The current distortion from simple transformer-rectifier-capacitor power
supplies contains a lot of third harmonic content. In a 3 phase system (as are
all distribution systems for commercial and industrial) the third harmonic ADDS
in the neutral, or creates circulating currents in a delta configuration. These
currents, as you mention, can get very large and were the cause of many
transformer explosions in cities as these power supplies became common. The
transformer designs had to be improved, but the PFC supplies make a big difference.
How many of you have looked at the power line waveform, especially in an
industrial or commercial area? Doesn't look much like a sine wave, does it? So
it's pretty funny to see audiophile outlets
(http://www.dedicatedaudio.com/power_outlets).
Peter
On 6/15/2013 6:56 PM, stan, W1LE wrote:
PFC to me is power factor correction, not only the classical power factor to
minimize (VAR) volt-amp reactive component,
but also to remove the harmonic load current imposd on the electrical power
system.
A '90's onward technique. in th 80's and 90's without the harmonic load
current reduction and having
a few 100 end items of equipment, each withtheir own a switch mode power
supplly,
it was not uncommon to find hundreds of amps of the third harmonc on neutra,
in the electrical power distribution system.
Could be a serious EMC problem if you were dealing with voice grade channels.
And people safety issues.
Stan, W1LE Cape Cod
On 15-Jun-13 5:52 PM, J. L. Trantham wrote:
Sorry for the interruption but what is 'PFC'?
Thanks.
Joe
-----Original Message-----
From: [email protected] [mailto:[email protected]] On
Behalf Of Poul-Henning Kamp
Sent: Saturday, June 15, 2013 4:09 PM
To: Robert Atkinson; Discussion of precise time and frequency measurement
Cc: Perry Sandeen
Subject: Re: [time-nuts] HP and other equipment failure
In message <[email protected]>,
Robert Atkinson writes:
While I agree with everything else you say, you CAN have too much
filter capacitance. At least where dc rectifier / filter (smoothing)
circuits are concerned. Increasing C causes increased ripple current
[...]
And ripple current can be a major source of power-line frequency noise in
all electronics.
The main reason why switchmode power-supplies today (can) outperform linear
power supplies with respect to noise, is because the legally mandated PFC
correction eliminates the bridge-rectifier ripple harmonics.
I would not hessitate to use a good quality switchmode to replace the linear
supply in a HP5370B.
I did some experiments a couple of years ago, with an audio-amplifier:
I put a standard PFC corrector chip on the secondary side of the trafo.
The overall result was not satisfactory, but the 50 Hz "sneer"
we all know and hate was absent, and the "Tzoing!!!!!" power-on mechanical
shock from the trafo was also eliminated, as was the consequent dimming of
the lights ;-)
The main reason not to do this, is that you need some physically gargantuan
coils for a 10A+ PFC-switcher.
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