On Sat, Jan 24, 2015, at 11:49 AM, Kirk Wallace wrote: > Thinking aloud... > > I have been playing with an MA860H stepper drive which seems to work > well enough for my mill. > http://wallacecompany.com/ma860h/ > http://www.wallacecompany.com/machine_shop/Shizuoka/ > > I have an Antek toroidal transformer that is feeding 75 VAC to the > drive. The drive has an input limit of 80 VAC or 110 VDC. Although, to > feed three drives, I'll need a bigger transformer to supply 15 Amps or > more. I can use a 60Hz big iron transformer, but these are big, heavy > and expensive to purchase and ship. I could use multiple toroids, but > these are expensive. > > Then I got to thinking. What about a buck converter to convert mains > (120 or 240 VAC) to mains DC to high Hz AC to let's say 90 VDC? Or more > simply, pump the useful part of the mains AC into a large capacitor in a > way that maintains the DC voltage I need? This sounds a little like an > SCR circuit, such as a light dimmer, universal motor speed controller, > or SCR DC welder. This might be a way to leverage a cheap second hand > commodity device to a specialty purpose. (But keeping in mind that > common converters don't like their outputs switched.) > > So, what are some ways of feeding roughly 90 VDC or 70 VAC at 15 Amps to > motor drives from 240 VAC mains? >
That big heavy 60 (or 50) Hz transformer is not only providing voltage matching, it also provides isolation. Most of the alternatives you are talking about will not isolate the 90V DC from the mains. That may or may not matter, it depends on your drives. If you need isolation, the 60Hz transformer is the simplest way to get it. You could also rectify the 240V line to give you about 325V DC, then chop that into high frequency AC and run it through a much smaller high-frequency transformer, then rectify it back to DC. That is basically building a switching power supply from scratch, and is a non-trivial task. If you don't need isolation, you could rectify the line to 325V DC, then build a buck topology switcher to drop the voltage to 90V DC. A buck is simpler than a fully isolated switcher, but an off-line buck at this power level is still not a beginner's power electronics project. The MOSFET phase controller that Peter describes avoids the need to do high frequency switching, but brings its own challenges. The current peak will occur immediately before you turn off the FET. Stray inductance (including the inductance of the power input cable) will generate overshoot at the FET, probably enough to kill it, unless you snub it somehow. Neither of those projects is simple. There is probably a lot more literature out there on the buck converter approach though. The phase control approach is somewhat off-the-wall. Both the buck converter and the phase controller share one major disadvantage: when FETs fail, most of the time they fail shorted. In an isolated switcher, a shorted FET blows fuses on the primary side, but the secondary side voltage usually just drops to zero. Replace FET and fuse and maybe a bit of other primary side stuff and you are back in business. If the FET fails shorted on the buck or phase controlled supply, the output goes to 325V or so. The stepper drives will probably blow before the fuses, unless you build some kind of a crowbar into the supply. You'll be replacing a lot more $tuff. John Kasunich jmkasun...@fastmail.fm ------------------------------------------------------------------------------ New Year. New Location. New Benefits. New Data Center in Ashburn, VA. GigeNET is offering a free month of service with a new server in Ashburn. Choose from 2 high performing configs, both with 100TB of bandwidth. Higher redundancy.Lower latency.Increased capacity.Completely compliant. http://p.sf.net/sfu/gigenet _______________________________________________ Emc-users mailing list Emc-users@lists.sourceforge.net https://lists.sourceforge.net/lists/listinfo/emc-users
