On a sunny day (Mon, 04 Jan 2010 07:53:30 +0200) it happened Paul Keinanen wrote in :
Yes, the ones I have dealt with also had a huge heavy inductor beteen the rectifiers and the filtercaps. L ========= -------------------------^^^^^^^^^---------------- + | | | | --- --- --- | / \ / \ / \ | --- --- --- | | | | |
--- | | | | | | | ===
--- ) ------| | | | | --- C
--- ) ----- ) ----- | | | | | | --- --- --- | / \ / \ / \ | --- --- --- | | | | | -------------------------------------------------- - The inductor smoothes the main current, and reduces capacitor ripple current. For something like 4kVA and up.
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
------------------ To bridge ( A---------- ( ------------------ To bridge
B----------- ) ) ) C-----------
Since the voltage between B and C is at 90 degrees to the voltage from A to ground, the voltage on the secondary only needs to be the tan() of the angle you are shifting the phase by.
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
--
Deep Friar: a very philosophical monk.
Website: http://webpages.charter.net/dawill/tmoranwms
... Or maybe too young. Back when rectifiers couldn't handle the power, you needed more diodes and a way to share the current so doing this didn't add parts.
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
--
Deep Friar: a very philosophical monk.
Website: http://webpages.charter.net/dawill/tmoranwms
I think you missed that the source is 3 phase power and we are constructing more phases with the transformers. If so the rest won't be needed as you will already see it.
Imagine that you have a synchronous motor running on 3 phase. The 3 phase power makes a rotating magnetic field that is of constant strength.
Now imagine that the motor is moving an infinitely fine commutator under some brushes such that the contacts are always at the voltage peaks. This is the infinite number of poles case.
--
"Electricity is of two kinds, positive and negative. The difference
is, I presume, that one comes a little more expensive, but is more
durable; the other is a cheaper thing, but the moths get into it."
(Stephen Leacock)
The way true and apparent power were calculated should include the effect of harmonics.
--
"Electricity is of two kinds, positive and negative. The difference
is, I presume, that one comes a little more expensive, but is more
durable; the other is a cheaper thing, but the moths get into it."
(Stephen Leacock)
While is most certainly does include the power in the harmonics, it does = not=20 calculate the spectral power of each of the various harmonics. I am=20 interested in the harmonic power by harmonic number.
It is often better to start off with knowing the real power. Stripping that out of the waveform makes the wave form you need to process much easier to deal with. Only if the FFT in spice works on exactly an integer number of cycles does it make the FFT into a single spike. The tail of the skirt on the fundamental shifts the other harmonics up.
Whilst you are investigating that, it would be interesting to evaluate the effect of different line reactors on harmonic currents in the supply.
--
"Electricity is of two kinds, positive and negative. The difference
is, I presume, that one comes a little more expensive, but is more
durable; the other is a cheaper thing, but the moths get into it."
(Stephen Leacock)
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