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Energy Savings For Medium Voltage Applications Using VFDs.

Years ago, we sold thousands of drives in the HVAC market retrofitting existing across-the-line applications. VFDs were “Green”, and we rode that wave as long as we could. At the time many across-the-line applications were still out there in commercial HVAC, so the payback in energy savings paid for the drive in a very short amount of time (our sales guys LOVED those calculations). Nowadays, very few commercial HVAC systems are sold new without VFDs and most of the old ones have been retrofitted by companies like EMA. It is a well-known fact within that industry that VFDs save money and commercial building owners have taken advantage of it for years. However, there are many more applications where the same principles apply, yet drives are not as commonly used. Medium voltage applications, like HVAC, are comprised primarily of fans and pumps, which means the energy savings potential is massive.

The basic affinity laws (which is a mechanical concept) of centrifugal loads help to explain it better:

  1. Flow is directly proportional to speed.
    • This makes sense if you think about it. If you want more liquid or airflow out of the motor, you increase the speed. If you want less flow, you decrease the speed; it is a direct relationship.
  2. Torque & Pressure is directly proportional to the square of the speed.
  3. Power is directly proportional to the cube of the speed.

In a nutshell this tells us: with variable torque loads, a small reduction in speed is a massive reduction in the power consumed.

Reminder: This only applies to variable torque loads; the affinity laws of centrifugal loads does not apply to constant torque applications.

If the initial engineer did his job right, the motor was sized for maximum load demand. However, many centrifugal applications rarely need/use maximum demand. Instead of using mechanical methods to restrict or bypass flow, the motor can be slowed down using a VFD to meet the exact demand requirements. The results are not just better process control, but major energy savings!

The Numbers Don’t Lie:

Let’s use an example of a fan application on a 2000 HP, 4160V motor running 24 hours a day, 6 days a week. By simply reducing the speed 10%, the projected energy savings are $228,753 per year (this is based on the national industrial average price of 8.10 cent/kwh per Electric Power Monthly). If you reduce that to 85%, the savings increase to $335,139! Using a liberal estimate project cost of $500,000 to purchase and install a new 2000 HP drive, we’re looking at a payback in under 2 years!

Note: I used energy savings predictor software for these calculations, similar to one available at Yaskawa’s website. Actual savings will vary depending on load and motor characteristics.

Peak Demand:

Many utilities will also charge “peak demand” prices where they charge users based of the highest power usage within any demand interval. Basically, if you go to a certain power usage, even for a short amount of time, you’re billed like you’re using that power for the entire interval (this sounds like a rip off). If you use across-the-line motors, starting them up (especially at the same time) will cause a massive, instantaneous current draw, increasing your peak demand pricing. Utilizing a drive not only saves energy by slowing the motor down, it also reduces potential peak demand charges as well by “softly starting” the motor; this is a very common application for medium voltage variable frequency drives.

Bonus Mechanical Advantages:

Aside from energy savings on medium voltage variable frequency drives running centrifugal fans and pumps, there are mechanical advantages to using VFDs on any type of load. The above mentioned softstart not only reduces inrush current, it also reduces wear and tear on seals, couplings, belts, and all other mechanical elements that are stressed by the sudden across-the-line start. Medium voltage motors are rated for only a certain number of starts per hour, which is eliminated using VFDs, reducing downtime.

You Can Benefit

Could you benefit from the energy savings potential of a medium voltage VFD? Let us help you find out! At EMA we deal with VFDs all day, every day and have a large stock of new and refurbished medium voltage variable frequency drives ready to roll!


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