How To Size A Variable Speed Drive

VSDs (Variable Speed Drives) come in a bewildering array of sizes and alternative names. They are sometimes also called Variable Frequency Drives, Adjustable Speed or Frequency Drives, Microdrives, AC Drives and Inverters. Even if you limit yourself to the one name, the more important issue involves sizing a Variable Speed Drive for the appropriate application. At Rowse Automation we offer a full range of VSDs in many size and voltage permutations, and we are always happy to help you choose the right product for your needs.

What Is A VSD?

A VSD is a control mechanism that changes the speed of an electric motor by varying its input voltage and the frequency at which it is supplied. This frequency (measured in hertz) relates directly to the speed of the motors rotation, which is measured in RPM (Revolutions Per Minute), so the RPM increases in direct correlation to the magnitude of the frequency.

Put simply, a VSD increases or decreases the motor speed in accordance with the requirements of the application it is intended to drive, so the first step to sizing a Variable Speed Drive is to determine what application you need it for, and the speed at which its motor needs to run. VSDs are used for all sorts of applications, including compressors, pumps, fans, heaters, coolers and ventilators, as they help to maintain flow, level, pressure etc. They are designed for specific frequency conversion ratios, so it is important to recognise that a VSD produces neither torque nor horsepower, but simply supplies current. With the correct frequency and voltage, this current makes the application's motor turn at the desired speed.