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How Variable Frequency Drives (VFDs) Can Save Energy!

I wanted to discuss Variable Frequency Drive (VFD) technology and how energy savings can be utilized in certain applications. Electric motors, specifically induction motors, account for a significant amount of energy consumptions in industrially developed nations and large developing nations. In fact, statistics shows that electric motors are generally responsible for about 66% of industrial power consumption

in each nation, or about 40% of overall power consumption. VFDs can be utilized to reduce energy consumption for many residential, commercial, and industrial processes. VFDs can provide energy savings by utilizing the affinity laws for variable speed applications. However, constate horsepower loads offer little to no energy savings due to their constant speed application.

Benefits of VFDs:

· Energy saving opportunities through constant torque applications where some constant speed systems include reciprocating compressor, conveyors hoists, positive displacement pumps where 50% speed reduction equates to a 50% power reduction: a linear speed to power relationship.

· Energy saving opportunities through variable torque applications. Common applications for a variable torque type load include fans and blowers (primary air fans, cooling tower fans, ventilation fans, dryer fans, etc.) and centrifugal pumps (chilled water pumps, pressure boosting pumps, cooling tower pumps, wastewater pumps, etc.). Variable torque applications abide by the affinity laws. The affinity law for a variable type of load is given by, Power α Pressure× Flow, where Pressure α Speed^2; and Flow α Speed. Therefore, Power α Speed^3. Therefore, from the affinity laws for a variable type of load, a fifty percent reduction in speed reduces the energy consumption by 87.5%.

· Example: When a centrifugal fan or pump is used with mechanical flow control, converting the application to VFD can save energy if the fan or pump is designed to operate between 40% to 80% of full flow. The same principal can be applied to variable speed HVAC systems if heating and cooling loads don’t require the compressor and fan to run at full load 100% of the time.

Disadvantages of VFDs:

· Little to no energy saving opportunities through constant power applications such grinders and winders. VFDs are not always the silver bullet for energy savings. For example, a properly sized pump always running at constant pressure and flow continuously, then a VFD is not beneficial. In this application, a traditional motor starter, or soft start (for in-rush and mechanical benefits), is more practical.

· Constant-speed motors will usually benefit from a soft start controller instead of a VFD. The benefit from a soft start is that it will eliminate the in-rush current, which significantly reduces the heat build-up thereby extending motor lifetimes—particularly of those motors which are stopped and started frequently.

· VFDs can cause damage to AC motors from increases in shaft currents. VFDs creates damaging voltages on the motor shaft unless they find discharge pathways to ground. There are serval ways to mitigate damage to shaft bearings, thus increasing rate of premature failure, from the increased shaft currents.

There are plenty of opportunities where VFDs can reduce energy consumption and save money. However, not all applications will benefit from installing VFDs and may actually end up costing more money in the long run.

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