Old 02-06-2010, 05:27 AM
  #38  
kyleservicetech
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Originally Posted by MustangMan View Post
Commercial/Industrial 3-phase motors are indeed a bit different in detail from our little motors but operate using the same general principles. Smaller 3-phase motors use squirrel cage rotors just like the smaller 1-phase motors use. In these motors the "field" magnetization of the rotor is "induced" by the slip between the theoretical rotation of the stator fields and the mechanical rotation of the rotor at a slightly slower speed. That's why induction motors never achieve true "synchronous" speeds. Larger motors, which operate synchronously, have field windings on the rotor which creates the field magnetization. These must be fed DC current through clip rings whereas our motors have permanent field magnets.
If you want a demonstration of that "slip" between the three phase rotating magnetic field, and the squirrel cage rotor, try to locate an especially powerful magnet, samarium cobalt or similar.

I've got some that are 3/8 by 1 1/2 by 3 inches (They pull 400 pounds on a one inch thick steel block) Take that magnet, and swipe it very rapidly near an aluminum sheet, at least 1/4 inch thick. (Be danged careful with magnets like this. I got a finger smashed between one of those magnets and a steel magnetic structure. That finger didn't stop bleeding for 8 hours.)

The magnetic drag caused by the magnet inducing short circuit currents into the aluminum sheet is very noticeable. In fact, I was able to measure the voltage generated across a 1/2 inch sheet I've used on my oscilloscope. The scope showed a few millivolts across that sheet, indicating the short circuit current generated in that aluminum sheet was something around 1000 amperes.

But, 1000 amperes and a few millivolts still is only a few watts. :o
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