Rural Water - Quarter 3, 2015 - (Page 26)

Variable Frequency Drives d A Great Strategy for Energy Savings and Process Improvement BY lOnnIe ruSSell, CertIFIeD energY Manager, SOuth CarOlIna rural water aSSOCIatIOn During the course of performing energy audits on water and wastewater systems, I frequently recommend installing Variable Frequency Drives (VFDs). Sometimes there is resistance to this, but often it is based on horror stories of the VFDs of yesteryear. I hope, in this article, to help alleviate those fears and show that VFDs are an excellent business choice for more reasons than just energy efficiency. Throttle with a VFD, not a valve or damper If a pump is pumping more water than is wanted, typically a valve is partially closed to limit the flow. A good analogy to this is driving your car at 3,000 rpm and controlling speed with the brake. VFDs control the speed and torque of an AC induction motor by controlling the frequency and voltage supplied to the motor (without a VFD, full voltage and frequency are always applied to the motor). This would be the same as using the accelerator to control the speed of the car verses the brake. This is much more efficient! How much more efficient? A natural assumption is that if the speed of a motor driven pump is reduced by 50%, the power used is also reduced by 50%. This direct relationship is true for the flow (50% reduction in motor speed reduces the flow 50%) but not for power. The relationship of speed to power is a cubic one (power of 3). This means that if speed (and flow) is reduced by 50%, power would be reduced by 87.5%. This is theoretical, but actual savings usually are to the 2.2 power (instead of 3), so the reduction would be 78%. That is still pretty impressive. If this math escapes you or you hate exponents, do not fear, the chart below makes it easy to understand. Pump Affinity Laws Wheel Velocity Constant, Wheel Diameter Changing 9 8 Pressure, Flow, Power 7 6 flow pressure power 5 4 3 2 1 0 0 0.5 1 Relative Wheel Diameter 26 thIrD Quarter 2015 1.5 2

Table of Contents for the Digital Edition of Rural Water - Quarter 3, 2015

From the President
Innovations for Every Utility
What’s Ahead for Water Technology?
Is Your Utility Prepared to Fight Misinformation on Social Media?
Variable Frequency Drives
Available EPA Tools and Resources for Small and Rural Utilities
Rural Water Assists Mount Zion with Water Loss, Finances
Regulatory Update
Throwing My Loop
Index to Advertisers/
From the CEO

Rural Water - Quarter 3, 2015