The usual method to limit inrush that is used in almost all electronic devices is to place an NTC thermistor in series with the AC mains supply. If you take apart any TV, stereo amp, or PC power supply that was built this century you find them inside.
They are really easy to use. They act just like a resister and limt current just like a resistor but the resistance changes with time. When you first hit the "on" switch the thermistor might have a resistance of 100 Ohms, then after a second it falls to about 2 ohms. Ametherm is a major manufacturer of these and will give anyone a handful of samples and there is a ton of information on their web site about how to select and use these devices. https://www.ametherm.com The trick to using these is to match their specs to your needs. Yu need to use one that has the correct resistances, and time constant. It is like building an RC lowpass filter. Yes, you could simulate a thermistor with some switches, a timer, and load resisters or just ask Ametherm and they give you a dozen samples. There is a technical article on the above web site that is titled "limiting in-rush current in DC motors". They work out a problem where you place an NTC Thermister in series with a large DC motor. In this case the resistence starts at about 1 Ohm and falls to about 0.01 Ohms when the motor is running full speed. With motors they place them in series with the motor. In a power supy they place them in series with the AC mains or in series with the transformers winding that has the most current. The best thing is that these things are so simple that the can't fail and they are way-cheap. On Tue, May 19, 2020 at 10:51 AM Gene Heskett <[email protected]> wrote: > Greetings all; > > Having destroyed one 40 amp SSR by turning on the switching psu;s for the > two running the X&y motors and the builtin supply of the even bigger > drive running the Z axis, already, I am now contemplating how to do a > soft start to limit the initial in-rush of charging up those 3 power > supplies. > > And doing it in a lower powered version of how I am doing the huge analog > supply that runs the spindle motor. > > There, I have two timers which are controlling two 40 amp SSR's with a 50 > ohm 200 watt resistor, so it starts charging that supply with its > thousands of u-f's by applying the power thru that big resistor, then > the 2nd SSR comes on applying it directly 3 seconds later. This has > worked flawlessly for about 4 years now. > > Now I have obtained two more used genuine crydom 40 amp 480 volt SSR's, > and am considering doing the same thing in essence, only with a much > smaller ohmage R to limit the in-rush this batch of switchers in a > similar manner. > > But switchers, I have heard, need that initial bump to get started > correctly. To that end I've also ordered a pair of 10 ohm 70 watt > resistors, which if need be can be paralleled for 5 ohms in series with > these supplies for the first 1/2 to 1 second. > > But I intend to use one of these in another analog supply, starting the > spindle psu in The Little Monster too, putting all its heavy power under > LCNC's control. But that is a different project. > > These switchers are, or s/b all fused to protect them in the event they > don't start. So I don't expect a start failure to do more than blow the > fuse. > > Has anyone else any experience with this, that can add gotcha's to watch > for in such a current limited startup? 10 ohms in series with the power > to protect the SSR's is the basic idea. 10 ohms would limit at 12.5 amps > of in-rush, which seems reasonable. > > Thanks all. > > 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) > If we desire respect for the law, we must first make the law respectable. > - Louis D. Brandeis > Genes Web page <http://geneslinuxbox.net:6309/gene> > > > _______________________________________________ > Emc-users mailing list > [email protected] > https://lists.sourceforge.net/lists/listinfo/emc-users > -- Chris Albertson Redondo Beach, California _______________________________________________ Emc-users mailing list [email protected] https://lists.sourceforge.net/lists/listinfo/emc-users
