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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