>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 It takes over twice as long for the starting surge to decay. I could still detect it past 8 seconds. This analysis shows switching at 0V is the best option. If you wish to do further analysis, the LTspice .ASC and .PLT files are at http://www.pst.netii.net/misc/48b96262.zip The wikipedia article states "To avoid magnetic inrush, only for transformers with an air gap in the core, the inductive load needs to be synchronously connected near a supply voltage peak." https://en.wikipedia.org/wiki/Inrush_current Clearly, from the above LTspice waveforms, switching at the peak gives the highest inrush surge that is possible to obtain. It also takes the longest time to decay. MRM _______________________________________________ time-nuts mailing list -- [email protected] To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts and follow the instructions there.
