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RE: 3 Phase Full rectification, doide protection, HV Inductor needed, advise urged.!



Original poster: "Leigh Copp" <Leigh.Copp@xxxxxxxxxxx>

Jim (both of you I guess),

A few thoughts on the HV rectifier game:

I'll agree that while the tubes are a little more resilient, they are
pretty expensive to replace.

Not putting voltage sharing networks across the individual diodes
however would not be something I would recommend, but your mileage may
vary ;)

Unless you measure your diodes to determine their reverse breakdown
characteristic, and ensure that they are all at the same temperature,
they will not share voltage, and in a fault condition when the first
diode in the string fails, the little inductive snap and the step change
in voltage across the rest of the string will cause the spike to go
through the rest of your series devices like poop through a goose.

The internal junction capacitance and resistance of these guys is not
necessarily consistent either - yet another reason to balance the
unequal impedances with relatively low impedance parallel paths.

If my hastily rigged voltage divider and scope had survived at the time
(young, poor, and stupid at the time - enough said) I could show you the
waveform from one particular episode of the sort I am describing above.

And speaking of transients, and reverse recovery time, an R-C snubber
circuit across the individual diodes also reduces dv/dt handily to help
protect against such things.

100k ohms in parallel with 0.1 uF is very common when we have a 6-40 amp
DC supply to work with (BTW: my frame of reference is 50-800kW plate
supplies). The resulting losses may be unacceptable with the size of
your dc supply (1.4 amp diodes were mentioned?), however, so you may
need to increase "R" and "C".

Reverse recovery time is not really the issue as was said, we just want
to buy time when we have a fault, and things start to ring (like that
big choke!)

With that much series inductance btw, consider a 7th diode stack across
Vdc as close to the rectifier terminals as possible. -This- rectifier
could benefit from reduced turn off time.

Cheers,

Leigh



-----Original Message-----
From: Tesla list [mailto:tesla@xxxxxxxxxx]
Sent: September 21, 2006 2:18 PM
To: tesla@xxxxxxxxxx
Subject: Re: 3 Phase Full rectification, doide protection, HV Inductor
needed, advise urged.!

Original poster: Jim Lux <jimlux@xxxxxxxxxxxxx>

At 07:15 AM 9/21/2006, you wrote:
>Original poster: "Jim Mora" <jmora@xxxxxxxxxxx>
>
>Hello Group,
>
>I am getting antsy to power up my 3 phase DC coil PS. The PS is from
>Raytheon, Nikie era. It puts out 24KV DC. I think I posted a picture of
the
>beast once. Bert Hickman called it "Knurly", (California speak) and it
is!
>
>In its former Cold War Radar/tracking configuration, it used (6) 8020
vacuum
>tubes for 6 pulse rectification. It has a 150 H inductor on the output
which
>does not play well by way of Richie's Burnett's equations. If my math
was
>not wrong, I would have a max 180 BPS (too small) and .4uf cap (too
big). A
>more ideal inductance would be say 75H so two of these in parallel or a
>smaller one would work.
>
>I don't know if there is any value in DC reasonance charing or just
brute
>force. The design is cool. The tube insulators step up to longer
insulators
>namely 4" to 10" with the anode caps.
>
>Anyway, most tube suppliers suggest going to solid state diodes which I
plan
>to do. Where's the beef,... getting to that? I have (36) HDb7.5 puck,
stud
>diodes which are 7500v 1.4 amp rated. I would use 6 per leg (delta) wye
>input.


Are the 8020s dead?  If not, I'd use them.  They're a burly tube that
has long been used in HV power supplies.. I think the specs are like
40kV, 300mA.. but more to the point, tubes have a bit more cushion
against accidents.  A bit of over voltage, and they survive. A
transient overcurrent, and they survive.

OTOH, if they die, replacing them at $200 each (or probably more..
that was 10 years ago that I priced them from Richardson) is a good
reason to switch to semiconductors.



>So here are the questions:
>
>1. Is there any point in resonance charging or for that matter can it
be
>avoided in some form?

With a 6 pulse rectifier, you've got pretty steady DC.  You'd just
wind up drawing a power arc, because there's nothing to quench it.


>2. What should be across the individual rectifiers in the strings to
add
>failure prevention and equalization?

These days, with very consistent devices, the usual strategy is just
to be conservative with how many diodes you put in the string.  Say
you've got 6kV PIV diodes and you've got a 30 kV application.  Use
8-10 diodes in series rather than squeaking by with 5.  The concern
is with reverse recovery time (turn off).  The diode that turns off
first sees the entire voltage across the string.   In a sinewave
application, though, the reverse voltage comes up slowly.

Throw away all those old ARRL handbook pages that talk about
capacitor/resistor networks for equalizing diodes in series strings,
unless you're using diodes you bought loose at 10 different hamfests
over the past 20 years.


>3. Shall I bother with a de-queing diode string? (6 more).

With resonant charging, you need it.