Streamline - Winter 2011 - (Page 25)

Variable Frequency Drives Can Save You Money WOULD A SAVINGS of $0.03/kWh help your bottom line? Would reducing the frequency of replac- ing motors reduce your overall operating costs and reduce the necessary manpower requirements? A variable-frequency drive (VFD) is a device that allows a motor to reduce the starting current, run at lower frequencies that the power company provides and control the speed of the motor to operate only at the speed the load requires. We buy electricity by the kilowatt-hour, a measurement of electrical power over time. If we can start the motor with less current, we are saving power, thus reducing energy costs. If we run the motor at only enough speed to move the load, we are saving power. And if we reduce the frequency of the applied electricity, we reduce the average current, again saving power. All of these savings of electrical power are functions of using a variable frequency drive. The primary objection to VFDs is initial cost. A VFD cost can be calculated roughly at $100/hp, which would make a VFD for a 10 hp motor cost around $1,000. However, if using a VFD gives us twice the life of this $1,000 motor and reduces the power usage by 16 percent, the payback is quite short. So, how does a VFD work? An electric motor will draw approximately three times the run current to provide the initial torque when first starting. This quickly comes down to normal run current as the motor comes up to operating speed. A VFD starts the motor at a greatly reduced speed and increases the speed as the load is started. This reduces the amount of electrical energy used each time the motor starts. Think of a typical wastewater lift station or a pressure booster pump on a water distribution system. Each of them start and stop the pumps numerous times each day. The VFD can be set to adjust the speed that the motor runs to only that speed required to meet the need. Electric motors are usually BY FRANK NADEAU WASTEWATER TECHNICIAN I The U.S. Department of Energy estimates that 89 percent of the electric motors used in the United States are oversized. single-speed devices by themselves, but a VFD makes their speed variable. The U.S. Department of Energy estimates that 89 percent of the electric motors used in the United States are oversized. Engineers size motors to meet the maximum load that can be put on the motor with the result that many motors operate much less efficiently during normal operation. If by the use of a VFD we can only use the amount of power necessary to function closer to the maximum efficiency of the motor, we can save electric power. A VFD accomplishes this by reducing the frequency of the power going to the motor, thus reducing the average current, reducing the power and saving us money. The normal frequency of electrical power used in the U.S. is 60 Hz. A VFD can reduce that frequency down to close to half-frequency. I know this all sounds very technical and maybe even far-fetched, but it’s real and can help change your bottom line from red to black. 25

Table of Contents for the Digital Edition of Streamline - Winter 2011

From the President
From the Executive Director
Farm Tours for Source Water Protection
Preparing Yourself for an Emergency Response
One Step Ahead: Tappahannock Wastewater Treatment Plant Upgrade
Improving Virginia Waters – VACS
Variable Frequency Drives Can Save You Money
City of Emporia Water Plant Upgrade
The Woodstock Wastewater Treatment Plant
Bird Contamination and Your Water Tank
VRWA 24th Annual Exposition Preview
Throwing My Loop
eLearning Benefi ts
Membership Application
Do You Know What Your VRWA Benefi ts Are?
VRWA Mailbag
Welcoming New Members
Training Calendar
Board Of Directors
VRWA Committees
Index To Advertisers/Ad.Com

Streamline - Winter 2011