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Using Inexpensive A.C. Motor Drives in an Introductory Power and Controls Course

Abstract The induction motor is generally cheaper and more rugged than a dc machine. Thus, ac variable- frequency drives (VFD) have made induction motors the first choice for variable-speed applications in industry. As a result, ac motor drives are an important topic for introductory power or motors courses. The newest generation drives offer the ability to program the drives, either with a human interface module or via a link to a computer. One difficulty with incorporating variable-frequency drives into a laboratory portion of the course is the expense. “Name-brand” drives can cost more than $2,000, even for fractional horsepower motors, and software to communicate with the drives can cost several thousand dollars. Fortunately, the internationalization of power equipment manufacturing has brought about the opportunity to obtain much less expensive drives (about $250) that offer the same functionality as the high- priced brands. The drives were connected to PCs using internet protocols, which allowed them to be programmed “remotely” as well as from the front panel. This paper begins by describing some of the capabilities of modern VFDs, in particular the “generic” drive that was chosen, and how it is used in lab. It then discusses how the drive was used in lab by the students and shows some typical results. Experimental procedures for the laboratory are included.

Introduction For the past 15 years or so, we have included a lecture and a laboratory session on variable-speed induction motor drives in our introductory electrical power and controls course1. This is a very important topic because most of industry now uses variable-speed ac motor drives rather than dc motors2-5. When we first began including the laboratory exercise, we used a rather basic drive that literally had two knobs–one to change the frequency and one to go forward, reverse, or stop. We used them because we were able to purchase them at a significant discount, but that was still about $1000 per drive. Of course the capabilities of drives increased dramatically and those drives were become obsolete after a few years. They did, however, have one interesting feature, namely they operated with a single-phase input and provided a three-phase output. As a result we still use those drives for a small portion of the lab, which will be described later.

About 10 years ago, we were fortunate enough to receive a donation of new, state-of-the-art, name-brand variable speed drives. Those drives were three-phase in and three-phase out and featured programming capabilities through a human interface module. We used these quite successfully for eight years, but eventually they too were becoming dated and noticeably larger than new drives on the market. In addition, we were unable to obtain a donation of software to interface the drives to the lab computers and the software was priced at several thousand dollars per station. Thus, we began looking at replacing the drives. Initially we approached the company that had donated the previous drives, but we were unable to obtain a new donation. We then investigated purchasing “name brand” replacement drives. Unfortunately, we found that new drives were about $2000 apiece and software to connect them to the computers was even more. That was not an option, given that we have eight lab stations, so we began looking at other

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Skvarenina, T. (2008, June), Using Inexpensive A.C. Motor Drives In An Introductory Power And Controls Course Paper presented at 2008 Annual Conference & Exposition, Pittsburgh, Pennsylvania. 10.18260/1-2--3659