On Wed, May 25, 2016 at 9:59 AM, Mike Monett <timen...@binsamp.e4ward.com> wrote:
> >Am 23.05.2016 um 05:15 schrieb Jim Palfreyman: > >> As far as a remedy goes we are going to try a solid state relay that > only > >> switches on at 0V in the AC waveform. This should slow the inrush > current, > >> and hopefully the magnetic impulse. > > > >In the context of transformers and motors, switching on at 0V is > >actually the worst point in time. > > > >< https://de.wikipedia.org/wiki/Einschalten_des_Transformators > (German) > > > >< > > > http://electrical-engineering-portal.com/practical-considerations-of-transformer-inrush-current > > > > > > >regards, Gerhard > > LTspice shows switching at 0V is the best point in time. With no > flux in the magnetics, the inrush current is limited by the circuit > resistance. The magnitude is given by Ohm's law: I = E / R. > > Switching at 0V, > > I = E / R > = 0 / R > = 0A > > However, switching at the peak of the voltage can give very high > inrush currents. You can verify this with LTspice. The schematic is > at > > http://www.pst.netii.net/misc/48b961a6.gif > > There are two circuits. They are identical except the second has the > applied voltage shifted by 90 degrees. The inductors have 1 Ohm > series resistance (not shown.) > > The waveforms are at > > http://www.pst.netii.net/misc/48b96217.gif > > The currents are IL1 and IL2. > > Switching at 0V, IL1 starts at zero. It rises to a peak of 900mA, > then falls back to zero. The DC offset takes 3 or 4 seconds to > decay, then the current is stable at zero +/- 450mA. > > Switching at the peak, IL2 is > > I = E / R > = 169.7 / 1 > = 169.7A > Er, no. It's an RL circuit for which: V(R) = E(1-e^(-(R/L)t) so I(R) = I(L) = (E/R)(1-e^(-(R/L)t) for t close to zero where E is essentially constant, I is going to rise with the time constant of R/L which for a 1H inductor and 1 ohm resistance is 1 second. I suggest running the simulation with a pulse voltage source, Tdelay 1ms, period 1s, Ton 0.5s (Period and Ton are fairly arbitrary, the Tdelay is important). You only need to run the simulation for a few ms. You will get a very different result. Now as Poul-Henning Kamp suggested, an induction motor is anything but an ideal inductor... _______________________________________________ time-nuts mailing list -- time-nuts@febo.com To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts and follow the instructions there.