Torque motors optimised for machine applications

Torque motors are a special class of brushless permanent-magnet synchronous motors. Since the payload is directly connected to the rotor without the use of transmission elements, torque motors are classified as direct drives.

Depending on your perspective, 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 conventional torque motors to attain high torque at moderate speeds.

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 optimised according to the required performance or purchased as part of an assembly.

Torque motors produce high torque at moderate speeds and even when stationary or stalled'. Contrary to traditional drives, the sizing and selection of a torque motor is purely based on torque, not power. Fundamentally, the peak torque determines the maximum torque that the motor physically produces, and the continuous torque defines the amount of torque the motor can continuously supply. The duty cycle of the application will define the dependency on peak or continuous torque.

Reduced cost of ownership

Direct coupling of the payload to the rotor eliminates the need for mechanical transmission elements such as gearboxes, timing belts, speed reducers and worm gear drives. Unlike brushed rotary motors, there is no contact between rotor and stator; therefore, there is no mechanical wear resulting in excellent reliability and long lifetimes. Fewer mechanical parts also minimises maintenance and reduces the system cost.

ETEL's torque motors are available in a wide range of sizes and can be easily adapted to most applications. ETEL's standard product offering includes motors with external diameters from 140 mm to 1,290 mm. The use of magnets and limited air gap results in a large hollow shaft or bore for easy integration of cables, cooling tubes, or other application related equipment.

Dynamic performance is significantly improved by using direct drive due to the very high control loop bandwidth that can be achieved on the overall system. The direct coupling of the load and position feedback to the motor has the advantage of eliminating all phenomena that limit the dynamic performance on non-direct driven machines. Eliminating long-time drift, elasticity, and backlash is a huge advantage for machine performance and lifetime.

Depending on the specifics of a system's duty cycle, the peak torque, continuous torque, or both will drive the selection of a motor. An application with a light duty cycle that requires high peak torque for a short period of time will typically utilise an air-cooled motor like ETEL's TML series. As the continuous torque requirement increases so will the temperature of the motor. The integrated water-cooling capability of the ETEL TMB+ and TMK motor provides an efficient means of temperature control ensuring high dynamic performance that can be maintained even for the most demanding applications.

The torque motor is part of a complete direct drive solution which includes a position controller. High-end digital controllers like the ETEL position controllers which have been designed specifically for direct drive applications provide excellent control loop quality ensuring optimum stiffness, smooth motion, and excellent velocity control with low torque ripple.