Controlling a dc-motor by keeping electrical power input constant
Generally motors are controlled for the purpose of controlling the driven machine or process. The control mode is determined by the characteristics of the process and some desired performance. Controlling motor speed usually provides the desired performance. Controlling motor torque is sometimes desirable, but there is usually an outer process-variable control loop or even a speed loop when torque is controlled. When speed is controlled, there may also be an outer process-variable control loop. The key concept is that the motor and motor-control can not be viewed aa an independent system. It is only half of the equation.

The relationship between input power and the ultimate process performance rarely results in a need for controlling input power. Input power has been sometimes been controlled in mixing processes. If the objective is to thoroughly mix a batch of material that starts out quite viscous and thins out as the mixing progresses it may be desirable to let the impeller operate as fast as it can without heating up the material too much. In that case, an operating power is set and the mixer gradually increases speed as it thins out and gets easier to mix. When mixing starts, the torque is high and the speed is low. As the mix thins, torque decreases and speed increases, but the power remains constant.

There are other constant power operations such as winding material onto a spool that is on a driven axel and cutting material with a machine tool using tools of various diameters. In those cases, the motor experiences decreasing torque with increasing speed, but other process parameters are used to set the motor speed or torque.