On 3/4/2020 10:15 PM, Jim Brown wrote:
Power factor is a quantity associated with mains power (and similar
power distribution systems). In the old days (when I went through EE),
it was, indeed, related to the phase angle.
I meant to add that power factor now includes the triplen harmonics
resulting from non-sinusoidal current with power systems that are mostly
electronic loads. Current flows mostly in short peaks at the positive
and negative peaks of the mains waveform to charge the filter capacitors
in the power supply; as a result, the voltage waveform is rounded off
and the current is rich in harmonics of the mains frequency, 50 or 60 Hz.
Nearly all power distribution uses 3-phase wiring, and "triplen"
harmonics (those whose number is divisible by 3) add in the neutral, no
matter how well balanced the load between phases. This turned into a
major problem roughly 50 years ago, when a very high fraction of current
in these systems was delivered to electronic loads, everything from
fluorescent lights to computers to anything with an AC to DC power
supply. The most dangerous side effect is that triplen currents in the
neutral of 3-phase systems can be almost twice the current in the
phases, which can cause destructive failures in older systems, where the
neutral used conductors and hardware rated for half the current in the
phases! The movie "The Flaming Inferno" is based on the true story of a
massive fire in a high rise that was started by that neutral current,
spread through the building in vertical "riser" spaces, and that was
fueled by the insulation on the cables themselves! Sometime in the last
century, a major TV station almost went off the air because excessive
heating of their power distribution was close to catching on fire! I
attended an SBE (Society of Broadcast Engineers) meeting led by the
consultant who worked with the station engineers on the problem.
MAJOR changes were made to standards for mains wiring after this event,
including the rating of transformers and neutral hardware to handle the
peak currents and the harmonic content, conduit requirements for the
wiring, and requirements for insulation on the wiring that does not
contribute to flame spread and that does not produce toxic fumes.
BTW -- the triplen harmonics also appear on equipment ground, and are
the reason why we mostly hear power line noise in the audio as "buzz"
(triplen harmonics of 50/60 Hz) rather than "hum" (pure 50/60 Hz).
There's a not-too-technical discussion of all of this written for hams
in http://k9yc.com/GroundingAndAudio.pdf
and a more detailed one written for sound and video professionals in
http://k9yc.com/SurgeXPowerGround.pdf
73, Jim K9YC
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