What is torque control?
Hello there,
(For ac motor speed control systems)
What is torque control?
In ac motor control inverter drive systems, regardless of the motor speed, it is called torque control, which is mainly closed-loop working in order to keep the total torque of the drawn load constant, and can exhibit a high holding performance up to 200% even at zero speed.
Its biggest advantage is the fast torque response. Torque control is done together with the motor + load combination.
For a variable frequency controlled drive to control torque, it must measure the total motor current, separate the load current and the magnetizing current, and adjust the motor speed to maintain a constant load current, assuming the magnetizing current remains the same.
Note: Dear friends, if you truly want to understand torque control, please study the example below carefully. Don't let your mind be confused by urban legends and other inconsistent information on this topic. Our website operates for the purpose of maximum benefit.
Unlike a simple frequency controller, this is a closed-loop control system; that is, we have to know if it has reached a setpoint or value that we want. In this case, a torque reference calculated from the load current and magnetizing current will act on the output current.
The driver compares both and adjusts the output frequency accordingly. So it can be considered to work like any loop system but it is not exactly the same control format. As we explained earlier in vector control system, there are gain and integral factors to be used to stabilize the control system.
Torque control can work in V/F control to vector drives, but it will be much more successful if we run the drive in Vector control. A previous article on vector control explained how torque and flux control are important parts in a vector control system and that makes sense too.
In Vector Control settings, although load calculations are not usually required, there are several load adjustments we can make to stabilize torque control.
One factor that can be important is to give the driver an idea of the load's moment of inertia. If we are trying to control the torque applied to a high inertia system such as a flywheel, the control parameters (gain, integral factors) will be different from those of a small winder system.
The usual way to give this information to the driver is to enter the ratio of the motor to the load inertia. This is a difficult number to calculate or even predict, but by adjusting it (upward in case of high inertia applications) system stability can be easily improved.
The torque control formation is not only used for unwinding type applications. Torque sharing on long conveyors allows load sharing between motors and drives. Or, if there are more than one motor connected to the same shaft, it is mandatory to use torque-controlled drivers for the motors not to draw excessive current and for healthy control.
However, in a few applications it is sometimes useful to control the torque of the motor. Torque is the rotational equivalent of linear force; the more torque at a given speed, the more power you get.
Torque, Speed and Power are related as:
Power = Torque × Speed × 2π / 60
Where the speed is in rpm. Both electrical and mechanical losses must also be taken into account. The torque produced in an electric motor is a product of flux (magnetizing current) and load current. If there is flux, the load current increases and the torque increases as you load the motor.
Although there are many ac drives on our website, we have a model that can actually control torque (see application devices)