The torque motor is a special motor with a large number of poles, which can continue to run when the motor is at low speed or even stalled ( that is, the rotor cannot rotate ) , without causing damage to the motor. In this working mode, the motor can provide stable torque to the load ( hence the name torque motor ) . Torque motors can also provide torque ( braking torque ) in the opposite direction of rotation . The shaft of the torque motor does not output power with constant power but with constant torque.

Torque motors include : DC torque motors, AC torque motors, and brushless DC torque motors. It is widely used in machinery manufacturing, textile, paper, rubber, plastic, metal wire and wire and cable industries.

DC torque motor

A special form of DC servo motor. Most of them use permanent magnet excitation. Its basic requirements are similar to DC servo motors. In order to obtain large output torque and low speed, it is generally made into a flat structure. The ratio of motor armature core length to outer diameter is very small. In order to make the motor work stably, the output torque of the motor cannot exceed the maximum stall torque. The latter is also known as peak stall torque. The armature current corresponding to the peak stall torque is called the peak stall current. If the armature current exceeds the peak stall current, the motor is demagnetized and needs to be re-magnetized to function properly. In order to reduce the pulsation of torque and speed , the stator adopts multiple pairs of magnetic poles , and the armature adopts more slots, commutation pieces and series conductors. The DC torque motor adopts a flat structure with a large inner hole, which is beneficial for the motor to be directly placed on the load shaft, which improves the coupling stiffness of the system, enables the system to respond quickly, widen the frequency band, work stably, and meet the requirements of dynamic performance.

The overall structure of the DC torque motor can be divided into two types: split type and assembled type. The sub-assembly type is composed of stator, armature and brush holder , and the rest of the supporting parts are selected by the user according to the installation requirements of the whole machine . The assembly type is the same as the general motor, and the motor is an independent whole.

In order to overcome the disadvantages of DC torque motors using brushes and commutators , a brushless DC torque motor was developed in the late 1970s . Its principle and structure are the same as those of brushless DC motors.

AC torque motor

Its basic requirements are the same as those of AC servo motors. A wide speed range and softer mechanical characteristics 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 larger, and the stall current is much smaller.

The AC torque motor generally adopts a cage rotor structure, and obtains a low speed by increasing the number of pole pairs . It mainly operates in a state of heavy load and low speed , and can also be operated in a stalled state for a short or long time.

The speed control of the AC torque motor is realized by two methods: the speed feedback of the tachomotor and the control of the tension.

The operating principle of the AC torque motor is exactly the same as that of the general squirrel cage asynchronous motor, but the structure is different. It uses materials with high resistivity (such as brass, constantan , etc. ) as the guide bars and terminals of the rotor. ring. Therefore, the rotor resistance of a torque motor is much greater than that of a normal squirrel-cage motor. The mechanical characteristics of torque motors are different from ordinary squirrel cage motors.

Due to different design parameters, the mechanical characteristics and uses of torque motors are divided into two categories : torque motors with winding characteristics and torque motors with constant torque characteristics. Winding characteristic torque motor, realizing constant tension transmission, suitable for occasions that need to maintain constant tension of the product and wind the product on the roller at a constant line speed during the production process, for example, on printing and dyeing machinery, winding fabrics Roller, as the fabric is wound onto the roller, the diameter of the roller gradually increases and the load increases accordingly. It can be seen from the mechanical characteristics of the torque motor that when the load increases, the speed automatically decreases, thereby maintaining a certain line speed and tension, and achieving the process requirements. This kind of torque motor is often used in textile, printing and dyeing, paper making, rubber, wire and cable, etc.

The constant torque characteristic torque motor can make the torque basically constant in a generally wide speed range, and is suitable for transmission occasions that require constant torque when the two speeds change. For example, in printing and dyeing machinery, when several rollers are used to convey the fabric, since the fabric is not wound on the rollers, but only attached to the surface of the rollers and driven by them, the diameter of the rollers does not change. The motor with constant torque characteristic is used to ensure that the torque is constant and the tension of the fabric is constant at any speed.

The torque motor allows long-term low-speed operation ( even if it is blocked ) . Its heat is very serious, and an external blower is usually used to force air cooling. When using the torque motor, please pay attention to check whether the operation of the blower is good, and there should be a good ventilation environment around it, and dry and inflammable materials are not allowed. Inflammable dust or volatile combustible oils, etc. are near.

Due to the different use conditions, the types and specifications of the materials wound or transmitted by the machinery driven by the torque motor are different, and the required tension is different. It is required to adjust the torque of the torque motor, or to change the speed within a certain range, usually by adjusting the applied torque. voltage on the motor to achieve these requirements. To change the input voltage of the torque motor, a voltage regulator is usually used. In the case where the hardness of the mechanical characteristics and the adjustment accuracy are required to be improved, the thyristor speed negative feedback control circuit can also be used for stepless speed regulation, but the system is more complicated.

The torque motor can also be flexibly applied according to its various characteristics : it can partially replace the DC motor, it can be used in the opening and closing gate ( valve ) door and the drag system with large resistance torque, and it can also be used in the device with frequent positive and negative rotation Or other similar actions on various machines.