A DC motor have the performance with fast response , a large starting torque,
it can provide rated torque from zero speed to rated speed . But advantages of the DC motor is what its shortcomings, because the DC
motor produce constant rated load torque , then the armature magnetic field and rotor magnetic field must be constant maintenance of 90 ° , which would produce sparks, toner form carbon brush and commutator in the motor rotation , it will not only make group damaged, but also the using is limited.