How To Size A Variable Speed Drive

Post By: Tom Rowse On: 26-09-2019 - Automation & Control

Variable Speed Drives

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 motor’s 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.


Sizing a Variable Speed Drive should therefore always be based on motor voltage and current, not torque or hp. This can be determined by reading the motor faceplate, which will have a chart displaying input voltage in volts (V) and full load amps (FLA). These specifications will often appear in two values, for example (V) 230/460 and (FLA) 13.4/6.7. This indicates that the motor will use 13.4 FLA when running at 230V AC, but only 6.7 FLA at 460V AC.

However, while the motor can be rated for more than one input voltage, it is the input voltage required for the application that is critical. VSDs are rated on one design-specific voltage range, such as 200-240V or 380-480V, so it is vital to assess the application for the amount of current it will draw and the speed at which it will be operating. A VSD also offers better overload protection for your motor, including overvoltage, undervoltage, phase protection, electrothermal overloads, etc.

Energy Efficiency

Older types of motor were designed to run at a constant speed and output, but modern technology has introduced many applications using variable speeds, especially to promote improved productivity and energy savings. Some such applications do not need to operate at full speed, so a VSD allows the speed of the equipment's motor to be matched to the required load, offering up to 70% energy saving. Running a motor at a more efficiently matched speed will also cut down on faults and equipment downtime, thus increasing overall production levels. Using a VSD to govern motors driving belts and conveyors makes the process smoother, eliminating jerky start-ups and sudden stops, and reducing wear on parts so that equipment will last longer.

Full Load Amps

The FLA rating reflects the number of amps that could be drawn by the motor when operating at full speed and full load. At a lower speed or reduced load, the actual amount of FLA drawn by the motor may be lower than that specified on the faceplate – but on the other hand it may be higher. It is vital when sizing a Variable Speed Drive that the motor FLA must also account for any additional overload required by the application, as such an additional overload rating could actually push the size requirement of the VSD into the next FLA bracket.

Service Factor Amps

The operational margin offered by a motor's service factor is sometimes taken into account in, for example, some pumping applications, where the service factor boost is necessary to get the pump to perform in certain conditions. If such conditions are so frequent as to render the service factor amps as normal operating conditions, then this measurement must be used to size the VSD. It will sometimes appear on the motor faceplate, but if not, it is critical for ensuring proper VSD sizing that the running amps be multiplied by the service factor. The VSD's overload rating will usually have the VSD running into its rating for intermittent output current (which it can do for 1 minute in every 10, depending on the manufacturer), so when sizing a VSD this overload operation should be treated as its continuous amp rating.

High Overload Usage

If the VSD is going to be subject to high overload usage, it is necessary to know for what application the motor will be used. More demanding applications such as cranes, crushers, conveyers, blowers, belt presses and positive displacement pumps operating at constant torque may require a much greater motor current overload, to overcome start-up inertia or during heavy load operations. In such cases, up to 150-160% overload of motor current may be demanded when the application is in an overload condition. When sizing the VSD, it is therefore critical to find out as much as possible about the motor's operating conditions, and to recognise that different terms are used by different manufacturers to indicate the overload condition, such as constant torque, high overload or industrial/automation VSD.

Normal or Standard Overload

Other applications may only demand 110-120% overload current (for 1 minute in 10), to overcome start-up inertia or during heavy operating conditions. Fans, centrifugal pumps, blowers or other loads with variable torque may require a standard or normal overload for pumping viscous materials or solids. Modern VSDs for such applications have variable torque measurements as their primary sizing.

Other Factors

There are many factors to take into consideration when sizing a Variable Speed Drive, such as the weather and temperature conditions in which the motor operates and what type of motor is used. Submersible motors, single-phase only power input and high-altitude installations all require different operating parameters, while extremes of heat and cold significantly affect the running temperature of the VSD. In these cases, no fail-safe rule applies, but at Rowse we are always happy to help you determine the appropriate sizing of your Variable Speed Drive for your application.

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