On Monday 11 January 2016 13:10:10 John Kasunich wrote: > SSRs can fail shorted, I don't think this buys you much in terms of > reliability and safety. > > I think there is a little over-kill going on. > > Do you really need a fast bleeder (<1 minute to safe)? Or would a > five minute bleeder be OK? > > My understanding is that you have two 68000uF caps in series, charged > to about 130V. (If this is incorrect let me know - the thread is long > and rambling and refers to other thread. I'm basing this on something > written in the first message of this particular thread.)
Actually 10 6800 u-f in parallel, working at their labeled 63 volts ratings. But yes, you have the right idea. :) > > Series caps divide, so you have 34000uF. Let's say that "safe" is > 20V. So you need to discharge from 130V to 20V, that is to 15% of the > original voltage. Ln(0.15) is -1.9, so it takes 1.9 RC time constants > to get to a safe voltage. Call it 2.0 for easy math. If you want to > get there in 5 minutes, then one time constant is 2.5 minutes = 150 > seconds. T = RC, solve for R, gives you R = T/C = 4411 ohms. At 130V, > that would draw 29mA and burn 3.8 watts. Next lower 10% value is > 3.9K, that would draw and burn 4.3 watts. I like to derate power > resistors by 50%, so you want a 3.9K 10W resistor. I actually have, atm, 6 ea 5k 10 watters in parallel clipped onto it right now that I was going to use once the relays get here. Thats 833 ohms which would drain it even faster than just the one. And, somewhere on an undermanned galley in the pacific, is a 250 ohm 250 watt resistor which would do the bleed-down even faster, and the current peak would be nominally .5 amps in that case. I can't see that as being capable of welding a relay contact. > If you do indeed have two caps in series, they ought to have balancing > resistors anyway, so you could split the bleeder into two sections. > Make each section 2K or 2.2K and 5 watts. > That too, I was contemplating, but the 51 ohmmer would need to be dismounted to gain access to the pcb bus connecting them all together. Since I have them already 2 each of the 5k's across each bank would seem to be about right. They are well discharged atm, and I could do that yet this afternoon. If my math is correct, at 5k and 63.5 volts=0.80645 watts per, so if I spread them out on the busses, the heat shouldn't hurt the caps. And my existing SSR lashup would be fine when the rest of the parts to make a decent charge-pump-detector to drive the SSR's with do arrive. I can do that yet this afternoon if I can get high enough on a ladder to turn the unit face up so the soldering is easy. There's a 4 footer currently parked 3 feet away. :) The problem is the floor real estate to spread it out. > Something like this can be chassis mounted and wired to the caps: > http://www.digikey.com/product-detail/en/RH0052K000FE02/RHRA-2.0K-ND/1 >166266 Cost is $5 each, total $10 > > Even cheaper would be to parallel up some 1W leaded resistors: > http://www.digikey.com/product-detail/en/PR01000101002JR500/PPC10KW-1C >T-ND/597177 $2.06 for ten - five in parallel will make 2K 5W, put one > set of five across each cap. Might be a nuisance to mount. > The cap busses are nominally a foot long, 3/8" wide 2oz copper > If you have can type caps with screw terminals, use power resistors > with solid axial leads, crimp/solder them to ring lugs, and mount > right on top of the caps: > http://www.digikey.com/product-detail/en/SQP10AJB-2K0/2.0KW-10-ND/1878 >9 $0.64 each, total $1.28 > > I realize that Digikey might not be your supplier of choice due to > shipping costs, but my point is that an always-on bleeder is simple, > safe, and at this power level, cheap. As you get to larger energy > levels (more volts or more uF) or if you need a much faster discharge > time you might be able to justify a switched fast bleeder. But that > will ALWAYS be less reliable and less safe than an always-on bleeder. > Even if you have a switched bleeder it would be wise to have an > permanent bleeder with maybe a 10 minute time constant. At 10 > minutes, the power and cost are negligible, and it serves double duty > as the voltage balancing resistors for your series caps. > > Inrush limiting is a separate issue. Separate resistor, bypassed by a > contactor or relay for normal operation. Put it on the AC side so you > don't have any issues with DC contact ratings. That is what I am doing right now with the SSR's, one to switch on the power thru that 51 ohm resistor, and a second SSR about 5 seconds later to short out the resistor. > The main risk is that > if you try to run with the bypass relay open you can fry your > resistor. One way to avoid that is to make the "resistor" a 100W > incandescent light bulb. Which rigid tapping would quickly make history. The spindle turnaround, even if I have it rate restricted, still pumps the supply high enough to blow a C7 bulb in 4 or 5 pecks. Jon's pwm servo amp is effectively a full 4 quadrant controller, so it dumps motor energy back into the supply when it reverses the motor. I haven't actually measured the voltage as my digital meters aren't near fast enough to catch the true peak. I have about a 1 second rate ramp in the hal file to ease that to about 1 second from 2500 to -2500, without that its positively brutal. But thats yet another problem that I should stretch the allowed time on. One setp in the .hal. > Using bulbs for bleeders is risky because they run all the time, and > when they burn out they leave the caps charged with no indication of > the risk. But using a bulb for charging is different. The bulb only > lights for a few seconds when you first apply power. If it is burned > out the caps won't charge. If you try to run with the bypass open the > light will glow to let you know. > > A 100W 120V bulb has a hot resistance of 144 ohms. With 34000uF the > charging time constant is 4.9 seconds. As the bus charges the voltage > drop across the bulb goes down, it cools down, and its resistance > decreased - that is good, it make the "long tail" of the charging > curve go faster. The inrush during the first cycle or so will be > based on the cold resistance of the bulb, but if your switch and > diodes can handle a 100W light bulb you know it can handle any size > cap bank fed thru that same 100W light bulb. Some how, given the quality of 100 watt light bulbs today, I trust this 50 yo 51 ohm 200 watt resister a lot better. And that gives me a T=RC= 1.734 seconds, so at 5 seconds, the current delay in then .hal file, they should be well charged. I'll go put the 5k's, 2 in parallel per bank of caps, in yet this afternoon & report back. 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> ------------------------------------------------------------------------------ Site24x7 APM Insight: Get Deep Visibility into Application Performance APM + Mobile APM + RUM: Monitor 3 App instances at just $35/Month Monitor end-to-end web transactions and take corrective actions now Troubleshoot faster and improve end-user experience. Signup Now! http://pubads.g.doubleclick.net/gampad/clk?id=267308311&iu=/4140 _______________________________________________ Emc-users mailing list Emc-users@lists.sourceforge.net https://lists.sourceforge.net/lists/listinfo/emc-users
