On Wednesday 13 February 2013 21:11:23 Stephen Dubovsky did opine: > On Wed, Feb 13, 2013 at 11:59 AM, Gene Heskett <[email protected]> wrote: > > My point was, and still is, that relays & switches, unless mercury > > wetted, are generally dirty at both operations > > Contactors do not 'bounce' on open. Esp w/ inductive loads;)
In 60 some years of watching magnetically driven contacts open and close, or any mechanical switch, open and close, the open /may/ be prolonged by maintaining conduction through the arcing of an inductive load, but I have also seen the contact open, and the voltage rise in the next microsecond or 10, high enough to re form the arc, effectively closing the contact again, 5 or 6 times in one instance. In that particular instance, platinum contacts at about $300 1965 dollars a set of 4 were being destroyed in 6 months. And arc snubber made out a a T shaped string of 2200 ohm 2 watt carbon resistors, and a stack of NE-2 neon lamps across the contacts resulted in zero failures over the next 57k hours of operation. Probably longer as I went down the road for a better job > An enable signal to the drive is NOT safe w/ DC servos. When the > transistors fail, they will apply full voltage to the motor. True, and also true for steppers except the stepper is frozen by the transistor failure but the method I suggested would still result in the power being removed in a short time, 50 milliseconds perhaps. In your case, you are saying the driver was fubar first, but my experience says the drivers transistors were probably the second failure, not the first. Something disturbed the circuit, and a 600 to 1000 volt rated hexfet was destroyed by the surge, in a circuit that was supposed to be running on 180 volts. In circuits such as this, so called infant mortality failures are generally going to be in the first minute of operation, if not considerably less. Old age failures are more than likely not age of the semiconductor related, but age of the capacitors used for energy storage in these circuits. ESR developing in the capacitors allows ever higher amounts of noise to build up until the semiconductor lets go. This is NOT the fault of the semiconductor. And if you replace the semiconductor without replacing those usually bad electrolytic capacitors, the semiconductors _will_ fail again in a relatively short time. There is a guy who was in Omaha NE, but whom I believe has moved to Kansas City in the last 10 years, who makes a device called a Capacitor Wizard, <http://midwestdevices.com/> which only does one test, it measures the capacitor's Equivalent Series Resistance with an applied voltage of about 50 millivolts worth of 100 kilohertz signal. At that low a voltage, the semi's aren't turned on so even "in circuit" you are looking only at the capacitor you are probing. Only 2 ohms of resistance is sufficient to take the needle out of the "green is good" area on the meter scale. This is now selling at $229, and as a Certified Electronics Technician, I have tried other versions of this circuit, all have been found wanting in comparison to this one. This one Just Works(TM). > The only > surefire way to stop them is disconnect the motor or disconnect DC power > from the bridge. See above, my method, if a transistor failure isn't being dealt with, is much easier on the drivers. If a transistor has failed, it will still be shut down in another few milliseconds. The difference, ignoring the potentially damaged part, is in how much it costs to restore the failed driver, and how long that restoration lasts. If you have to do it again 2 weeks later, then the driver restoration wasn't done correctly. > SMD Cheers, Gene -- "There are four boxes to be used in defense of liberty: soap, ballot, jury, and ammo. Please use in that order." -Ed Howdershelt (Author) My web page: <http://coyoteden.dyndns-free.com:85/gene> is up! My views <http://www.armchairpatriot.com/What%20Has%20America%20Become.shtml> I can't stand squealers; hit that guy. -- Albert Anastasia I was taught to respect my elders, but its getting harder and harder to find any... ------------------------------------------------------------------------------ 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 [email protected] https://lists.sourceforge.net/lists/listinfo/emc-users
