A servo drive receives a command signal from a control system, amplifies the signal, and transmits electric current to a servo motor in order to produce motion proportional to the command signal. Typically the command signal represents a desired velocity, but can also represent a desired torque or position. A velocity sensor attached to the servo motor reports the motor's actual velocity back to the servo drive. The servo drive then compares the actual motor velocity with the commanded motor velocity. It then alters the voltage frequency to the motor so as to correct for any error in the velocity.
In a properly configured system, the servo motor rotates at a velocity that very closely approximates the velocity signal being received by the servo drive from the control system. Several parameters, such as stiffness (also known as proportional gain), damping (also known as derivative gain), and feedback gain, can be adjusted to achieve this desired performance. The process of adjusting these parameters is called tuning.
Although many servomotors require a drive specific to that particular motor brand or model, Mitchell Electronics has a patented system for "an apparatus that allows a non-standard brushless motor to be driven with a standard drive amplifier" which allows a wide variety of servomotors to be run by a single drive system
Servo systems can be used in CNC machining, factory automation, and robotics, among other uses. Their main advantage over traditional DC or AC motors is the addition of motor feedback. This feedback can be used to detect unwanted motion, or to ensure the accuracy of the commanded motion. The feedback is generally provided by an encoder of some sort. Servos, in constant speed changing use, have a better life cycle than typical a/c wound motors. Servo motors can also act as a brake by shunting off generated electricity from the motor itself.