A twelve-pulse transformer/rectifier series system offers the advantage of a
DC output whose ripple is about 1% without any capacitive filtering but
requires a very well designed 3phase transformer and the diode legs in the
rectifier unit have to be properly dimensioned.
A twelve-pulse transformer/rectifier paralleled system is quite complex
(also an extra winding is required if my memory doesn't fail) but the ripple
is one third 0.3% of the series version and the diodes voltage/current
requirements are not so demanding.
Althoug theorically not required, with twelve-pulse transformer/rectifier
series system it's a common practice to use a small
capacitive filter (abt 2 uF) across the output.
If the 3Phases transformer has not ben specifically designed for a WYE delta
series (parallel) rectifier system I wouldn't use it in such configuration.
73,
Mauri I4JMY
----- Original Message -----
From: "Einar Persson" <sm3mxr@swipnet.se>
To: "David Kirkby" <drkirkby@ntlworld.com>
Cc: <amps@contesting.com>
Sent: Sunday, January 07, 2001 9:05 PM
Subject: Re: [AMPS] 3 phase HT power supplies.
>
> Hi David (and all)
>
> If you already have the 3-phase supply, use it.
> But I wouldn't do it without at least a small capacitor across the HV.
> I tried a 3 phase 6-pulse supply without cap a few years back, with bad
result.
> It worked fine without a capacitor, but shutting it down was a surprise.
> I had the supply and it delivered abt 3kV DC @ 0,6A for a pair of 813's.
It
> worked very fine with a 16uF capacitor across the output.
> (16uF was what I had from an old BC tx)
> One day I wanted to know if there would be any hum in the tx without the
cap so
> I disconnected it.
> Switching on was no problem (soft start). No one heard any hum in my tx,
so
> everything was fine until I switched the supply off.
> 2 rectifier "legs" in the 6-pulser blown to pieces. Rectifiers were
10x1N4007 in
>
> each "leg" at the time.
> I had the power supply away in the basement so no way to see what happened
so
> when I switched it on again I also smoked the soft start resistors.
> I thought the rectifiers were to tiny so I tried with 10x1N5408 in each
leg with
> about
> the same result. Some "legs" in the rectifier in pieces.
> (bought 1N5408's for 1 SEK each, about 14 for a £ at the time, so it
wasn't a
> very
> expensive try :-)
> After repairing the 6-pulser I connected the capacitor again and
everything
> worked
> fine again. No more diodes blown to pieces.
>
> I didn't make a serious investigation about what caused the failures, but
my
> guess is that a
> spike in the transformer, when switching off, destroyed the diodes and
that the
> capacitor,
> when there, takes care of the spike.
> It must have been some hefty spikes to break 10kV worth of diodes so I'm
quite
> confident
> one can also destroy a transformer.
> After that experiment I built another 3-phase 6-pulse HV supply. (5.2kV DC
@
> 1.35A CCS)
> It's more or less the HV ps from a commercial PHILIPS FM 88-108MHz
transmitter
> from
> the late fifties or early sixties. (Philips 8FZ 705)
> It has a 3-phase transformer, 60x1N5408 in the 6-pulser and a 16uF/5.5kV
oil
> filled capacitor
> connected across the output. (That's the original HV cap from the PHILIPS
tx)
> In series with the +HV line to the amp I have a 50 or 60ohm 160w resistor
to
> hopefully save the
> tube in case of an arc over in it. (Also original resistor from the
PHILIPS tx)
> No chokes in the supply.
> The supply works very well with no problem so far.
>
> 16uF is a lot more than needed, just a few uF would do the job, and is
> preferable because of
> the smaller storaged energy.
>
> Your thoughts about using 3 single phase transformers in 3-phase
configuration.
> A friend of mine used that concept several years, in a 4CX1000A 2m amp,
with
> good result.
> The trannies were 220V on the primary and if I remember correct 1600V
secondary.
>
> He used the Y configuration on both primary and secondary. Each
transformer
> primary connected
> between a phase and "zero" and a 6-pulse rectifier on the secondary.
>
> My opinion is that it's better to split the power between all tree phases
if
> possible. The advantages
> are better voltage regulation = less twinkling lights, less ripple and if
> needed, more power than from
> a single phase supply.
>
> Hope I've answered some of your questions.
>
> 73 // Einar // SM3MXR
>
>
>
>
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>
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