Torque motors are a special class of brushless permanent-magnet servo motors commonly referred to as permanent-magnet synchronous motors or brushless DC motors. Since the payload is directly mounted on the rotor without the use of transmission elements, torque motors are classified as direct drives.
Depending on your viewpoint, a torque motor is either a rolled-up linear motor or a classic servo drive with a large number of poles. It is the large number of poles that enables torque motors to attain high torque at moderate speeds. Another attractive feature is their compact design that includes a narrow lamination stack and a large hollow shaft.
As with linear motors, torque motors are a type of "frameless" motor. This means that the motor does not include a housing, bearings, or feedback device. These components can be selected by the machine builder and optimized according to the required performances, or purchased as part of an assembly.
Torque motors produce high torque at moderate speeds or when the motor is stationary or stalled. Contrary to traditional drives, the sizing of a torque motor is based purely on torque, not power. The peak torque determines the maximum torque that the motor physically produces. Continuous torque defines the amount of torque the motor can continuously supply when all three phases equally share the load. The duty cycle of the application will define the dependency on peak or continuous torque.