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MOTORS: A LOST ART

by Bob Perrin

Start ı A Few Words on Words ı The DC Motor ı Polyphase AC Motors ı Single-Phase AC Motors ı Winding Down ı Sources and PDF

About three years ago, I was working for a company that made silicon wafer processing equipment. The products were robotically automated chemical process stations. A typical station was 15' long and cost upwards of half a million dollars.

One of our big customers had our tools in silicon fabs all over the planet. Quality and process consistency were high priorities with this customer. To ensure all of their fabs had identical processes, our customer demanded that every tool we delivered be "copy exact." This meant every tool we built for our customer had to have exactly the same componentsıright down to the plating on the screws.

Normally this wasnıt too much of a problem. But, when the customer began putting our tools into fabs in countries with 50-Hz power (instead of the 60 Hz found in North America), certain difficulties did arise. The most troubling was associated with pumps that were driven by AC motors. As the line frequency changed from 60 Hz to 50 Hz, so did the speed and torque characteristics of the pump motors. That impacted the delivery rates of certain chemicals (most notably de-ionized water) to the chemical process tanks.

I was a design engineer assigned to a group that was given the task of fixing this problem. My first suggestion was to go find a motor that was intended for operation on 50-Hz mains and that had the speed/torque characteristics we needed. That was shot down immediately by the customer who recited, "copy exact, copy exact, everything must be copy exact."

As it turned out, the customer was willing to retrofit 20+ North American based tools with "the fix" as long as "the fix" ensured the existing 60-Hz tools would contain hardware identical to the new tools and that all tools would work identically on 50 Hz and 60 Hz.

The biggest difficulty I faced was my complete ignorance of motor theory, construction, and operation. I had not a clue how we were going to get identical performance out of 1-hp motors operating at different line frequencies.

Fortunately, I was just one member of a team, and a relatively junior member at that. The group was called the Special Products Group, and these guys were always given the companyıs tough-nut problems. Over the years, the engineers on the team had become seasoned pros and they knew how to solve the problem.

My supervisor informed me that all we needed was a VFD in front of the motor. That way, the VFD would be responsible for controlling motor speed. The VFD wouldnıt care if it was operating on 50 Hz or 60 Hz, and as an added advantage, the VFD would allow the customer to tweak pump-flow rates to optimize certain processes. My boss got the customer to sign off on this approach.

That was good and all, but I had to ask for clarification on exactly how a vacuum fluorescent display (VFD) was going to control the motor speed. My boss about split a gut at that. When he finally regained control, he explained that a VFD, in this context, referred to a magic little box called a variable frequency drive.

To make a long story short, I called T. B. Woods company, explained the type of motor we had and what the input mains looked like. The T. B. Woods engineer selected a nice little VFD. We tested it and specıd it in the BOM. T. B. Woods sold us a small fortune of VFDs. The customer installed them in existing tools, and we integrated them into new tools. All was right with the world. Except, I still didnıt know a thing about motors, their theory, their construction, their operation, or their control.

Motors, like slide-rules seem to be an art lost to many engineers. Well, thatıs not good enough for this engineer. My curiosity and conscience finally got the best of me a few months ago. I bought a few books on motors and went down to the local surplus store and picked up a box of various motors for $40. I read the books and disassembled the motors. Hereıs what I learnedı.

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