Date: Mon, 23 Sep 2013 09:07:30 -0700
From: Jim Brown <jim@audiosystemsgroup.com>
To: amps@contesting.com
Subject: Re: [Amps] ?Direct rectification of AC mains to drive the
amp,
On 9/22/2013 10:10 PM, Jim Thomson wrote:
> Has anybody actually measured the PF on a transformerless supply, with
> a big C filter ???
> I cant see it being any worse than your typ kw tube amp....with the
> typ xfmr + FWB / Doubler +
> high C filter.
No, but the issue is not reactance, it is the fact that current is not
sinusoidal, but rather is drawn in pulses at the peaks of the cycle to
charge that cap. Back when most of us went to school PF was simply
related to the phase angle between the sine wave of voltage and the sine
wave of current. Now, the definition of PF has been expanded to include
that impulsive current.
Also the current is impulsive,so it is rich in harmonics, and "triplen"
harmonics add in the neutral of 3-phase systems. Because the loads on
electrical systems are now dominated by equipment of all sorts with
capacitor-input filters, and triplen harmonics add in the neutral of
3-phase systems, it has become all too common for currents in the
neutral of 3-phase systems to be equal to or greater than the phase
currents. Remember "the towering inferno?? The cause was overheating
neutrals, and flames were burning insulation that spread up raceways.
In EU, power factor is regulated by electrical codes. In North America
it is not, although I think I've heard that power companies may offer
discount rates for maintaining a better PF.
73, Jim K9YC.
## This is all old news. Typ ham tube type amplifiers with high C filters are
the
original pulse train power supplys. Its easy to see these high level
pulses every
8.3 msecs, using ducans Psud software. Or with a scope across a small fixed
resistance.
## Im well aware of the excessive high neutral current in the towering inferno
movie....true story btw.
The neutral current typ runs at 2.1 x normal. However, this is ONLY when you
use 120 vac single
phase loads on one leg of a 3 phase system. IE: your typ 208/120 v 3
phase power fed to
condos and apartment buildings..and commercial buildings. In one recent
study, in seattle, wash,
they had hundreds of pcs..all running on 120 vac single phase, on each of 4 x
floors. From the street
it was 14.4 kv 3 phase... dropping down to 600 v 3 phase...with the usual
vault xfmr. Then a 600v
3 phase riser cable going to all 4 x floors. Each floor had its own 600
V down to 208/120 step
down xfmr... a 50 kva unit. The load on each xfmr was only 38 kva, but the
xfmrs were ready to burn up.
## The normal fix is to replace the 50 kva xfmr with a larger 75 kva unit.
The real fix these days is to use\the newer
style amphorus xfmrs..which have a 3rd winding in them..which eats the
harmonics. Then you can easily get away with
a 50 kva xfmr.
## IF EU style PF regulations were ever imposed on ham gear,.. there is NO
way you could ever use the typ
HV supply config, with the FWB /doubler + high C filter. You can easily see
the ODD harmonics on any of this
stuff, using the HP harmonic analyzer, that’s made for this purpose. It
will display both even + odd harmonics out to the
21st harmonic. The single biggest problem in each case is the 3rd harmonic,
followed by the 5th, the 7th is way down... and the
9-13-15-17-19-21 is a non issue. There is no easy way to implement PF
correction, unless the load is steady state, like a AC motor.
With stuff like high HP motors, thel oad is inductive..and the usual huge PF
correction caps are used. Even then, the value of the cap is
only correct for a specific load. Change the load on the motor, and the cap is
the wrong value. In pulp mills etc, you will see
banks of caps, with dozens of contactors used to switch em in and out.... to
obtain the best PF.
## On a varying capacitive load.,...like what you get from a SSB /cw ham tube
type linear amp with a xfmr + high C filter cap, its impossible to
implement PF correction. With a steady load like RTTY /FM, an inductor can
be used for PF correction, but it has to be sized exact for the
steady state load. Even then, the best you can do with either a motor load +
cap..... or a HV supply + inductor....is to kill the 3rd harmonic.
You cant kill the 5-7-9th harmonics..so you still wont get unity PF.
## In NA, for commercial use, you get penalized for bad PF. They want to
see at least .9 PF and better. In some states in the US, if your
PF is below .85 they are not obligated to provide power..and will read you
the riot act. Install PF correction, or get your power shut off !
## The new style xfmrs wit the 3rd winding came about after the Bush jr,
energy bill came out. It is easier to use a xfmr that handles harmonics vs
the end user trying to implement PF correction. This all came about b4
switchers that also had PF correction implemented. Most switchers don’t have
PF correction.. but that is changing..esp with EU regs.
## I went through all this PF correction with the late Peter dahl several
times over a 2 year period. The consensus was..with the typ HV xfmr + FWB
/ doubler + high C filter.... don’t attempt trying to implement PF
correction. Dahl told me he had tried several times....with poor results. I
tried several
configs myself, with the help fo a ham electrician buddy...and we ended up
either making the PF worse.. or a tiny bit better. It can be done on a
steady state load,
but not on a varying load like ssb /cw. In the end, I gavc up. The good
news is, dahl sez his xfmrs will easily handle lousy PF. They are designed
for it.
Put in all the C you want. He was correct of course. 800 uf doesnt cause
any additional heating of the xfmr.
## having said all that, direct rectification of the 240 vac line is still
feasible. The PF will be no worse than if a Xfmr + high C filter was used.
You wont
get sky high neutral currents since the neutral is not being used. But a
50vdc @ 65A CCS switching type supply, 3.3 KW CCS,, WITH PF
correction is cheap at $450.00 to $500.00
Jim VE7RF
_______________________________________________
Amps mailing list
Amps@contesting.com
http://lists.contesting.com/mailman/listinfo/amps
|