andy pugh wrote: > I think that the failure mode is that, in the case of a power glitch, > the crowbar relay switches and is discharging 300V DC at a fair > current, then the power returns and the NC contact tries to break 300V > 10A and welds the NC contacts closed. Then the NO contacts close and > we now have the crowbar resistor connected directly across the > rectifier output. The crowbar resistor is not sized for continuous > operation. > If this is the case, the contacts will still be welded, so open up the relay and examine it. Normal relays are severely derated for DC, and breaking 300 V DC cannot be accomplished by any standard relay. It probably doesn't take a glitch on the mains, just after so many on/off cycles, you will get a failure as the contacts degrade. One way to solve the fire problem is to put a fuse (correctly rated for 300 V DC) in series with the source, and strap it to the resistor. When the resistor overheats, it will melt the fuse. But, that will be a safety hack, rather than a fix. Use double-break contactors with large contact spacing for 600 V+, or use a FET to control the dump resistor.
Jon ------------------------------------------------------------------------------ October Webinars: Code for Performance Free Intel webinars can help you accelerate application performance. Explore tips for MPI, OpenMP, advanced profiling, and more. Get the most from the latest Intel processors and coprocessors. See abstracts and register > http://pubads.g.doubleclick.net/gampad/clk?id=60134791&iu=/4140/ostg.clktrk _______________________________________________ Emc-users mailing list Emc-users@lists.sourceforge.net https://lists.sourceforge.net/lists/listinfo/emc-users
