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Your Guide to Types of Servo Motors

Your Guide to Types of Servo Motors

By Jade York

There are three main classifications of servo motors. They come in many sizes by many different manufacturers, but their base functions and structures are similar. Learn more about the three main types below.

AC Servo Motors

An alternating current servo motor (AC servo motor), sometimes called a brushless servo motor, is based on the design of conventional AC motors.  An AC motor is a type of electric motor that runs on alternating current. It is made up of two basic parts; an outside stationary stator that has coils supplied with alternating current to produce a rotating magnetic field, and an inside rotor attached to the output shaft that is given a torque by the rotating field. AC motors are more commonly used than DC motors.

The AC servo motor is similar, with the addition of an amplifier and a feedback device. A typical 3-phase AC servo motor has a magnetic rotor and a field consisting of three phase windings. An AC servo amplifier, or linear power amplifier, feeds the control winding. Three-phase servo motors can be used at high-speed to drive the load, but are usually used as indexers for part placement. They are also frequently used for repetitive motion applications in robotics and CNC machining centers.

Because it has low inertia, an AC servo motor can start and stop on a dime, then change direction.

DC Servo Motors

A direct current (DC) motor is an electric motor that uses electricity and a magnetic field to produce torque, which causes it to turn. It requires a minimum of two magnets of opposite polarity and an electric coil, which acts as an electromagnet. The repellent and attractive electromagnetic forces of the magnets provide the torque that causes the motor to turn.

A DC servo motor, also called a brushed servo motor, is based on the design of a conventional DC motor, with the addition of an amplifier and a feedback device, such as a tach or resolver encoder.

Servo-Spindle Motors

A servo-spindle motor is an induction motor. Unlike an AC servo motor, the spindle has no magnetic rotor. Instead, it has an induction rotor. It does share a commonality with an AC servo motor, in that it has feedback and runs with an amplifier/drive mechanism. Spindle motors are low vibration, 3-phase, induction AC motors. They are specifically designed for grinding machines and low vibration applications.

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