On Monday 16 July 2018 05:53:53 andy pugh wrote: > On 16 July 2018 at 02:21, Dan Bloomquist <[email protected]> wrote: > > <http://lakeweb.net/three_phase/> > > Interesting. > > Many years ago (before I knew much about electronics, before the > Arduino was invented and before I taught myself C in order to write > 3-phase drivers for LinuxCNC) I made a voltage-doubling rotary > converter that very nearly worked... > > The idea was to connect 240V single phase to one leg and the > star-point of a three-phase motor and to then take 440V three-phase > out of the ends of each leg. I found that I needed to spin the motor > up with a single-phase motor initially, but that it would then > continue spinning on its own. This was with no active control at all. > It didn't produce particularly symmetrical 3-phase and would tend to > bog down when a load motor came on line, so wasn't really a success, > but did seem to have some promise. > > I wonder if this setup, with active control, could be made to work?
I think it could, useing 40 amp SSR's for the switching needed. Timeing of the drive signals would be required of course as the switching both ways would need to be done at the zero crossing point when turning the individual SSR on, else large, likely destructive current spikes would otherwise flow as a cap was brought into the circuit. Its an idea that needs checked out. > > While on the subject of voltage-increasing, apparently it is possible > to re-wire the input stage of a 440V VFD into a doubler circuit to > allow it to run (derated) off of 240V. > https://en.wikipedia.org/wiki/Voltage_doubler#Delon_circuit Due to the physical size of the capacitors involved, I'd think it impossible to shoehorn it into the 440 volt VFD. It would take a separate box, but if it was big enough, the derate would not be noticeable. However, rather than huge high voltage electrolytic's, I think I'd prowl ebay looking for a big enough transformer, and just give the VFD the voltage it wants. Freight on a can big enough to run a 10 horse would apply, and someplace to put it that is code acceptable would also run the cost up. But it would be a one time cost. And we shouldn't forget that these VFD's come with a recommendation to replace its big caps at (usually) 5 year intervals. Probably more important if feeding them single phase like I am as that increases the ripple currents flowing considerably. Over specing it by using a smaller motor will extend that somewhat as that also reduces the ripple currents. In my case a 1 hp motor and a 1.5 rated VFD. I should probably note that the capacitative + diodes voltage doublers are outputing DC, and that the VFD's are cap voltage rated for at least 1.41x the name plate source voltage, (which is above the 525 volt surge rating of the best electrolytic's) so if feeding them from such a DC source, you should figure on a tripler circuit as you'll need nominally 620 volts in that case, but thats above the voltage withstand of the electrolytic chemistry since its dependant on the thickness of the alox film on the foil, and leakage currents rise rapidly above 525 volts for the best ones. Thats a big pile of caps that will need to be fan cooled. And replaced from time to time. So while its possible, I'd stick with the big can solution. Much cheaper in the long view. The only "rent" you pay on the vfd's then is the cap replacements at the recommended interval. -- Cheers, Gene Heskett -- "There are four boxes to be used in defense of liberty: soap, ballot, jury, and ammo. Please use in that order." -Ed Howdershelt (Author) Genes Web page <http://geneslinuxbox.net:6309/gene> ------------------------------------------------------------------------------ Check out the vibrant tech community on one of the world's most engaging tech sites, Slashdot.org! http://sdm.link/slashdot _______________________________________________ Emc-users mailing list [email protected] https://lists.sourceforge.net/lists/listinfo/emc-users
