How Does a VFD Save Energy and Money?

Provided by our friends at The GreenBuildTV Story VFDs (Variable Frequency Drives) are basically a green energy savings product that matches the amount of work or load on a motor to the amount of energy it needs to power that amount of work. This reduces excess energy from being wasted. Watch video, or continue reading after the break.

We use a lot of energy in this country and most of that energy is used to move air and water around a building. About half the electricity in commercial buildings is just used to move air and water around, so a VFD is a big way to save energy there. If you look at a typical pump motor the life cycle cost of a pump, 90% of its life cycle costs is the energy it consumes and only 10% the actual cost of the pump motor… a VFD can cut that in half.

Most motors are oversized to deal with your worst-case scenarios- your peak loads. A VFD allows you to run that motor at the load it needs to be instead of running it at peak load all the time. Another benefit is it has a built-in soft start capability. So those combinations of things are going to give you savings; not only on energy, but also extending the life of the motor.

So what is a VFD? Some of the different names that are used in the industry are a variable frequency drive, an adjustable frequency drive, a variable speed drive, or an adjustable speed drive. The technology has been around for quite a few years but only has started to make some headway in HVAC and pumping applications in the last several years. The size and cost of electronics has made VFDs applicable to a wider range of motors and increased the opportunity for savings.

All VFDs are going to take 3 phase AC power and convert that 3 phase power to DC power inside the drive and pulse it out in a simulated AC wave form to the motor. The motor still thinks it has AC power but the DC power conversion now lets us control the speed of the motor without harming it. Now we are in control to save energy and money.

The basic concept with the savings for VFDs is your speed and your flow are more or less proportional. But the energy consumption is cubed. If you’re running your motor at full speed 60 Hertz, you don’t have any savings- but any reduction pays the reduction cubed.

But if you’re even able to take that oversized motor down to 90%, maybe even still running at a constant volume let’s say, cause you haven’t changed out the entire system; but you just don’t need to run at full speed most of the year. However, if you can run at 54 Hertz or a 10% slow down output from the VFD, your savings is 27%. How is that? Well, if you cube 90% or .9 by multiplying .9 X .9 X .9 -the net result of that is .729. So now you’re only using 72.9% of the energy you were using. The difference between that and 100% is your savings so with a 10% reduction in speed you save 27%. If you’re at half speed or 30 Hertz it’s even more. Multiplying .5 X .5 X .5 is .125 so you’re only using 12.5% of the electricity. Below this you’re really not going to get any significant additional savings.

What a variable frequency drive does is match the amount of energy your motor needs to match the amount of work that is being done. This saves energy- a lot of energy! Great applications are air handling units ,industrial process cooling pumps, chilled water pumps, hot water circulation pumps, cooling tower fans, return air fans, chillers, air compressors, circulation and supply pumps, combustion blower fans, injection molding machines- there a lot of good applications and you might even qualify to get one with no investment and have it paid for out of savings through a Green Energy NegaWatt Savings Agreement. The first step is for an energy service consultant to review your various motor opportunities and get an audit to determine savings.

Tags: energy efficiency, net zero energy, variable frequency drive, VFD

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