I want to know if some one can direct me where I can find information
on how to implement a power factor correction on a three phase
alternator with permanent magnets. I have a small alternator with high
inductive winding and it's not efficient if I use only rectifiers to
make my DC bus.
I don't know if I can use the same principles used with a three phase
transformer when I have variable frequency due to the speed variation
of the alternator.
Well, Pino, I don't exactly follow you. But I will mention that vehicles
have small alternators from which DC is derived. Maybe you can find
information on techniques used in those alternators, to guide you.
On 1/26/06 9:57 PM, in article rDiCf.337351$2k.4048@pd7tw1no, "Don Kelly"
While I agree that the original question is nonsense, the use of rectifiers
can lead to reduced power factors (form factor). The answer is harmonic
filtering rather than conventional power factor compensation.
-- Ferme le Bush
It seems no one have an idea about what I'm talking. So, I'll try
to explain it in more details.
First, I have a small (just above 1 kVA) three phase permanent magnet
alternator. The output of this alternator is of course AC. The rotation
speed of the alternator is variable and produces sine wave from 50 to
AC into DC and follow by a filtration stage.
Beside that my power source is an alternator, this configuration is
exactly the same as if I was using a typical three phase transformer.
As it known from every one how work with that king of circuit, the
power factor is usually better in three phase then single phase but it
is less then unitary.
My question is :
Can some one tell me where I can find some literature on the effects of
the variation of the frequency and the reactance of the power source on
the efficency of a VIENNA or a full bridge active power factor
p.s. By definition, PFC is an AC to DC converter whish is view from an
AC power source as an almost resistive load (unitary power factor).
Sorry if I skip some details in first but I didn't think to make a
OK (though I don't know the particulars of 'Gaetz' but I presume it's not
A typical three phase transformer transforms AC to AC. You are converting
AC to DC. This is not the same. Maybe you should try a word other than
You sure think highly of dc converters. The King of Circuits.
Until someone corrects me, I will accept this on your word. For my info,
are you talking about bad power factor due to fundamental displacement or
because of distortion?
Darn, that started out sounding real promising. I don't know VIENNA. I
would imagine source frequency and reactance effect active PFC circuits
differently, depending on the type of active PFC circuit. I'm sure there
are many types which would react differently to the changing parameters you
mention. By the way, where did the full bridge active power factor
correction circuit come from, I thought you were making a dc supply with a
P.S. That's not the definition of power factor correction. Not by a long
shot. And if the goddamn thing has a unity power factor why did you tell me
everyone how work with that king of circuit known that the power factors are
less than unity? Or are you saying you have a circuit with (a) an
alternator as a source, (b) a dc power supply feeding some load or no load,
and (c) an active PFC circuit to fix the poor power factor of the dc power
supply? Or maybe you have a circuit with (a) an alternator as a source and
(b) a circuit (VIENNA, possibly?) that is a dc power supply but that also
appears as a resistive load because it actively corrects the current
waveform that it draws? No I don't know of any specific literature
discussing the efficiency of active PFC with varying frequency / source
Right now you're not half as sorry as I am. You owe me 15 minutes.
If you mean you are using a three-phase alternator to feed your diode bridge
and filtration instead of a three-phase transformer, then yes, I can see
that. Except your alternator has a widely varying frequency and a
three-phase transformer would be a fixed frequency at the local supply
The 'power factor' is actually two components. It can have a time-based
component as is seen by inductive or capacitive loads on an AC system.
These loads cause a phase shift between the applied voltage and the
resulting current, resulting in a power factor equal to the cosine of the
phase shift. But there can also be a harmonic component to power factor
caused by applying AC to a non-linear load.
In your case, you have a diode bridge that will only conduct when the diodes
are forward biased, and that only happens during that part of each phase's
sine wave when the AC voltage exceeds the DC voltage on the output side of
your diode bridge. This results in no current flow for parts of the sine
wave when the instantaneous voltage is low, and then high current flow in
that part of the sine wave when the instantaneous voltage is higher than the
diode bridge output. The current waveform has a lot of harmonic content
that produces a poor power factor.
Well I have to admit I hadn't heard of a Vienna Rectifier before, so I went
and Googled to find this.
I must say, it seems like an ingenious circuit to rectify AC to DC without
generating a lot of harmonics typically generated by conventional diode
bridge rectifiers. Although a lot of complexity to avoid harmonic
distortion. Since your alternator is supplying just this load, why is
harmonic distortion so much of a concern? Getting too much heating in the
I haven't worked with this before (didn't even know it existed), but I
notice that the PWM of the MOSFETS must be synchronized with the AC supply.
Also the AC input has an inductor in each leg and so obviously the reactance
will vary quite a bit with your varying supply frequency. In order to
maintain the power factor, your 'fuzzy logic' control would have to
compensate for the increased reactance as frequency rises. Exactly how, I
Sorry can't be more helpful, but perhaps if someone else knows more about
Vienna Rectifier's they could contribute.
Finaly some one how know about what I'm talking.
This is the question.
No apologize, you help me a lot by explaining my problem in a "clear
By the way, I know that VIENNA topology is not the only way to do
active power factor correction in three phase. But it seem to be
common when I searched for A.P.F.C..
So, I'll continue my search elsewhere.
This is completely out of my field of expertise. However, the first two
solutions to think about with engineering problems is to (1) buy
something off the shelf, or (2) make sure it's a problem before you fix
I know that there are commerical power factors correction units
available, so that's one thing you might want to look at. Second, it
doesn't particularly matter, per se, if something is inefficient as
long as it does the job, but if it doesn't do the job it might be
simplier and cheaper just to get larger components. The power company,
incidentally, won't charge residential customers anything extra for a
low power factor. The power factor in my computers' power supply is
probably only about .6 or .7 and I don't worry about it at all.
No you aren't. He was correct. Residential customers only pay for real power
(the "useful" power). The reactive power which results in lower power factor
is free. To be technically correct, you will have slightly more real power
consumption due to greater line losses, but the difference is insignificant
compared to the total.
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