Genaral Specification:

Servo Motors

Like stepper motors, servo motors have many implementations. Let’s consider the most common design, which incorporates a rotor with permanent magnets and a stationary stator with the windings. Here too, the current creates a magnetic field distribution that acts on the rotor to develop torque. Servo motors have significantly lower pole counts than stepper motors, however. As a result, they must be run closed-loop.

In general, servo motors are more sophisticated than stepper motors. They run significantly faster than stepper motors, with speeds on the order of several thousand RPMs. This enables servo motors to be used with gearboxes to deliver much higher torque at useful speeds. They also deliver more consistent torque across the speed range of the motor. Unlike stepper motors, they do not have holding torque per se.

Closed-loop operation enables the controller/drive to command that the load remain at a specific position, however, and the motor will make continual adjustments to hold it there. Thus, servo motors can deliver de facto holding torque. Note, however, the zero-speed torque scenario depends upon the motor being sized properly to control the load and prevent oscillation about the commanded location.

With closed-loop feedback, servo motors deliver high accuracy positioning coupled with better speed and acceleration than stepper motors. The trade-off is increased cost, size, and complexity.

Servo motors typically use rare-earth magnets while stepper motors more frequently use less expensive conventional magnets. Rare-earth magnets enable the development of higher torque in a smaller package. Servo motors also gain a torque advantage from their overall physical size. Servo motor diameters typically range from NEMA 17 all the way up to 220 mm. As a result of these combined factors, servo motors can deliver torques of up to 250 foot-pounds.

The combination of speed and torque enables servo motors to deliver better acceleration than stepper motors. They also deliver improved positioning accuracy as a result of closed-loop operation.