AC Servomotors

AC servomotors are typically permanent magnet synchronous motors that often have low torque-to-inertia ratios for high acceleration ratings. AC servomotors have an output shaft that can be positioned by sending a coded signal to the motor. As the input to the motor changes, the angular position of the output shaft changes as well. Generally, AC servomotors are small but powerful for their size and easy to control. Induction and gearmotor types are some common forms of AC servomotors.

AC servomotors vary by AC voltage and frequency. In North America, the standard residential or commercial voltage for 60Hz power is 115 / 120V; however, 208-230 / 240V, 460 / 480V, and 575 / 600V are also used. International voltage levels, such as those common in Asia and Europe, use 50Hz power. AC servo motors and other components using 400Hz power are primarily used for aerospace applications.

AC servomotors are either single phase or three phase. Standard commercial and residential power is single phase, meaning one sinusoidal or other alternating voltage pattern. Three phase power contains three simultaneous sinusoidal or other alternating voltage patterns, typically 120° out of phase with each other. Higher power efficiency and smoothness of operation is possible with three phase operation. Three phase power is most typically used for industrial or high power motors.

AC servomotors vary according to shaft speed, continuous current, continuous torque, and continuous power output. The shaft speed is the no-load rotational speed of the output shaft at a rated terminal voltage. The continuous current is the maximum rated current that can be supplied to the motor windings without overheating. The continuous torque is the output torque capability of the motor under constant running conditions. The continuous output power is the mechanical power provided by the motor output. With multi-speed AC servomotors, motor speed can be continuously adjusted or set at discrete speeds within the operating range. With reversible AC servomotors, motors can be run in both clockwise and counterclockwise directions with approximately the same operating characteristics.

AC servomotors use integral encoders, integral resolvers, and integral tachometers for feedback signals. Integral encoders contain attached encoders for angular position signals, and may include absolute or incremental encoders and a number of different encoder signal types. Integral resolvers contain an attached resolver to indicate the angular position.  Resolvers often rely on magnetic fields and are typically very robust; they are sometimes specified for harsh environments. Integral tachometers produce an output indicating rotational motor speeds.

AC servomotors are either cylindrical or square, open or enclosed, and available in a variety of housing sizes and diameters. Drip-proof motors contain ventilation openings that are designed so that drops of liquid or solid particles falling vertically from any angle within 15 degrees cannot enter the motor. Dust-proof AC servo motors protect against dust infiltration with features such as total enclosure and labyrinth seals for shafts.

Some AC servomotors are designed for special environments such as clean rooms. Others have a cryogenic rating or vacuum rating. Explosion-proof motors have totally enclosed housings that are constructed to withstand internal explosion of a specified gas, vapor or dust. Radiation-hardened AC servomotors are designed to withstand high-energy gamma radiation.  

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Part Numbers for AC Servomotors