Here's the answer

May 07, 2020   photos  

Because DC current is more stable and controllable in low speed/high torq motors. There's less oscillation and the losses of energy are nearly zero. One more important thing is that for safety of the people because DC current is less dangerous then AC current. Finally, DC series motor used for high starting load areas. For the high starting torque and the constant speed at a very high load. So, we can say that constant torque at any speed. This is mainly because of it depends on load. DC series motor produces high starting torque.