i mean in the sense of working ,construction wise ..........
dc motors are generally permanent magnet type and rotate at high speeds 4000 to 6000 rpm. In many cases, for automatic control, such high speeds are not required. So there will be a gear train to reduce the speed. This gear train might introduce avoidable errors such as due to backlash. In short a dc servomotor is a geared dc motor. there are however torque motors which have very low inertia. These are also dc motors with special construction. Low inertia is essential for servo applications. In ac motors, stepper motor is popular. A variation of this is the ac servomotor, which has fairly low speed. Otherwise, speed is in the range of 1500rpm to 3000 rpm. Changing the speed of an ac motor is no joke. One has to provide adequate voltage, but lower the frequency! You cannot control speed well by reducing the voltage as torque decreases substantially.
AC means alternate current DC means direct current. DC motors give a continual and standard flow, while AC motors are used in varying current flows. DC motors are self starting, whereas AC motors need effective starting equipment. AC motors work well for hard systems as these need a lot of upfront power. On the other hand, DC motors do not perform that well at producing power over extended periods of time.
