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Helpful Resources - VFDs

Variable Frequency Drives

A Variable Frequency Drive (VFD)[1] controls the speed of an AC induction motor by controlling the power (voltage and frequency) that supplies the motor. By reducing the motor speed to match the needs of the application, VFDs have the potential to significantly save energy and reduce operating costs for a variety of motor driven applications[2]. Motor systems that should be evaluated for energy savings through the use of VFDs include:

  • variable load motor systems where output is throttled or damped below full rated speed
  • centrifugal fan, pump, or blower systems
  • systems that meet either criteria above that operate frequently (e.g., > 2000 hrs/yr)

The potential energy savings derived from reducing a motor’s power to manage its speed are approximated by the Affinity Laws[3]. These engineering laws are used to express the relationship between flow, head, and consumed power in relation to shaft speed for pump and fan applications and can be summarized as follows:

  • change in flow is proportional to the change in shaft speed
  • change in head (pressure) is proportional to the square of the change in shaft speed
  • change in power consumed is proportional to the cube of the change in shaft speed

Energy Savings Example

To illustrate the relationship between motor speed and consumed power, assume a 50hp centrifugal pump (95% efficiency) operating 4,067 hours per year with a 75% load factor, and electric costs at $.07 / kWh, is using a throttling valve to regulate flow to 70% on average. Applying the third affinity law, the yearly electric cost of the motor running at full rated (100%) shaft speed (or flow) would be:

Annual Energy Cost (Throttling Valve) =

(hp/Emotor) * LF * 0.746 kW/hp * (% full rated shaft speed)3
* (annual operating hours) * (cost of electricity)

Or

50hp/0.93 * 0.75 * 0.746 kW/hp * (1.0)3 * 4,067 hrs/yr. * $.07/kWh = $8,564 per year

The same system is represented below, except an ASD (97% efficiency) replaces the throttling valve to achieve the same flow regulation by varying the motor's rotational speed.

Annual Energy Cost (ASD) =

(hp/Emotor) * LF * 0.746 kW/hp * (% full rated shaft speed)3
* (annual operating hours) * (cost of electricity) * (1/EASD)

Or

50hp/.93 * 0.75% * 0.746kW/hp * (0.7)3 * 4,067 hrs/yr. * $.07/kWh * 1/0.97 = $3,028 per year

Using the information from each scenario, potential savings are calculated: replacing the throttling valve with the ASD can achieve approximately 5,500 in annual energy cost savings. Nearly 65% energy cost savings is gained as a result of a 30% reduction in shaft speed (or flow). In other words, small reductions in speed and flow can lead to significant energy cost savings.

For additional summary information, please see CEE Motor Efficiency, Selection, and Management:  A Guidebook for Efficiency Programs.

 

VFD Energy Savings Resources

The following resources highlight the energy savings benefits of motor control through VFDs.

General Resources and Market Data

• “Application Guide for AC Adjustable Speed Drive Systems,” National Electrical Manufacturers Association (NEMA), 2001. Publication developed to assist users in proper selection and application of drives.

• “Best Practices: Motors, Pumps and Fans” and “Motors, Pumps, and Fans Publications”, U.S. Department of Energy EERE ITP, 2010. Links to tip sheets, sourcebooks, case studies, technical publications, and software, fact sheets, market assessments, and repair docs.

• “United States Industrial Motor Driven System Market Assessment: Charting a Roadmap to Energy Savings for Industry”, U.S. Department of Energy EERE ITP, 2009.  Summarizes US industrial motor system energy use by application, industry sector, large plant, etc.  The report additionally showcases savings achieved through DOE’s Motor Challenge program.

• “United States Industrial Electric Motor Systems Market Opportunities Assessment”, U.S. Department of Energy, 1998, revised 2002. A market overview of US industrial motor systems inventory, opportunities for energy savings, and motor system purchase and management practices.

• “Improving Motor and Drive System Performance: A Sourcebook for Industry”, U.S. Department of Energy, 2008. A technical overview of motor and drive systems, performance opportunities, costs and economics.

Other

• “VFDs – Energy Efficiency Reference Guide”, Natural Resources Canada, 2009. Technical application considerations including load characteristics and comparison with conventional control methods.

Drivepower, E Source, Technology Atlas Series, 1999. Comprehensive market and technical overview of motor and drive market structure and penetration, end-use data and program strategies, technology and efficiency, sizing, selection, maintenance and repair. Requires subscription.

Primary Industrial Control and Adjustable Speed Drive Index, NEMA, 2010. Relative reported sales activity of 90% of the U.S. domestic market from 2001-2010.

Variable Frequency Drives, NEEA, 2000. Comprehensive U.S. market study of VFD market penetration, sales, and opportunities by industry and application type.

Energy Savings Brochures, Fact Sheets and Video

Each of the following brochures provides a short introduction to VFD technology, including a definition and suitable applications for VFDs, as well as calculated energy savings examples and case studies.

General Applications

• “Control Your energy costs with VFDs”, Wisconsin Focus on Energy, 2009.

Fans and Pumps

• “Pumps Optimization: Energy Efficiency within Industrial Facilities”, BC Hydro Power Smart, 2011.

• “Use of Variable Frequency Drives for Fan and Pump Control”, Natural Resources Canada, 2002.

• “Energy Tips – Pumping Systems”, Pumping Systems Tip Sheet #11, U.S. Department of Energy ITP program, 2007.

Compressed Air

• “Compressed Air Guide to Savings”, Efficiency Vermont.

Energy Savings Calculators

VFD Tech Tool, New Jersey's Clean Energy Program. Spreadsheet that calculates overall facility-wide energy savings estimate for a range of facility types and multiple motors.

ASD Calculator, Bonneville Power Administration. Spreadsheet tool for estimating the energy savings and simple payback for ASD installations on fans and pumps.

Case Studies

Presentations

• “Variable Speed Drives Make Cents”, Hydraulic Institute (HI). Registration and download are free.

• “VFD Applications”, Ken Rackowski, Bluestone Energy Services, presented at NSTAR Technology Show, June, 2009. Introduction to drive technology, applications, benefits, controls.

Online Periodicals That Cover VFDs

Drivesmag - industry forum that provides specially developed content written by industry experts regarding electronic speed control of electric motors.

Electrical Construction & Maintenance - technical publication for electrical professionals in electrical contracting firms, industrial plants and commercial & institutional facilities.

Maintenance Technology - a resource for manufacturing, process and service operations for technical, business and professional-development needs of C&I engineers, managers and technicians.

Reliable Plant – Published by Noria Corp., provides plant maintenance and reliability leaders resources including technical tips, lean and best practices, safety, and energy management resources.

Plant Engineering / Control Engineering – Published by CFE Media LLC (Content for Engineers), provides the engineering community with research and information.



[1] Variable Frequency Drives (VFD) are sometimes referred to as: Adjustable Speed Drives (ASD) and Variable Speed Drives (VSD).  ASD and VSD are more broad categorizations that include mechanical, electromechanical, hydraulic, and VFD controls.  VFDs uniquely control the frequency of the electrical power supplied to a motor.

[2] Actual savings depend on the application. When used in constant speed, high torque, sequencing, and soft start applications, energy costs may not change or actually be higher with a VFD.

[3] Affinity Laws express an ideal relationship and do not take into account energy losses in the motor system. More information visit The Engineering Toolbox.