Torque Motor: Structure, Features, Types, Application

Figure 1: Torque motor.

Torque motor is a special motor with a large number of poles, which can run continuously without causing damage to the motor even when the motor is at low speed or even blocked (i.e. the rotor can't rotate). In this working mode, the motor can provide a stable torque to the load (hence the name torque motor). Torque motors can also provide torque in the opposite direction of operation (brake torque). The shaft of the torque motor does not output power with constant power but with constant torque.

1. Structure of Torque Motor

A torque motor can be regarded as a torque-optimized motor. The difference between torque motors and general motors is that they allow very high torque. Torque motors are generally designed with brushless DC motors, but sometimes switched reluctance motors and induction motors are also used. Torque motors are sometimes called low-speed motors or high torque motors.

The torque motor has a doughnut-shaped structure and is divided into an outer rotor motor with the rotor on the outer ring and the stator on the inner ring, and an inner rotor motor with the stator on the outer ring and the rotor on the inner ring. Because the outer rotor can generate a larger torque under the same size, the outer rotor motor is more commonly used.

Some torque motors can only work within a specific angle range and cannot make one rotation. Such torque motors are called limited angle torque motors or swing motors. This characteristic of torque motors has been considered to be introduced into linear motors.

Figure 2: Structure diagram of permanent magnet DC torque motor.

2. Features of Torque Motor

Torque motors are characterized by soft mechanical properties and can withstand stall conditions. When the load torque increases, it can automatically reduce the speed and increase the output torque at the same time. When the load torque is a certain value, the speed can be adjusted by changing the motor input voltage, but the speed adjustment rate is not good.
Therefore, a tachometer device can be added to the motor shaft, and equipped with a controller to automatically adjust the terminal voltage of the motor by comparing the output voltage of the tachometer device with the voltage given by the controller to stabilize the motor.

The torque motor has the characteristics of low speed, high torque, strong overload capacity, fast response, and small torque fluctuation. It can directly drive the load without the transmission gear, thereby improving the operating accuracy of the system.

In order to obtain different performance indicators, torque motors have three different structural forms: small air gap, medium air gap, and large air gap type. The small air gap structure can meet the general use accuracy requirements, and its advantage is lower cost.

Large air gap structure: due to the increase of the air gap, the cogging effect is eliminated, the torque fluctuation is reduced, the nonlinear change of the reluctance is basically eliminated, the motor linearity is better, the electromagnetic air gap is enlarged, the armature inductance is small, and the electrical time constant is small, but the manufacturing cost is high.
The performance indicator of the medium air-gap structure is slightly lower than that of the large-gap structure motor, but much higher than that of the small-air-gap structure motor. Its volume is smaller than that of the large-gap structure motor, and its manufacturing cost is also lower.

Figure 3: Permanent magnet DC torque motor.

3. Classification of Torque Motor

Torque motors include: DC torque motors, AC torque motors, etc.

3.1 DC Torque Motor

DC torque motor is a special form of DC servo motor, most of which use permanent magnet excitation. Its basic requirements are similar to those of DC servo motors. In order to obtain large output torque and low speed, it is generally made into a flat structure.

The working principle of the DC torque motor is the same as that of the ordinary DC servo motor, but they are different in the structure and the proportion of external dimensions. Generally, in order to reduce the moment of inertia of the DC servo motor, most of them are made into a slender cylindrical shape.
In order to generate relatively large torque and low speed under the same volume and armature voltage, the DC torque motor is generally made into a disc shape, and the ratio of armature length to diameter is generally about 0.2. Considering the rationality of the structure, it is generally made into a permanent magnet multi-pole type. In order to reduce the fluctuation of torque and rotational speed, more slots, commutator segments and series conductors are adopted.

The overall structure of the DC torque motor is divided into two types: split type and built-in type. The split-type structure includes three parts: stator, rotor and brush holder. The casing and shaft are selected by the user according to the installation method; the built-in type is the same as the general motor. The housing and shaft have been assembled by the manufacturer.

Figure 4: Outer rotor torque motor.

1—Permanent magnet 2—Inner magnet 3—External magnet 4 coils 5—Spring

3.2 AC Torque Motor

The basic requirements of AC torque motors are the same as those of AC servo motors, and they can be divided into two types: synchronous and asynchronous type. At present, squirrel-cage asynchronous torque motors are commonly used. They adopt a cage-type rotor structure and obtain low speed by increasing the number of pole pairs. They mainly run in the state of large load and low speed, and can also run in the locked-rotor state for a short or long time, featuring speed and high torque.

Its working principle and structure are the same as those of a single-phase asynchronous motor, but due to the large resistance of the squirrel-cage rotor, its mechanical properties are softer. A wide speed range and softer mechanical properties are obtained by increasing the rotor resistance. Compared with the general asynchronous motor of the same frame size, the output power of the AC torque motor is several times smaller, the stall torque is large, and the stall current is much smaller.

Figure 5: ORIENTAL MOTOR AC Torque Motors & Gear Motors.

4. Application of Torque Motor

4.1 Winding

Winding is a very important process in the processing of wire, cable, textile, metal, paper making and so on. When the product is wound, the diameter of the reel gradually increases. It is very important to keep the tension of the rolled product unchanged during the whole process, because excessive tension will thin or even break the wire, or cause uneven thickness of the product. If the tension is too small, loose winding may be caused.

In order to keep the tension constant during the winding process, it is necessary to increase the output torque of the motor driving the reel when the reel diameter increases as the product is wound onto the reel. At the same time, in order to keep the line speed of the winding product unchanged, the speed of the reel must be reduced accordingly, and the mechanical characteristics of the torque motor can just meet this requirement.

4.2 Unwinding

Unwinding is also known as unreeling, uncoiling, unrolling, etc. In industrial production, it is sometimes necessary to transport the product wound on the roller to the next process. During the conveying process, it is required to apply a tension opposite to the transmission direction, and at the same time, it is required to keep the linear speed and reverse tension of the product transmission constant with the change of the cylinder diameter, which requires the motor to have the characteristics of braking constant power.

Figure 6: Spinning and winding workshop.

4.3 Motor Stall

In some special occasions, the motor is sometimes required to maintain a static torque for a period of time, such as the initial stage of cable winding to maintain tension. Due to the large impedance of the torque motor, its locked-rotor current is small, and forced ventilation is used, so it can meet the locked-rotor requirements for a certain period of time. The allowable locked-rotor time should be calibrated on the nameplate. If a longer locked-rotor time is required, a larger torque motor can be selected, , by reducing the terminal voltage of the torque motor to obtain.

4.4 Other Application

Torque motors can also be used flexibly according to their various characteristics. For example, they have the DC series excitation motor characteristics and can partially replace the use of DC motors.
Another example is that according to the high resistance characteristics of the rotor, the starting (locking) torque is large, so it can be used in the opening and closing gate (valve) and the drag system with large resistance torque. It can also be used in various machines with frequent forward and reverse rotations or other similar actions by taking advantage of its large starting (locked-rotor) torque, small starting (locked-rotor) current, and high mechanical strength of the solid rotor.

Okmarts can provide various torque motors, such as high torque motor, and high torque DC motor. If you are interested in that, please contact us!