carry almost of the rated current
during starting condition. But why so much current flows through
the circuit of DC motors during the starting condition?
This is because when the motor is at stand still condition, there
is only a small armature resistance present in the circuit so no
back emf is generated. That is why when the motor is started with
full supply voltage across its terminal, there is a huge current
flow through the motor, which may damage the motor because of heavy
sparking across the commutators and a huge amount of heat is
generated. That is the reason why some precautionary measures are
taken during the starting of the DC motors.