A couple of years ago, in engineering school – the project was such: We were given a bunch of odds and ends – a few paper clips, some wire, a film cannister, rubber bands, string, a wooden spool and a metal rod – and we were to partner up and design a rotor (the part of a motor that rotates). This is easy, but we also had to fine tune our design such that our motor ran at a speed of 1200 rotations per minute. Many of us figured that we might be able to get this through a lot of trial and error, but there was also a preliminary report that was to be handed in and graded. We had to EXPLAIN how we were going to do this.
Somehow, some formula to calculate the length of wire we needed for our rotor to run at that speed emerged from among the teams and was passed around. Most used it in their reports. It was due soon, and my partner and I had nothing. I asked friends where the equation came from and what its logic is. “How should I know? Elliott man, just use the formula.” But it was sketchy, so we didn’t.
It’s the night before the report is due, it’s late, and we still have nothing. My partner and I are on MSN trying to figure out what we’re going to do. I’m surfing the web for ideas. We’re clueless. I start reading up on AC motors, even though our class hasn’t touched on them yet. I’m trying to understand AC Synchronous motors, and I read that the speed of this motor relates to the rate at which the input voltage alternates. Typically, for household electricity, the voltage goes from positive to negative and back at a rate of sixty times per second. A simple motor plugged into this would make one full rotation for each of these cycles. 60 rotations per second = 3600 RPM, a multiple of 1200! We might be able to tweak it such that it makes a 1/3 rotation for each full rotation it would normally make. How cool is that, I thought.
It was a risky plan though. It wasn’t using anything we had yet learned in class, and the project outline sheet indicated that it would be powered by 12 volts, which people interpreted as direct current. Alternating current usually fell in the realms of 120 or 240 volts. Maybe the prof was expecting a DC design and would automatically flunk an AC one. But we had nothing, and this was something.
We stayed up real late preparing. I missed class the next morning, and when I did come in it was straight to the computer lab to get it printed out. Ank (my partner) and I didn’t want others to see it – our idea was either so stupid it was embarrassing or so brilliant that we didn’t want to give out ideas. But that kind of flew out the window:
“What’s this, is this your report? Dude, you’re making an AC MOTOR? What are you, STUPID?”
“Well, it didn’t say that it had to be DC. It just said twelve v–”
“Hey guys, the motor has to be DC, right?” A number of classmates were in the room, and they were nodding. And it wasn’t like we had time to come up with a new plan. The report was printed and I headed to Dr. Little’s office.
“Here’s… uh… our… report.” It was awkward enough that I hadn’t been to class. I was also passing in a report that could jeopardize my final grade. And my partner’s.
“Uh, ok.”
“I have a little… issue… uh… the motor is AC.”
“Fine.” He took the report and dropped it on top of the others. He wasn’t impressed.
“It didn’t say it had to be DC. It just said 12 volts.”
“What type of design will use use?”
“AC uhh… synchronous?”
He paused, held his pen up across his mouth and leaned back in his chair. “How many poles?”
“Six.”
Another pause. He mumbles some math “Thirty si… six… take thre… sixty hertz… Right!” Suddenly he sits forward in his chair, “That way you can get it at exactly 1200! It would be tough, because usually these motors are big and you would have three windings of 3 loops each on that little spool which doesn’t leave you much room and you would want it plugged into a three phase source. I’d really be curious to see how it works out, though. I’d like you to do it, and I’d like to work with you on it. It’s your call.”
I didn’t really have a clue what he just said, but I was happy. Ank was equally pleased. The next week, our report came back and was marked 9/10. Boom shakalaka.
But our design still didn’t make complete sense, even though the concept was right. I talked to the prof a few times to straighten things out, but by now, our peers had already spent hours fine-tuning their designs. Their biggest struggle seemed to be getting it to go the right speed. Nobody’s was running at 1200. “How fast is your’s, Dave?” “896, but we’re going to add a bunch more wire now.” “That’s not bad! We’ve used up most of our wire and we can’t get it faster than 600 RPM. Celine’s is going at 1700, but they can’t seem to slow theirs down.”
After long hours of redrafting our design and wrapping our little film cannister in a complex 9 loop arrangement, our EVAC (our initials put together) 1200 was put to test. Owing to the complexity of our design, we needed to use our lab equipment to power it. Three wires were plugged into the massive power machine, and each of these ran to separate fancy “commutators”, contact disks that we had crafted on our rotor. Since our little device could ONLY run at 1200 RPM, my poor buddy Ank had to try to hand-start the motor, lawn-mower-that-doesn’t-work style. Finally, after many spins, we got the motor to start running on its own. Sparks flew out from the electrical contact points. It was rotating at a steady speed, which we measured by pointing a little electronic instrument at it.
One two zero zero.
It was a glorious moment. This is one of the reasons I enjoy my field of study.
Our project ended up employing a similar, but much more crude, design to this:
