However, in some applications it is useful to control the torque output. Torque is the rotational equivalent of force, and at a fixed speed the more torque, the more power.
To control torque, the drive measures the output current, separates the current into the magnetising (flux generating) and load components, and then varies the output frequency to maintain the load current constant (keeping the magnetising current constant).
So a torque control system is a closed loop control system similar to those described in an earlier article, and therefore has adjustable parameters such as gain and integral settings. Torque control is more easily operated using Vector Control, as Vector Control uses Torque and Flux control systems in any case.
Torque control is used in applications such as winding (to control tension) and similar applications. Torque control is very useful to share loading among motors which are mechanically linked, such as on a long conveyer. The first drive is operated in normal frequency control, subsequent drives run in Torque control, taking the Torque output of the master drive (via the Analogue output for example) as a reference. So all the drives run with the same torque.
An easy way to do this is to use Invertek’s master/slave system. This will ensure all drives run with either the same frequency or Torque, simply by connecting the master to the slaves via an Ethernet cable and setting a couple of parameters. The frequency or Torque can be scaled for each slave, and local trim controls can be set up using the analogue input (See application note AN-ODP-2-027).