Speed Control and Drive Classification are the Drivers where the driving motor runs at a nearly fixed speed are known as Constant Speed or Single Speed Drives. Multi-speed drives are those which operate at discrete speed settings. Drives needing stepless change in speed and multispeed drives are called Variable Speed Drives. When a number of motors are fed from a common converter, or when a load is driven by more than one motor, the drive is termed as multi-motor drive.
Speed range of a variable speed drive depends on the application. In some applications it can be from rated speed to 10% of rated speed. In some other applications, speed control above rated speed is also desired, and the ratio of maximum to minimum speed can be as high as 200. There are also applications where the speed range is as low as from rated speed to 80% of rated speed.
A variable speed drive is called constant torque drive if the drive’s maximum torque capability does not change with a change in speed setting. The corresponding mode (or region) of operation is called Constant Torque Mode. It must be noted that the term ‘Constant Torque’ refers to maximum torque capability of the drive and not to the actual output torque, which may vary from no load to full load torque. The Constant Power Drive and Constant Power Mode (or region) are defined in the same way.
Ideally it is desired that for a given speed setting, the motor speed should remain constant as load torque is changed from no load to full load. In practice, speed drops with an increase in the load torque. Quality of a speed control system is measured in terms of speed-regulation which is defined as
If open-loop control fails to provide the desired speed regulation, drive is operated as a closed-loop speed control system.