How does the inverter control the torque of motor

In most of the tape processing machines like slitting machine, rewinding machine, laminating machine, spooling machine and etc ac motor is used. Hove you ever thought about a question that as long as the tape roll diameter is changing, how to keep a constant line speed of the machine? And also when unwinding and winding motor revolution speed changes, how to control the torque of the motor which effect the tension control? In this article, we will make a brife introduction to above questions.

There are two ways to control the tension of the inverter. One is to control the output torque of the motor, and the other is to control the speed of the motor. The open-loop control mode of the inverter conforms to the first method, and it does not require tension feedback. The so-called "open loop" means that there is no tension feedback signal to the inverter. The inverter controls the output frequency or torque to achieve the control purpose, which has nothing to do with whether there is an encoder. The torque control mode means that the inverter controls the motor torque instead of the frequency, and the output frequency changes automatically according to the speed.

If the torque of the reel changes according to the change of the reel diameter, the tension of the material can be controlled. This is the basic mode of open-loop tension control.

The inverter can accurately control the output torque of the machine in the closed-loop vector control mode. In this control mode, the VED must be installed with an encoder (the inverter has a PG card). The closed-loop tension control mode adds a tension feedback regulator on the basis of the open loop. The tension feedback signal and the tension set value are used to form a PID closed-loop control to adjust the torque command output by the VFD to achieve higher-precision tension control.