How Can Variable Frequency Drives Save You Money—or Not?

EnergywithwordsIt’s been said that, if all you have is a hammer, everything looks like a nail. When applied to energy, these essentially means that we need to understand that every motor does not require a drive. So while a VFD will oftentimes save a significant amount of energy and money, this isn’t always the case.


What is a variable frequency drive?

Variable frequency drives (VFDs) are used to vary the speed of electric motors by changing the frequency, which is measured in hertz (Hz), of the electric power feeding the motor. VFDs only work on three-phase power. Motor speed, which is measured in rotations per minute (RPM), changes in direct proportion to the Hz that is supplied to the motor. For example, if the frequency of the motor is measured at 60 Hz, the motor speed is 1,800 RPM. If the frequency is only 30 Hz, the motor speed is 900 RPM.


How does a VFD save energy and money?

Motor power consumption varies as a function of the cube of the motor speed. For example, a 100 horsepower (hp) motor operating at 1/2 speed consumes only 12.5 hp or 1/8 of the normal wattage. Here’s the math:


Power consumption = 100 hp x (1/2)3

= 100 hp x 1/8

= 12.5 hp


By varying the motor frequency, and therefore motor speed, with a VFD, you are able to cut down on motor power consumption and save energy and money.


Tips, tricks and considerations:

VFDs are beneficial in various applications, including:

  • Air and water moving systems that have a wide range of flow required that would allow the motor to operate at reduced speed for extended periods of time.
  • Situations where a soft start is preferred to avoid inrush current and wear and tear on belts, bearings and motors.
  • In some cases to balance the total air delivered on constant volume systems. This can be a worthwhile strategy if constant volume and space pressurization is important. VFDs can handle any variations in system pressures, such as clean vs. dirty filters. Otherwise VFDs can be a very expensive balancing tool.


However, some applications of VFDs should be avoided, including:

  • Constant volume air or water systems where there is only one start/stop per day.
  • Undersized systems that are required to operate near 100% capacity at all times.
  • Installation on a non-inverter duty motor. A high-efficiency motor will be required and (to some degree) matched with the VFD.
  • With motors and bearings that are not grounded properly, which could cause arcing at the bearings and considerably reduce the bearing life.


Rules of thumb (be careful with this as every application is different):

  • A typical office building operates 3,432 hours per year (12 hours/day x 5.5 days/week x 52 weeks/year).
  • For every 10 hp reduction in load, there is a corresponding savings of approximately 28,447 kWh and $2,276 in an average office building.


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