On Wed, Feb 13, 2013 at 9:59 AM, Stephen Dubovsky <smdubov...@gmail.com> wrote:
> Disconnecting a motor from drive/vfd isn't the problem. Reconnecting it > while its still powered is what typ lets the magic smoke out. Gene had a good point, that a mechanical contact never goes monotonically from 'connected' to 'open' due to contact bounce, so you end up disconnecting and reconnecting over and over--but I can see that different output stages will be affected to a different degree. Today I learned how a HVDC breaker work: it's clever and maybe relevant because that's the kind of trouble one has to go through in power applications: ----lowSpeedContact-----+----highSpeedContact---lowVoltageSolidState---+------- |___highVoltageSolidState__________________| it starts by having all switches closed; the hVSS switch has high resistance so most of the current goes through the hSC and lVSS. First, they open lVSS, which causes the current to go through hVSS, which starts to heat up like crazy, so they quickly open hSC (it has no current flowing so it won't arc). Then they open hVSS before it melts down, and finally open lSC to cut off the leakage current. ------------------------------------------------------------------------------ Free Next-Gen Firewall Hardware Offer Buy your Sophos next-gen firewall before the end March 2013 and get the hardware for free! Learn more. http://p.sf.net/sfu/sophos-d2d-feb _______________________________________________ Emc-users mailing list Emc-users@lists.sourceforge.net https://lists.sourceforge.net/lists/listinfo/emc-users
