What is the difference between Category DC-1 & Category DC-3?
Category DC-1: This category applies to all types of d.c. load with a time constant (L/R) of less than or equal to 1 ms
Category DC-3: This category applies to starting, counter-current braking and inching of shunt motors.
Time constant less or equal 2 ms.
On closing, the contactor makes the starting current, which is about 2.5 times the rated motor current.
On opening, the contactor must be able to break 2.5 times the starting current at a voltage which is less than or equal to the mains voltage. The slower the motor speed, and therefore the lower its back e.m.f., the higher this voltage. Breaking is difficult.