Kirk, it seems that your frequency converter is not grounded.
I had the same problem with some reportedly defective converters about a year ago. I connected them one after the other, some worked, some not. I wanted to look at the waveform. It occcurred to me that I might encounter the voltage problem we are talking about with my scope. Now, when I connect a small motor to a converter, it will most probably wired in Y. The common center of the three windings is floating, but should represent ground potential, given the windings are equal in resistance and inductivity and there is no insulation defect. Measure the voltage between this center tap of your motor and real ground, preferably using an old, low resistance meter. If the voltage is steady and high, e.g. near the mains voltage, there may be a fault in the insulation of the converter or in the wiring, if you read zero volts, you got it made and can proceed connecting these grounds. Else, connect a 100 Watts light bulb between these "ground" points. If its lights brightly, there is surely a fault. If not, try a lower wattage bulb. Decrease this load until it seems safe to connect the two "grounds" directly. Now you will have three ground referenced secondary voltages, ready to watch with your scope. This way, you can avoid dangerous DC potential on the secondary side. The safety elements in the converter won't detect faults on the secondary side in most cases. Never leave elements electrically floating on power circuits. If your motor is wired in triangle (delta), grounding of the center is not possible. In this case, simulate a virtual common with three resistors (temporarily) between the phases of the motor, sufficiently small to minimize the effect of current splitting. Induction heater: I bought a used "third choice" household heater, 1800 watts, from ebay cheaply just for testing and experimenting. I measured the coil (113 µH), frequency (28 kHz) and mains current (3-8 amps at 230 V). I made a cylindrical 17-turn copper tube coil (4" diam.) for melting aluminum which, unfortunately, has much less inductivity (15 µH) than the heater's original plane coil. Now I am going to test which influence the reduced inductivity has and how I can compensate for this. Maybe a larger capacitor (1.3 nF instead of the original 0.3 nF) will do the trick, although the overall impedance will decrease to one fourth. I don't have much experience with these power applications because it takes a braver man than me to encounter high power flashes and thunder in our kitchen. Best greetings Peter Blodow >> Hello Kirk, >> I don't see why you should not be able to measure your mains phases with >> an ordinary oscilloscope, set up as usual. At least in this country, the >> three phases of mains current are ground referenced, 230 V effective >> voltage to neutral. Between the phases, as can be figured with a >> triangle formula or by trigonometry, there are 400 V eff. (not ground >> referenced, of course). Connect each phase with a high resistance probe >> or, if exact magnitude is not important, with a small capacitor. It's >> just measuring a ground referenced voltage, a little higher than usual, >> but still. No floating oscar, as long as you don't want to measure >> differentially between phases. >> > > My converter phases are not ground referenced, so I need differential > scope measurements or isolated inputs of some sort. > > >> E.g., right now, I am working on an induction heater which outputs 28 >> kHz at 800 V maximum, 500 V eff. No problem with my 40 year old oscar. >> Since this voltage is ground referenced, I can hook it up to the oscar >> input with the ordinary 10 MOhm probe, 1:10. I leave the probe's ground >> crocodile off to avoid cross ground currents through the oscar. I only >> have to watch the way the plug is connected to the wall outlet to make >> sure the leg I am measuring at isn't the grounded one :-) >> > > Induction heating is another thing on my to-do-someday list: > http://wiki.linuxcnc.org/cgi-bin/emcinfo.pl?Videos > > Also, I have an ultrasonic cleaner to fix. > > > The problem with Spring is all the gardening that won't wait. I should > have started my seedlings by now. > ------------------------------------------------------------------------------ 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
