On Tuesday, March 15, 2011 08:46:44 PM Kirk Wallace did opine: > In order to investigate three phase issues more, it would be nice to > measure at least two waves at a time. I have a two channel scope, but > the channels are ground referenced. I can float the scope, but I think > this will only allow me to measure one signal, or two if they happen to > have the same reference point. Plus I don't think floating the scope is > the safest practice for me or the scope. What might be good is to have a > box that floats like a DVM but can feed a scope channel. What sort of > keywords should I use to find such a thing, or is there a better way?
That depends on the scope Kirk. 30 years ago the phrase 'double insulated' was all the rage, even for scopes. When I was the CE at KIVA-TV, I hit the station up for what I thought was a decent scope in view of all the stuff I was in charge of, so I bought a dual trace, 35 mhz rated, double insulated Phillips scope. That double insulated came in handier than a button on the outhouse door on several occasions, most noteworthy be the slipping of 3 layers of cambric and heat shrink, not shrunk, over the probe lead where the transmitter door closed on it, with the ground clip of the probe clipped onto the high side of a 10 ohm arc absorbing 100 watt resistor which was sitting at +1500 volts, and the probe clipped onto the other end of the resistor. This was the 4CX5000A visual driver stage, and I was suspecting that the sync compression I was seeing on the air was in that tube. The 4CX5000A is a shadow grid tube, meaning the screen grids wires are in the electrical shadow of the control grid. That also means they had damned well better stay there else the screen grid current will rise and that heats the wire until it gets hot enough to become an electron emitter. In this case I could see that the 40 mills and rapidly rising with the drive level current on the transmitters meters wasn't what I had come to expect as 5 to 10 mills would have been much closer to normal. That scope hooked up, allowed me to see it in real time, and disclosed that the peak current was actually around a full amp, for the duration of the sync pulse. Without it, I would have been guessing, but likely would have reached the same conclusion within a few days. So I tuned not for peak power, but for usable power, getting about 90% with much less compression than before, but that convinced me the tube was about fini. It took us a couple of weeks to collect enough money for a fresh one, and by the time it arrived it had faded to about 60%. KIVA-TV in Farmington NM, FWIW, was at that time only one step up from the smallest market in the country, Miles City MT. Starvation was always just around the corner. Back to the question: If your scope is blessed with this 'double insulated' moniker, then it would be _very_ illuminating to float the scope, then hook the ground to the otherwise unused center connection of the motor when it is wired as a Y, not delta. Hooked to the crotch of the Y, and the probes then connected to the other coils, you will be able to see in real time, the exact phase angles the motor is seeing. Needless to say, the scope is to be sitting on a dry wood table, and you are to be standing on a dry foam rubber foot cushion mat, wearing rubber soles. With one hand jammed deeply into a pocket... -- Cheers, Gene "There are four boxes to be used in defense of liberty: soap, ballot, jury, and ammo. Please use in that order." -Ed Howdershelt (Author) <http://tinyurl.com/ddg5bz> <http://www.cantrip.org/gatto.html> If any man wishes to be humbled and mortified, let him become president of Harvard. -- Edward Holyoke ------------------------------------------------------------------------------ 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
