Torque motor vs servomotor

A question often asked is what the difference is between a servomotor and a torque motor. To give a quick and simple answer: there doesn’t have to be a difference. A torque motor can be a type of servomotor. A servo motor is a motor that gives high control over position, acceleration and velocity, because of a sensor that provides a feedback signal. This signal is used by the control mechanism of the motor to self-regulate its output. A so called “closed loop” mechanism. A torque motor can be fitted with e.g. an encoder to provide this feedback signal, making it technically a servomotor. But what then is the difference between a torque servomotor and other types of servomotors? 

Servomotors

Although servomotors consist of a wide range of types of motors, most commonly a servomotor is a permanent magnet synchronous motor (PMSM), fitted with a closed loop control mechanism. These servomotors generally have a long axis, small diameter and have an output shaft. These motors often have an added gearbox. Usually they have a NEMAstyle mounting to mount them onto the application. The term “conventional servomotors” will be used in this article for these type of servomotors.

Torque motors

Torque motors are also generally PMSM motors that can be fitted with a closed loop control mechanism, like conventional servomotors, but the concept and shape is different. A torque motor is toroid shaped, like a donut, and thus has a large diameter and short axis. These motors can be designed into the application, saving space as no motor has to be mounted on top of it. Also because of this it directly drives the application without additional mechanics, like worm wheels, belts or pulleys. Because of the difference in shape, the torquemotor generates a lot of torque, but its RPM is limited. A conventional servomotor on the other hand, can reach much higher RPM but generates far less torque. It is therefore often fitted with a gearbox to reach the desired speed to torque ratio, but this gearing does reduce efficiency and accuracy.

Which one to choose?

For some applications, like indexing tables, torque motors are mainly used because of its higher accuracy. Also in the food and health industry it is often preferred because it is “cleaner”. It has no lubricated mechanical gearing and can be designed into the product, directly driving the application. For high RPM, low torque applications, mainly ungeared conventional servomotors are used because they can reach the desired speeds. But for other applications both types of motors can be used and it is usually a matter of the best price-performance ratio. Conventional servomotors can be fitted with different gearboxes, making it relatively easy to generate the desired speed to torque ratio’s, using available motor designs. But when an existing torquemotor design is fitted to your application it often outperforms conventional servomotors. So the best solution depends on the application and what is already available on the market.

Conventional servomotor
Conventional servomotor
An image of a torque motor
Rotor and Stator of a Torque Motor