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From: dgj2y@kelvin.seas.Virginia.EDU (Dave Jacobowitz)
Subject: Re: How does a brushless DC motor work?
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References: <flipk.789971272@soclab.soc.iastate.edu> <D2IKG6.F5p@hpcvsnz.cv.hp.com> <3fu86e$2q8@sabre.goldsword.com>
Date: Sun, 22 Jan 1995 19:08:11 GMT
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Xref: glinda.oz.cs.cmu.edu sci.electronics.repair:5520 sci.electronics:116372 comp.robotics:17231

In article <3fu86e$2q8@sabre.goldsword.com>,  <danields@goldsword.com> wrote:
>>For most two pole motors, their speed at rated horsepower will be about
>>3500 rpm.  There is a 100 rpm slip between the magnetic field pattern and
>>the rotor.  This slip is needed to make up for the losses in the rotor
>>(an induction motor with a superconducting rotor would rotate at 3600
>>rpm).
>
>If the rotor is at the same speed as the stator field, how could current be 
>induced into the rotor circuit? What would the torque be at zero slip?
>				Dan

You are right, at zero slip the torque and rotor currents are zero. 
However, at infinitesimal slip the torque and rotor currents are 
infinitesimal, etc. If the rotor were superconducting AND the bearings
of the motor were completely friction free then the speed be
asymptotically "close" to the synchronous freq.

dave


