On 12/26/2009 07:34 PM, Terry Blanton wrote: > I have built a myriad of pulse motors and do not understand what CEMF > means. Can you explain what to look for? >
Voltage drop across a coil carrying a varying current, in a motor subjected to a varying load. For a stationary coil it's just -L*dI/dt. Of course if you're looking at battery voltage, you won't see the CEMF; and in fact that's the main objection to Sean's demo. He didn't find back EMF, but he didn't look very hard, either. To get a good measurement you would need to simultaneously measure voltage drop across the motor (or, better yet, an individual coil) along with current through the motor (or coil), and if the inductance is small, you'd need to look very carefully at the edges. Then look at how it changes as the RPMs and the load change. Sean appears to have been measuring battery voltage and current through one coil. The battery voltage droops slightly when the coil turns on. Inductance of the coil is indeed apparently small, so the back EMF as the B field builds up is hard to see in the scope traces the way Sean had the thing set up -- but since he apparently didn't say exactly where the probes were placed, and since I didn't study the comments carefully or listen to the original video (just read a transcript), my comments here should be treated with extreme caution. An experimenter who always distrusts his own results, as recommended by various people on this list, would have tried a great deal harder to get an accurate measurement of CEMF as the motor rotates than Sean appears to have done, if I can judge by what I gleaned of his demonstration. > Terry > > On Sat, Dec 26, 2009 at 7:29 PM, Esa Ruoho <[email protected]> wrote: > >> http://jnaudin.free.fr/steorn/index.htm >> >
