John-
I was afraid that things were getting long and confusing. The "Part 1" and "Part 2" comments in my reply refer to the posting immediately below mine. But to answer your fundamental objection. It is indeed true that if we place a resistive and capacitive load, measure voltage amplitude AND phase of the resulting voltage relative to the source (absolute phase), we can then calculate the source resistance and (inductive) reactance of the voltage source. However, if we cannot measure absolute phase of the resulting (loaded) voltage (phase relative to the source voltage), then we are stuck with magnitude-only calculations. Obtaining absolute phase needs some sort of power system clock or stable time reference. Measuring the phase between the current and voltage at the load tells us only about the phase characteristics of the load, and nothing about the voltage source. With the magnitude-only calculations, the resulting voltage equation for a resistive load contains terms in Rs², Xs², and Rs (Rs = source resistance, Xs = source reactance). For pure reactive loads, the resulting voltage equations contain terms in Rs², Xs², and Xs, with the coefficients for Rs² and Xs² in each equation equal. This last property allows one of the two voltage magnitude equations to be scaled such that when the two equations are subtracted from the other, the Rs² and Xs² terms cancel, leaving an equation with Xs as the only unknown. From there, Rs many be found. On the other hand, the voltage magnitude equation for the resistive load does NOT have equal coefficients for the Rs² and Xs² terms. Even if the coefficients were equal, the equation has the Rs term. Remember that the voltage magnitude equiation for the capacitive load contains an Xs term. Scaling and subtrating these two equations would yield an equation with Rs and Xs terms (two unknowns and one equation). Don Borowski Schweitzer Engineering Labs Pullman, WA John Woodgate <j...@jmwa.demon.co.uk>@majordomo.ieee.org on 11/05/2002 01:23:33 PM Please respond to John Woodgate <j...@jmwa.demon.co.uk> Sent by: owner-emc-p...@majordomo.ieee.org To: emc-p...@majordomo.ieee.org cc: Subject: Re: Measuring AC Line Impedance I read in !emc-pstc that don_borow...@selinc.com wrote (in <of32a3a512.366695dc-on88256c67.00802...@selinc.com>) about 'Measuring AC Line Impedance' on Tue, 5 Nov 2002: >Part 1. This measurement will NOT give just the resistive part of the >source impedance. Your post is so long that it is not clear what you are trying to correct. Also, by top-posting you make it more difficult to understand how your comments relate to previous texts. If we measure the voltage developed across a resistive load, we have one equation with three unknowns, e.m.f., source inductance and source resistance. If we measure the voltage developed across a pure capacitive load (or one with known losses), we also have one equation with three unknowns. If we measure the *open-circuit* voltage, we immediately have the value of one unknown, the e.m.f. or Thevenin equivalent source voltage. That leaves us with two equations and two unknowns, which are soluble. -- Regards, John Woodgate, OOO - Own Opinions Only. http://www.jmwa.demon.co.uk Interested in professional sound reinforcement and distribution? Then go to http://www.isce.org.uk PLEASE do NOT copy news posts to me by E-MAIL! ------------------------------------------- This message is from the IEEE EMC Society Product Safety Technical Committee emc-pstc discussion list. Visit our web site at: http://www.ewh.ieee.org/soc/emcs/pstc/ To cancel your subscription, send mail to: majord...@ieee.org with the single line: unsubscribe emc-pstc For help, send mail to the list administrators: Ron Pickard: emc-p...@hypercom.com Dave Heald: davehe...@attbi.com For policy questions, send mail to: Richard Nute: ri...@ieee.org Jim Bacher: j.bac...@ieee.org All emc-pstc postings are archived and searchable on the web at: http://ieeepstc.mindcruiser.com/ Click on "browse" and then "emc-pstc mailing list" ------------------------------------------- This message is from the IEEE EMC Society Product Safety Technical Committee emc-pstc discussion list. Visit our web site at: http://www.ewh.ieee.org/soc/emcs/pstc/ To cancel your subscription, send mail to: majord...@ieee.org with the single line: unsubscribe emc-pstc For help, send mail to the list administrators: Ron Pickard: emc-p...@hypercom.com Dave Heald: davehe...@attbi.com For policy questions, send mail to: Richard Nute: ri...@ieee.org Jim Bacher: j.bac...@ieee.org All emc-pstc postings are archived and searchable on the web at: http://ieeepstc.mindcruiser.com/ Click on "browse" and then "emc-pstc mailing list"
