Leigh's note about universal motors is correct, and I liked his induction
motor demos. I am planning to try those for myself and my students next
term. Thanks.
Leigh's description of the universal AC-DC motor reminded me of a lab we
used to do with general-education students fulfilling their science
requirement. We don't do it anymore because it took too much time and got
some students really frustrated. But I sure liked it. We had them build a
motor using nails and magnet wire. They made their own electromagnets for
both the field and armature, wired them in series, and hence made a
universal motor. I'm sure many of you have done this, and perhaps some of
you still take the time to have students do it.
Making good brushes was one of the frustrating parts. Students did not pass
the lab until their motor worked, and the brushes were often the initial
time-consuming part. Then we hit them with the killer question...
When they had the motor working they had to show it to the professor. After
seeing the motor run, the professor said, "That's great. Your motor is
running nicely. Now, can you please make it run the other direction?" Oh
my. Our students ran their motors from 6-volt lantern batteries. Of course
the first thing every student tried was to switch the polarity of the
battery. They were baffled to see the motor run the same direction. At
that point they were told, "Well, you go work on it for a while, and when
you show me that your motor can run either direction, you will have passed
this lab and you can leave."
Michael D. Edmiston, Ph.D. Phone/voice-mail: 419-358-3270
Professor of Chemistry & Physics FAX: 419-358-3323
Chairman, Science Department E-Mail edmiston@bluffton.edu
Bluffton College
280 West College Avenue
Bluffton, OH 45817
Michael's last note reminded me of something else. The "AC motors" in
Lionel's trains are actually more properly called "universal AC-DC
motors". They are like permanent magnet field DC motors, but the field
magnets are replaced with a winding which is in series with the rotor.
Thus the proper polarity of the field is always present to propel the
motor in the proper direction with AC, since the current in the field
reverses at the same time as the current in the rotor. To reverse this
motor one need merely reverse the leads of the field.
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