Variable Frequency Drives (VFDs)/Variable Speed Drives (VSDs)/Motor Control

One of the main advantages of using a VFD/VSD is the energy cost savings associated with speed control. In many applications, a motor runs at full speed; however, a VFD reduces the actual motor speed, which reduces the amps drawn by the motor. This reduces the amount of energy used—and saves you energy costs. By ramping up the motor speed slowly, VFDs also help customers save energy by reducing the in-rush and mechanical issues associated with starting motors across the line.

VFD/VSD Applications:

  • Pumps - fountains and water features (pools, spas and whirlpools)
  • Compressors
  • HVAC
  • Any machine or process that can be improved by varying speed or flow is a candidate for a VFD

When you are looking for energy efficiency and deep energy savings in your plant or operations, employing VSD / VFD technology will make tremendous impact in your energy efficiency strategy. It is very possible to get a Return on Investment of 2 years or less. When you couple the ROI with the reality that you have also upgrade to NEW EQUIPMENT, you can realize significant improvement to the bottom line.

What is VSD vs. VFD?

While the terms are somewhat interchangeable, the end result is very similar. Following is a basic overview of the basic information. For more in depth detail, Wikipedia has a great comprehensive article about VSD concepts.

VSD =Variable Speed Drive essentially means ANYTHING that varies the speed of a motor driven load,  Normally this term is for DC drive motors, but can apply to AC as well. There are several ways to accomplish this end goal, either by controlling the actual motor directly, or post drive mechanical devices that vary the output. Here are some examples:

  • AC VFD
  • DC drive
  • LSI drive
  • Cycloconverter drive
  • Mechanical variable pulley drive (Reeves Drive or Varidrive)
  • Eddy Current Clutch drive or even a hydraulic variable speed drive.

VFD = Varible Frequency Drive, a small subset of  VSDs that change the speed of AC induction motors by artificially altering the frequency and voltage together. There are further sub-divisions (mostly PWM Inverter drives for example), and then under that there are even more!

Without getting too complicated: All VFDs are VSDs, but not all VSDs are VFDs. Therefore we will use the term VFD.

Contact Ecogreen Solutions and discuss your VSD/VFD requirements

 Why use VFD Technology?

Simply put, by most any measurement using a variable-speed AC drive is one of the most effective ways to save energy. According to many manufacturers of VSD and VFD equipment, case studies prove out  that variable-speed drives frequently save double digit percentages on energy use over their non VFD counterparts.

It is estimated that electric motors use two-thirds of all electricity in industry. That is a BIG number. Obviously, any opportunity to reduce this usage at all is a great opportunity for significant energy cost savings. The best opportunities for savings with VFD technology are:

  • Fans and pumps: For example, replacing existing throttling valves and dampers with VSDs.
  • Conveyors: VSD can adjust to the changing load requirements when the flow of demand varies.
  • Compressors and chillers: Most compressors can take advantage of the energy savings associated with variable flow demand.

Example: VSDs and compressors

Following is a great description of a compressor scenario from ABB.

VSDs reduce the energy output of a compressor, by controlling the speed of the motor, ensuring it runs no faster than necessary. The traditional way of controlling a compressor is by running the motor at full speed and then stopping it when the air has been compressed to the correct pressure. It is then stored in a reservoir at a slightly higher pressure than is needed, to allow a hysteresis in the pressure. This “on-off” method is wasteful, because the motor keeps running at its nominal speed regardless of the requirement. Some compressors are designed with a bypass system which returns air from output to input, which is again wasteful.

It is not unusual for users to dismiss the promise of 50 percent energy saving on a 20 percent speed reduction as the exaggerated claims of a manufacturer. However the savings can be verified and the best way to start is with an energy survey. This will enable you to see the potential savings in black and white, enabling you to make the decisions that bring your company improved profitability.


HVAC  makes a constant use of fans AND compressors and are fantastic opportunities for big savings. It is not out of reach to see a 60% energy savings compared to conventional control methods when using this technology. Coupling HVAC improvements with other strategies such as window tinting, and other reductive techniques, you can increase savings throughout the whole system.

VFD and HVAC Systems

b 3 sustainability presentation Prepared by Alan Richardson