[Amps] Large Variac controlling HV power supply

[Jim Thomson]

Date: Thu, 31 Oct 2013 09:53:11 -0400
From: "David Jordan" <wa3gin at comcast.net>
To: <g8on at fsmail.net>, "Mark Beckwith" <r-emails at n5ot.com>,
<amps at contesting.com>
Subject: Re: [Amps] Large Variac controlling HV power supply



###   He said he has a 5000 V @ 3A  CCS  plate xfmr.   5000 x 1.414 = 7070 vdc
NO load...with a FWB +  high C filter.   Your shunt relay  to bypass the variac  is
just another method to step start the B+  supply.   The supply will still be running 
at max full voltage.  That’s fine..IF he wants a 7070 Vdc no load B+  supply.

##  If he wants say a 4000 vdc supply, your scheme wont do him any good, in which case,
the variac  must remain in the circuit at all times.... and simply dialed up to whatever
vdc he wants. 

##  BTW.... the variac  bypass scheme works  superb as the ultimate step start. 
It eliminates that huge secondary surge you get when the typ resistor step start is used.
That problem occurs when using big plate xfmrs with 1.5 to 2.0 A of magnetizing current...
and large values of filter C... like 50+ uf..... and 4- 8 kv.  
The magnetizing current flows through the step start resistor....creating a V drop across the
resistor.   The plate xfmr never sees the full  240 vac... hence the filter caps never charged up to
max no load B+.   Resistor gets shunted after 5-10 secs.... and bam, sec surge. You can minimize the
surge up to a point by using the lowest value resistor practical. 

##  But by using a SMALL  variac, like a 5A  unit, just for the step start..works superb.   Then you
can bring it up really slow..and if it has  a 0-270 vac output,  you can crank up the B+  so it’s a little
higher than normal NO load level when 240 vac is applied. IE: crank it up to  245-250 vac. 
A DPST relay or contactor is then used to  engage-disengage the variacs  output side from the xfmrs
primary.   A 2nd DPST relay-contactor is used to apply the main high current 240 vac path to the primary
of the plate xfmr itself. 

##  In actual usage,  relay #1 is turned on..and variac is dialed up slowly till B+ is a bit higher than normal.
Then relay #1 is quickly shut off, and relay #2 is quickly turned on....applying the main 240 vac to the xfmr primary
instead.   NO surge since the caps are charged to slightly beyond normal B+.   Ok, IF you forget to turn the variac
back down to zero for the night, when finished for the evening...OR  if you have a power failure... there is an easy
fail safe fix for that too.  A simple cam on the variac shaft is used to engage a microswitch when variac is almost
fully CCW.   Microswitch is used to engage a latching relay... or a normal relay whose contacts are wired to keep the
relays coil engaged.  Other contacts of the same relay apply the 240 vac to the INPUT of the small varaiac. 
If you forget to turn the variac  fully CCW for the night, no big deal.  With the variac cranked up high, it wont come on....
UNTIL   you rotate it fully CCW, and the cam engages the microswitch at almost zero volts.   Then the variac receives 
its 240 vac to its input.....at which point you simply crank the variac up...slowly till you get to just above normal B+ levels,
then do the relay # 1 and #2 sequence.

later... Jim   VE7RF


Once you bring up the power supply you could engage a shunt-relay to by-pass 
the variac.  I never did on my amp.  I determined the voltage drop was on 
the line, not the variac and I was using a 30amp rated variac.  Also, if you 
built a hefty enough HV supply a few volts drop on the input won't be felt. 
I was using 300Mfd for the 6KV, 30,000mfd for the 1KV screen supply with 
choke on the center tap.  The supply was rock solid.




Good Luck,
dave
wa3gin