On Tue, Mar 15, 2011 at 07:24:44PM -0700, Kirk Wallace wrote: > On Tue, 2011-03-15 at 22:08 -0400, gene heskett wrote: > > > > For voltage and phases you are correct as shown. > > > > Currents however will need toroid type current transformers installed so > > the motor lead is threaded through the toroid core. You get a voltage out > > that is proportional to the current in the motor lead. > > > > Dooh (hits forehead). A transformer in series with a motor lead would be > a bad idea (unless designed to be there). Small toroidal's are easy and > plentiful, but another time sink.
Kirk, the current transformers I have on PCBs in my junkbox all have one turn on the primary. A clamp-meter similarly has a split toroid, and clips around existing wiring (one turn), for non-contact AC current measurement. About 30 secondary turns on the toroid is what I typically see. Fiddling with one of them some time ago, I found it easier to see current waveform than measure absolute current. (But that was in a switch-mode application) DO NOT LEAVE THE SECONDARY OPEN CIRCUIT while in use. The size of the secondary load resistor (RL) dictates how big a voltage (proportional to current) you get. Infinity is not a good value. ;-) The one-turn primary presents only milliohms of DC resistance, but Z = RL/K^2 , where K = turns ratio. (i.e. the normal impedance transformation expected) So 10 ohm & 30 turns gives 11 milliohms in the current path. (IIRC) Instrumentation current transformers may have more turns, and they therefore can suffer more readily from electrical breakdown if the load resistor is absent. For voltage waveform observation, I like the idea of several AC-output plugpacks (wallwarts) from the junkbox. (My old filament transformers are inconveniently heavy, and too far back in the junkbox to be easily exhumed. :-) Erik -- "355/113 -- Not the famous irrational number PI, but an incredible simulation!" ------------------------------------------------------------------------------ Colocation vs. Managed Hosting A question and answer guide to determining the best fit for your organization - today and in the future. http://p.sf.net/sfu/internap-sfd2d _______________________________________________ Emc-users mailing list Emc-users@lists.sourceforge.net https://lists.sourceforge.net/lists/listinfo/emc-users
