What are torque motors and why do they work so well?

May 20, 2020

Today, direct-drive technology is recognized as a leading solution for the requirements of high-productivity, improved accuracy and increased dynamics of modern machinery. They are rigid, more compact, provide high dynamic performance, reduce cost of ownership, simplify the design of the machine and reduce wear and maintenance.

What is a torque motor?

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. Another attractive feature is their compact design, which includes a narrow lamination stack and a large hollow shaft or bore.

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 performance, or purchased as part of an assembly.

Torque motors produce high torque at moderate speeds and even when stationary. 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 designs.

Why adopt torque motors?

Torque motors have a lot to offer engineers. They are deployed as part of several mission-critical, high-profile applications from space exploration to high-production machine tools. Here are a few of the reasons they’re a popular choice.

1. 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. And 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 minimizes maintenance and reduces the system cost. The direct-drive technology intrinsic to a torque motor system results in an efficient and effective gearless assembly.

2. Easy integration

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. The ring-like configuration of a torque motor minimizes the volume required for mounting. This gives the machine designer great flexibility in locating the motor to work with bearings, feedback devices, and payload.

3. Dynamic performance

Dynamic performance is drastically improved by very high control loop bandwidth. 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. Torque motor applications have a wide range of dynamic performance requirements. Depending on the specifics of a system’s duty cycle, the peak torque, continuous torque, or both will drive the selection of a motor.

4. Wide torque-speed range

Direct-drive torque motors deliver high torque over a wide range of speed, from a standstill or low speed condition to high angular velocities. While torque motors can achieve high velocities (up to 5450 rpm), there is a trade-off in torque as the motor becomes limited by speed dependent losses increase. The performance of a torque motor over its velocity range is illustrated in its torque/speed curve.

The HEIDENHAIN Family of Brands has long been dedicated to the development of direct-drive technology. Through numerous innovations and patented designs, we continue to provide unmatched torque efficiency for the most optimized performance.



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