Asynchronous machines - Scalar control

Because of its low cost and robustness, the asynchronous machine is currently the most widely used machine for variable speed drives. The squirrel-cage asynchronous machine is structurally inferior to DC and synchronous machines. The fact that the machine is powered by a single armature means that the same current creates both flux and torque, so that variations in torque cause variations in flux. This type of coupling gives the asynchronous machine a completely non-linear model, which makes control of this machine very complex. A great deal of work has gone into developing efficient controls for the asynchronous cage machine.

Scalar control, the oldest and most rustic of the two, corresponds to applications requiring only average static and dynamic performance. Many drives equipped with this control mode are used, in particular, for industrial pumping, air-conditioning and ventilation applications. The corresponding installed powers are considerable.

Scalar control of the asynchronous machine consists of imposing the modulus of the voltage or current and the pulsation on the armature terminals. This is the simplest control method, but also the least efficient, especially at low operating speeds. However, at the installed power level, most drives don't warrant very high control performance. For drives whose speed range does not exceed a ratio of 3 or 4 between extreme speeds (ventilation, air-conditioning, centrifuges...) and for which there is no operation at very low speed and high load torque, scalar control gives satisfactory performance. It is therefore important to provide some information on asynchronous drives with scalar control.