When you made your worst-case estimate of power supply resistance, did you take into account the inrush limiting resistor in the supply?
The low measured inrush value could be due to line inductance, but a quick SPICE simulation says that the line inductance would have to be greater than 3mH before the peak would drop from 57A to 26A. Hmmmm ... were you using an isolation transformer or variac for these tests? On Fri, 11 Sep 1998 11:02:25 -0400, John Garrett <[email protected]> wrote: >I have a question with respect to the typical resistance of the mains >utility wiring; from the mains utility transformer into a residential or >industrial building. But first a little history on the problem: > >When testing a 30 W power supply at 120VAC, the inrush current at a phase >angle of 90 degrees measured 27A peak (4 unique sites were tested). >Calculating inrush current is a fairly straight forward application of ohms >law: > Inrush Current = peak voltage divided by the dc resistance ( I = Vpeak/R) >(where R is the series resistance from the power line into the p/s through >the EMI filter through the rectifier and the bulk capacitor back out the >other leg of the line) > >In this application the result of the above equation is I = (120 >VAC*1.414) / 3.00 ohms or I = 56.6 Amps... where 3 ohms is the worst >case resistance internal to the power supply (The assumption here being >there is little if any resistance external to the power supply that will >add significantly to this internal resistance). > > The problem is this: the calculated number (56.6 A) does not come close to >the measured (27A). If fact, when we look at the measured data it appears >as if the external resistance, i.e. the resistance of residential or >industrial wiring from the utility mains transformer to the building and >internal to the building, is adding a very significant amount of resistance >(approx. 3.0 ohms). This is very hard to believe! But it is repeatable. >We are checking these measurements even as I write, but my questions are >simple. First, am I missing something here with respect to second order >effects? Second, does anyone have a feel for the resistance of the power >lines from the utility mains xfmr to the service entrance and into a >residential or industrial building? Have any studies been performed that I >can refer to for this type of information? Any help would be greatly >appreciated. > > > > >John Garrett Principal Engineer >Hughes Network Systems Phone (301) 601-2699 >11717 Exploration Lane FAX (301) 428-2835 >Germantown, MD 20876 Email: [email protected] > > > >--------- >This message is coming from the emc-pstc discussion list. >To cancel your subscription, send mail to [email protected] >with the single line: "unsubscribe emc-pstc" (without the >quotes). For help, send mail to [email protected], >[email protected], or [email protected] (the list >administrators). -- Patrick Lawler [email protected] --------- This message is coming from the emc-pstc discussion list. To cancel your subscription, send mail to [email protected] with the single line: "unsubscribe emc-pstc" (without the quotes). For help, send mail to [email protected], [email protected], or [email protected] (the list administrators).
