Torque motor, how it works?

Torque motors are generally PMSM motors. Also they 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 directly drive 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.

In particular, there are two type of direct drive torque motors. One is the outrunner torque motor and the second type is the inrunner torque motor. Considering the outrunner motor, the rotor (the permanent magnet ring) is located on the outside of the stator (the electromagnet and lamination). In case of a inrunner motor, the rotor is located on the inside of the stator.  Both type of torque motors, each with their own advantages, function in a similar way.

The stator of a torque motor

The stator is the static part of the torque motor, which contains the lam stack and copper wire. Moreover, the lam stack contains “lam stack teeth” or “stator teeth”. On each of these teeth copper windings are wound. When three phase AC current is supplied to the copper windings, the stator turns into an array of electromagnets. As a result, an alternating rotating magnetic field is created around the stator, as shown in the video to the right. The orange and blue areas depict the south and north part, respectively. Further, the copper windings on the stator can be configured in different ways to create different performance parameters that can be important for various applications.

The rotor of a torque motor

The rotor is the moving or rotating part of the torque motor, which contains the permanent magnets. The permanent magnets are placed on the rotor in a north south north south pattern. The blue and orange areas depict the north and south part, respectively. This is shown in the video to the right. Consequently, the permanent magnetic field of the rotor will interact with the alternating magnetic field of the rotor. Further, between the stator and the rotor the magnetic air gap is located. 

The stator and rotor of a torque motor

In case of an outrunner torque motor, the rotor is put around the stator. As a result, the magnetic field of the stator starts to interact with the permanent magnet field of the rotor. Consequently, the permanent magnetic field of the rotor will start rotating synchronously with the stator magnetic field and torque (N) is created. This is shown in the video to the right. 

The torque of the direct drive torque motor is proportional to the current in the stator. In addition, the required voltage is proportional to the speed. 

Input: Electric power = current * voltage. 

Output: Mechanical power = torque * angular velocity. 

By adjusting the number of windings and / or winding configuration, the ratio between the voltage and current demand can be adjusted. This makes the motor usable for various application areas, such as semicon, robotics, food and packaging.