My my ...
That's what we used to call cycloconverters during the steam era. It good to
know there are still relics from my time in use, at least conceptually.
Alex 4Z5KS
-----Original Message-----
From: Amps [mailto:amps-bounces@contesting.com] On Behalf Of Manfred
Mornhinweg
Sent: Saturday, October 05, 2013 7:16 PM
To: amps@contesting.com
Subject: [Amps] Power factor correction
A quick note to all those who are still interested in that power factor
theme:
Yesterday I repaired a 22kW motor drive, like there are millions being used
in industry. This is a quite modern one, more modern than the ones I dealt
with at my former job. So I was expecting to find it having active power
factor correction - but it doesn't! Instead it has plain simple passive PFC
using nothing else than a swinging choke between the rectifier and filter
capacitor, and according to its specifications it achieves a power factor of
0.98 with that simple method!
This is a surprise for me. I would have expected something less good from
passive PFC, with 0.98 and higher being reserved for active PFC.
For those of you who don't know these "drives", as they are commonly
called: They are basically a line-connected rectifier, filter, followed by a
three-phase IGBT bridge, along with a microprocessor circuit that implements
quite sophisticated control, and drives the IGBTs. The output is three
phase, variable frequency, voltage and current, created by pulse width
modulation at a few kilohertz, that powers a motor, whose speed, torque and
even angular position can be accurately controlled.
These things exist in power levels from a few hundred watts to several
hundred kilowatts, and probably even higher. The one I repaired yesterday is
for 22kW, accepting an input of 380 to 480V, three phase.
For the doubters among you, all its power electronics are of course directly
connected to the mains. It is intended to be installed by competent
electricians, and used by anyone. It's made in Finland, and meets all
European directives applicable at the time of its manufacture, in 2007 -
including leadless solder, much to my pain, because with its higher melting
point it was a chore to unsolder the IGBT module with its
24 thick rigid pins rated for 50 amperes each.
There are some more good hints we can take from units like this. For
example, the input rectifier is rated at 1600V peak inverse voltage, and the
beast is rated for up to 480V AC input. That means, they are using less than
twice the diode voltage, relative to the peak AC voltage. And this is for a
highly reliable industrial application. That shows how exaggerated it is to
use 4 times higher diode voltage in a ham amplifier.
Also the current rating of the diodes: The bridge is rated for 53A
continuous DC output. The actual DC current used is up to 45A. So the
headroom they use is really small.
Despite that, this unit failed from poor installation, not from anything
attributable to its design or manufacture. The installer failed to use the
cable restraints provided by the equipment. He left the heavy cable hang
free from the contacts! Eventually a wire worked loose, made a short to the
chassis, creating a formidable arc that caused secondary arcs throughout the
power circuitry, killing essentially all semiconductors in the area.
Even the best design and construction of equipment cannot prevent users from
killing it through stupid or careless actions. And that's as true for
industrial equipment as it is for ham gear!
Manfred
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Visit my hobby homepage!
http://ludens.cl
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