In the right applications, VSDs save you a bundle

April 20, 2004
Do VSDs always generate energy savings?

Motor-driven equipment accounts for 64% of the electricity used in the U.S. industrial sector, according to the U.S. Department of Energy (DOE). Motors that run at fixed speeds regardless of actual demand waste energy. Variable-speed drives (VSDs) reduce energy consumption as much as 50-70% at partial loads by matching motor torque and speed to the load. They also provide soft-start features that reduce motor inrush currents and wear during startup.

Do VSDs always generate energy savings? A VSD is a system modification. Improperly implemented, it can increase energy consumption. For example, a process that requires continuous, full-rating output from a motor/load system can require more energy with a VSD.

What systems are best suited for VSDs? Any load with throttled output should be evaluated for VSD application. VSD retrofits will yield the quickest return on investment for loads that operate for longer periods, are highly throttled, and operate more of the time at throttled output.

The majority of VSDs are installed on loads where torque increases with speed. Constant-torque loads may also be suitable, although the VSD must be properly sized to ensure adequate starting torque. Custom-engineered solutions that manage heat may be necessary for loads where torque decreases as speed increases.

In multiple-motor applications, a combination of fixed drives and VSDs can be the most cost-effective solution. Using a standard-speed motor for the minimum base load saves energy while minimizing VSD equipment, harmonics filter, overhead and maintenance costs.

What tools can help me determine the level of savings I'll achieve?

    • The Bonneville Power Administration's new tool that calculates energy savings from installing ASDs on pumps and fans is a complement to their Industrial Audit Guidebook.
    • MotorMaster+ software, which the Washington State University Energy Program developed and the DOE funded, analyzes motor and motor system efficiency. Version 4.0 includes variable load calculations, which provide the data necessary for variable-speed drive retrofit analyses.
    • ASDMaster from the Electric Power Research Institute (EPRI) examines the entire system (electrical supply, motor, driven load, process, etc.) to aid in applying ASDs from a total system perspective.

For more information, see:

E-mail Contributing Editor Sheila Kennedy, Managing Director, Additive Communications, at [email protected] .

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