DC motor is powered by direct current while ac motor is powered by alternating current. From the constructure, principle of dc motor is relative simple with complex constructure, not suitable to maintain. But ac motor has complex principle with simple constructure. It is convenient to maintain.
From the aspect of pricing, the pricing of dc motor is higher than the pricing of ac motor in same power as well as adjustable speed device. At the same time contructure and maintenance has great difference. From the aspect of performance, dc motor has steady speed and can control the speed accurately which can not reach by ac motor. Under this condition, we only consider the adoption of dc motor not ac motor.
But we have to recognize ac motor is adopted widely since most of chemical plants use alternating current even though its adjusting speed is complex.