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Power factor correction

T

Tim Williams

Jan 1, 1970
0
PFC is a flyback thing. But flybacks suck over 100W, where forward looks
better. But forward sucks over wide duty cycle ranges. What do they do,
just bigger flybacks? Nothing at all?

Tim
 
F

Fred Bartoli

Jan 1, 1970
0
Tim Williams a écrit :
PFC is a flyback thing. But flybacks suck over 100W, where forward looks
better. But forward sucks over wide duty cycle ranges. What do they do,
just bigger flybacks? Nothing at all?

Tim

Flybacks work great in PFC but have pulsed input current which isn't so
great.
The usual PFC is a boost, but you don't get isolation and have to manage
inrush current.

I once did a 250W isolated sepic PFC with zero input ripple.
Worked great with none of the above pb.
I think I've made a couple of posts here. 't was circa 2001-2002.
 
T

Tim Williams

Jan 1, 1970
0
Jan Panteltje said:
For much higher power 3 phase rectifiers are used.
That leaves only a small ripple.
I am not sure if you need PFC in such a case.

Maybe, maybe not. You get 60 degree conduction, which isn't quite 90, so
maybe it would benefit some. Where cap input filters are used, conduction
can still be narrower and peakier.

What I'm most interested in is single phase, since there's a small band
between 2kVA-20kVA where single phase is available. My welder runs from a
240V, 50A circuit, for instance. Just imagine how many home machinists have
their shops wired with, say, a 240V 100A circuit, and have all their
machines running from a rotary phase converter because nobody mades single
phase motors over 1-2 HP. Three phase simply isn't an option for
residential in the U.S..

So I guess flyback (well, boost) is still the way to go? Just more phases?
Three phase I suppose would be a good start for this power level? I don't
like the idea of CCM, even at 200kHz those inductors will be bigger than the
rest of the circuit after it!

Tim
 
T

Tim Williams

Jan 1, 1970
0
MooseFET said:
You can also make the frequency rise as you go to the lower voltages.
The frequency and inductance can be optimized for near the peak
voltage. Since this is where most of the power transfers, the
efficiency is dominated by it. You can then raise the frequency as
you go to lower voltages to semi-optimize at each voltage point.

I was looking at the FAN9611 the other day. BCM is nice because it uses
"all" the inductor's capacity. Now, the datasheet says it's useful for
100-1000W, but do you think it'd be good for another decade beyond that?
What scares me is the frequency clamp stops at "only" 520-750kHz. I don't
think I want to deal with that much dV/dt and dI/dt just yet...

It doesn't look like Fairchild is making more than biphase chips... TI has
the UCC28070 which looks fairly equivalent (with constant frequency
instead). Offhand, I'm not finding anything with more than two phases.

Tim
 
A

Alan W

Jan 1, 1970
0
PFC is a flyback thing. But flybacks suck over 100W, where forward looks
better. But forward sucks over wide duty cycle ranges. What do they do,
just bigger flybacks? Nothing at all?

Tim

I've done a 2kW CCM PFC using a boost converter - the inductor was
pretty massive (several pounds at least), but then again it was running
at 50 kHz. (It was DSP-based, so the control loop couldn't run much faster.)

It was for a 2HP VFD, and there wasn't any size or weight constraint,
otherwise I would have figured out how to use a higher frequency with a
smaller inductor.

I seem to remember Bi Technologies made some power modules designed for
PFC converters in the 2-3kW range, and had some app notes for them.

- Alan
 
T

Tim Williams

Jan 1, 1970
0
Jon Elson said:
For home shop use, a VFD can't be beat.

Unless you're cheap, which most of rec.crafts.metalworking IS. ;-) When
you get a lathe for $200 and a case of beer, VFDs don't look so cheap.
They're definitely nice though.

As for electrical characteristics, VFDs are a fine example: what do they do
for PFC? Nothing? The VFDs installed in the school's power lab are 10 or
15HP, three phase input, and according to the Fluke PQAs, current is spikey
as hell, no PFC, big gulps of capacitor charging. Doesn't seem very nice to
me, and when you're losing half your VAs in harmonic current, it's just not
practical from a single phase supply.
The typical larger PFC scheme uses a boost converter that is modulated by
the voltage waveform, so the current draw matches the instantaneous
voltage, scaled by average current demand. This stage precedes the main
converter, which would usually be a push-pull design, and you can use the
topology and drive scheme of your choice in that converter.

Yeah, but that doesn't work well at let's say 400V and 30A. The inductor is
huge, and it's an awful lot of ripple on the capacitors. There's gotta be a
better way (multiphase helps).

Tim
 
J

JosephKK

Jan 1, 1970
0
PFC is a flyback thing. But flybacks suck over 100W, where forward looks
better. But forward sucks over wide duty cycle ranges. What do they do,
just bigger flybacks? Nothing at all?

Tim

What? Even i have done flyback at about 15 W and have heard of them usedin
the 10s of milliwatt range. I think i have seen PFC that was done with SEPIC
in the 20 watt range.
 
J

JosephKK

Jan 1, 1970
0
For much higher power 3 phase rectifiers are used.
That leaves only a small ripple.
I am not sure if you need PFC in such a case.

I am playing around with modeling 3-phase rectifiers. I think PFC
would be a real good thing for 3-phase rectifiers. May be more than
a tad interesting to implement though.

I am getting some interesting results. I would really like to
build some and measure them.


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And

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J

JosephKK

Jan 1, 1970
0
At modestly larger that 100W the flyback booster method still works
ok. The bulk of the power is transfered by the booster when its boost
ratio is small. To handle the very wide duty cycle dropping the
frequency as the mains nears its peak works.

Multiple boosters running out of phase will let you go up to higher
power levels and the ripple tends to cancel.

Other designs do use various sorts of forward converters. No matter
what you do, you need to have a core somewhere that has a significant
energy storage. To make the forward converter method work, the output
side inductor has to do the storing. Going up in frequency, keeps the
inductor's mechanical size smallish.

Not only that, i tripped over an alternate reading.
 
P

Paul Keinanen

Jan 1, 1970
0
For much higher power 3 phase rectifiers are used.
That leaves only a small ripple.
I am not sure if you need PFC in such a case.

The ordinary 3 phase 6 pulse rectifier is a quite nasty polluter of
the mains.

To reduce the harmonics, 12 pulse rectifiers have been used, with one
set of rectifiers connected to the wye windings and the other set from
separate delta windings. For high power systems, you still are going
to need a medium/low voltage transformer on site, so it is not a big
deal having separate wye and delta secondary windings on that
transformer.

If you want to get away with the heavy 50 Hz transformer, there seems
to be various more or less patented 3 phase systems using flyback etc.
systems to reduce the PFC, but these seems to be hugely complex and
have special floating requirements for the DC side.

If floating output or voltages other than those obtainable by simple
rectifiers are required, it might be easier to simply get three single
phase SMPS with PFC at the input and connect the DC outputs in
parallel.

If the input voltage ratings permit, instead of wye, connect the
inputs into delta to avoid any mains neutral polluting and also allow
higher voltage and hence higher power (1.7x) for a specific
semiconductor amperage.
 
F

Fred Bartoli

Jan 1, 1970
0
Paul Keinanen a écrit :
The ordinary 3 phase 6 pulse rectifier is a quite nasty polluter of
the mains.

To reduce the harmonics, 12 pulse rectifiers have been used, with one
set of rectifiers connected to the wye windings and the other set from
separate delta windings. For high power systems, you still are going
to need a medium/low voltage transformer on site, so it is not a big
deal having separate wye and delta secondary windings on that
transformer.

If you want to get away with the heavy 50 Hz transformer, there seems
to be various more or less patented 3 phase systems using flyback etc.
systems to reduce the PFC, but these seems to be hugely complex and
have special floating requirements for the DC side.

If floating output or voltages other than those obtainable by simple
rectifiers are required, it might be easier to simply get three single
phase SMPS with PFC at the input and connect the DC outputs in
parallel.

If the input voltage ratings permit, instead of wye, connect the
inputs into delta to avoid any mains neutral polluting and also allow
higher voltage and hence higher power (1.7x) for a specific
semiconductor amperage.

For Tim, which I suspect is asking this for his induction heater, it
might be easier to use his bridge so as to draw sine current.

The output power will be sine squared but who cares? And that'll put
more requirement on some components for the same average output power,
but it'll avoid a full PFC stage which won't do good on efficiency, and
this might have a higher overall efficiency.
 
T

Tim Williams

Jan 1, 1970
0
Fred Bartoli said:
For Tim, which I suspect is asking this for his induction heater, it might
be easier to use his bridge so as to draw sine current.

Yup. But it's also a good general question.
The output power will be sine squared but who cares? And that'll put more
requirement on some components for the same average output power, but
it'll avoid a full PFC stage which won't do good on efficiency, and this
might have a higher overall efficiency.

I may do this, and it even makes the coil hum with a pleasing growl. The
downside is it puts 120Hz into all my loop, and makes scoping the inverter
fairly useless. Constant current mode would have to be slower than 120Hz,
and I'm not sure how 120Hz will affect the PLL.

Tim
 
T

Tim Williams

Jan 1, 1970
0
MooseFET said:
With the coil, the current is near constant. This makes the harmonic
content less
but still significant. Adding a moderate sized transformer at each
phase and doubling
the number of diodes will lower the harmonics quite a bit.

Basically it is 3 of these
...

And of course, since you have two phases on two wires (let the other be
common), you have a linearly independent basis (not orthonormal, but so
what) from which you can span the entire vector space of voltage and phase.
So you could, for example, produce a 5-phase system with 10 pulses, or 120
if you wanted. You spend a lot on transformers, though.

What does that do for current, anyway? Current through each phase is
essentially a parabolic pulse. All those added up, in the ratios from which
they are generated, should distribute to a fairly constant current,
shouldn't they? Well, the sum of (three phase) currents is always zero, but
the sum of magnitudes isn't: that must oscillate at the 6th harmonic. So it
should be that, in the same way as a choke input filter causes relatively
more current draw on the flanks of the sine wave than overall, this
arrangement also causes more current draw on the flanks, resulting in an
inversely phased 6th harmonic. The curious part is, this nonlinear
conclusion was based on the linear construction of vectors: of course the
diodes, being nonlinear elements, are the reason, but the voltages don't
seem like they should do that.

Tim
 
T

Tim Williams

Jan 1, 1970
0
MooseFET said:
When the number of poles becomes infinite, the harmonics all drop to
zero.
Beyond the infinite number, there is no further improvement.
I hope this helps.

How can there ever not be harmonics, either voltage or current, input or
output, when transforming three phase to DC? I think a simple energy budget
shows this is necessary.

Ignoring the three phase source, if you start with n equal voltage, equally
spaced phases, you will get harmonics of 2n and higher, with a current pulse
roughly 1/n of the waveform per phase.

Tim
 
K

krw

Jan 1, 1970
0
On a sunny day (Mon, 4 Jan 2010 18:19:15 -0800 (PST)) it happened MooseFET
<b4f80fb0-b390-4a51-949b-00073266a8f3@s31g2000yqs.googlegroups.com>:


mmm I dunno,
designed something for the army that a had selenium rectifier in it..

I love the smell of burning selenium in the morning.
 
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