Looking at electric drive efficiency

variable-frequency drives (VFDs) provide dramatic energy savings in myriad applications with varying loads and speeds. While power and energy savings are key benefits, VFD efficiency is also of interest because drives can experience significant efficiency losses at partial load operation-much like electric motors.

07/01/2009


Five makes of VFDs tested had efficiency variations (bands) as shown for 50 hp (37 kW) units running at different speeds - along with larger characteristic efficiency drop-off at reduced torque and speed.

Few will dispute that variable-frequency drives (VFDs) provide dramatic energy savings in myriad applications with varying loads and speeds. While power and energy savings are key benefits, VFD efficiency is also of interest because drives can experience significant efficiency losses at partial load operation—much like electric motors. Yet motors have received predominant attention for energy efficiency, including development of recognized test procedures and standards. A broader look at “motor system” efficiency must recognize the effect of VFDs.

Recent progress in this area includes work done at the energy technology labs of Hydro-Quebec's (Canada) Research Institute. Findings of the underlying study, “Variable Frequency Drive Testing,” have been presented at Motor Summit 2008 (Zurich, Switzerland), MEPSA 2009 (Sydney, Australia), and 2009 Motor, Drive & Automation Systems Conference (Orlando, FL) by Pierre Angers, an engineer at Hydro-Quebec.

“Published VFD efficiencies at other than full rated power are scarce,” says Angers. “There is no recognized VFD efficiency standard for comparing different makes of drives.” Efficiency is the highest around 100% speed, which is just where you don't want to use variable-speed drives, hence most available efficiency values are virtually “useless,” Angers explains.

Goals of the project were to establish an efficiency testing procedure for VFDs, motors, and VFD-motor combinations—as well as to enable efficiency comparison between various VFD systems. Angers has tested five major makes of VFDs, which powered three sizes of induction motors (10, 50, and 100 hp/7.5, 37, 75 kW) at various loads and speeds. For each test run a series of 20 test points ranged widely over “% speed” and “% nominal torque” values (25-100% speed and 10-100% torque). Drive manufacturers' names remain proprietary. Test setup measured efficiency as a ratio of output to input power.

Preliminary results show 2-8% variation in VFD system efficiency, especially at low loads and speeds. Efficiency variation between different brands of VFDs was substantially less (see diagram). However, variability of VFD efficiency is further impacted by factors such as drive temperature, supply voltage (and voltage unbalance), switching frequency, and drive options like active front-end, reactors, or output filters.

One novel aspect of Angers' research is presentation of motor system efficiency results as a contour map, with percent nominal speed and percent torque serving as other variables. A typical conveyor application plots as a horizontal line (constant torque) across the efficiency contours, while a pump/fan application plots as a cubic system curve.

The work described here is the basis of a protocol for testing VFD efficiency. Two proposed standards are in draft status: Canadian standard (CSA C838) Variable Frequency Drives and International Electrotechnical Commission standard IEC 60034-2-3.

Users of VFDs obtain energy savings from matching motor speed to load requirements; in addition, they need to be able to compare efficiency of different electric drive brands for their applications.


Author Information

Frank J. Bartos, P.E., is a Control Engineering consulting editor. Reach him at braunbart@sbcglobal.net .




No comments
The Top Plant program honors outstanding manufacturing facilities in North America. View the 2013 Top Plant.
The Product of the Year program recognizes products newly released in the manufacturing industries.
The Engineering Leaders Under 40 program identifies and gives recognition to young engineers who...
The true cost of lubrication: Three keys to consider when evaluating oils; Plant Engineering Lubrication Guide; 11 ways to protect bearing assets; Is lubrication part of your KPIs?
Contract maintenance: 5 ways to keep things humming while keeping an eye on costs; Pneumatic systems; Energy monitoring; The sixth 'S' is safety
Transport your data: Supply chain information critical to operational excellence; High-voltage faults; Portable cooling; Safety automation isn't automatic
Case Study Database

Case Study Database

Get more exposure for your case study by uploading it to the Plant Engineering case study database, where end-users can identify relevant solutions and explore what the experts are doing to effectively implement a variety of technology and productivity related projects.

These case studies provide examples of how knowledgeable solution providers have used technology, processes and people to create effective and successful implementations in real-world situations. Case studies can be completed by filling out a simple online form where you can outline the project title, abstract, and full story in 1500 words or less; upload photos, videos and a logo.

Click here to visit the Case Study Database and upload your case study.

Maintaining low data center PUE; Using eco mode in UPS systems; Commissioning electrical and power systems; Exploring dc power distribution alternatives
Synchronizing industrial Ethernet networks; Selecting protocol conversion gateways; Integrating HMIs with PLCs and PACs
Why manufacturers need to see energy in a different light: Current approaches to energy management yield quick savings, but leave plant managers searching for ways of improving on those early gains.

Annual Salary Survey

Participate in the 2013 Salary Survey

In a year when manufacturing continued to lead the economic rebound, it makes sense that plant manager bonuses rebounded. Plant Engineering’s annual Salary Survey shows both wages and bonuses rose in 2012 after a retreat the year before.

Average salary across all job titles for plant floor management rose 3.5% to $95,446, and bonus compensation jumped to $15,162, a 4.2% increase from the 2010 level and double the 2011 total, which showed a sharp drop in bonus.

2012 Salary Survey Analysis

2012 Salary Survey Results

Maintenance and reliability tips and best practices from the maintenance and reliability coaches at Allied Reliability Group.
The One Voice for Manufacturing blog reports on federal public policy issues impacting the manufacturing sector. One Voice is a joint effort by the National Tooling and Machining...
The Society for Maintenance and Reliability Professionals an organization devoted...
Join this ongoing discussion of machine guarding topics, including solutions assessments, regulatory compliance, gap analysis...
IMS Research, recently acquired by IHS Inc., is a leading independent supplier of market research and consultancy to the global electronics industry.
Maintenance is not optional in manufacturing. It’s a profit center, driving productivity and uptime while reducing overall repair costs.
The Lachance on CMMS blog is about current maintenance topics. Blogger Paul Lachance is president and chief technology officer for Smartware Group.