John -
Try the test at a phase angle of 0 or 180 degrees. You should come a lot
closer to your calculated inrush current of 56.6. You are making the mistake
of seeing the input section of the power supply as a straight resistance
circuit. It is more inductive and will behave differently when the voltage
increases from a phase angle of 0 or 180 degrees than when it decreases from
a phase angle of 90 or 270 degrees.
Mike Campi
Corporate Compliance Engineer
Fujitsu PC Corporation
-----Original Message-----
From: jgarr...@hns.com [SMTP:jgarr...@hns.com]
Sent: Friday, September 11, 1998 8:02 AM
To: ieee pstc list
Subject: Inrush current and utility power line reistance
From: John Garrett@HNS on 09/11/98 11:02 AM
To: ieee pstc list <emc-p...@ieee.org>
cc:
Subject: Inrush current and utility power line reistance
Hello All,
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: jgarr...@hns.com
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