On Wednesday 06 January 2016 05:13:34 andy pugh wrote: > On 6 January 2016 at 02:05, Gene Heskett <ghesk...@wdtv.com> wrote: > > What do you think? Is that a safe and workable idea? > > I have seen NC SSRs. I don't know how they do it, and they are > expensive. > > I have been using normally-closed mechanical relays to do this, with a > 240V coil connected to incoming power. > > Ideally it needs to be a force-guided relay, with the main power feed > (or surrogate thereof) through a second set of terminals so that if > the cap-drain contacts weld shut you can't power up the system with > the drain resistor in circuit. > > You have to ensure that the PSU isn't powered up until the caps are > discharged. I do this in HAL because my drives feed-back bus voltage > to HAL.
That thought has crossed my mind too, but the power relays I have, with an aux set of contacts that could feed the interlock, are all NO types. NC versions seem rare, and pricy. $100 USD range. The $9 SSR will work, but I've no solid tally back to make sure its recovered other than enforcing a time delay that gives it plenty of time to recover at startup. As it takes time to home it, and do any touch-offs that may be needed to setup the job, I don't see that a 2 minute delay to a usable spindle would be a huge handicap. Checking the pdf of another brand, Fotek, also a Chinese part, fails to disclose a minimum holding current, just states its a zero crossing control. That gives me the idea of using a low voltage AC as the shutoff mechanism. One clue may also be the minimum applied AC voltage of 24 volts. Stated as: 24-380VAC. Using the LV AC in series with the load r's as a shutoff brings up the need to restrict the current to about the rated current of that transformers low voltage delivery because any higher than that might saturate the core with destructive primary currants flowing. So that transformer should probably be larger that the teeny 200ma rated one thats in that box furnishing control power now if I elect to use a lower value resistor. With these resistors, the peak flow would be 0.152400000061 amps. Now if T=RC, then T is 56.6666666644 seconds to drain it to 42 volts if my math is good, and another 57 seconds to 14 volts, so even with a 12vac in series, it looks like 3 minutes to be safe. Thats a bit long. Looks like I'd better go kick fleabays tires & see what might be available. That, and go get the morning coffee under construction if my lady hasn't, I just heard a cabinet door close and a pan rattle in the kitchen. No clue what she is up to though as she likes to have a variety, might be making tea. I get sent to the store for oddish stuff they never have quite often. After 26 years, I still don't have her 100% understood. :) 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> ------------------------------------------------------------------------------ _______________________________________________ Emc-users mailing list Emc-users@lists.sourceforge.net https://lists.sourceforge.net/lists/listinfo/emc-users
