[ARC5] SCR-274N Dynamotor Question

[Charles]

On 3/25/22 09:23, arc5-request at mailman.qth.net wrote:


Problem solved!

I initially looked at purchasing a used MIL 28V power supply such as the
PP-8474G. They are on eBay but total cost with shipping would be around
$450. So, that option is out.

I then decided to purchase #6 cable. Well, you need the connectors,
standoffs etc and that was around $75. I almost pulled the trigger... but
$75 is a lot of money.

What kept lurking in the back of my mind is that Chris and Scott kept
telling me they use floated batteries across a 28V supply and it works
great. I tried that and it didn't work very well at all. I decided to
revisit this option and emailed them.

Chris got back to me right away. I have an Astron LSRM-25A-BB 28V power
supply (18A 25A ICS) with battery backup output. I had my 12V tractor
batteries (2 in series) across the battery backup terminals as that's what
I was told to use. That was my mistake. They need to go across the main
28V output terminals.

Chris also cautioned me that the batteries need to be fully charged before
each use and disconnected after each use. I charged the batteries and put
them across the main 28V output.... and Wow! It worked great!

Key up voltage was 27.9 and key down was maybe 0.1 volts lower. The plate
voltage on the ARC-5s is now about 560V and my power output is 50W !!

The chirp is almost completely gone.

Cost: $0 ? which ain't too bad!

73 Mark K3MSB
==============================================

Glad you found a solution that worked for you ;)

I needed 24-28v at up to 115 amps to run a 2 KVA 115v/400Hz aircraft 
rotary inverter. As I'd previously posted, I decided to build the PS big 
enough rather than mess with lead-acid batteries that are heavy, 
expensive and require maintenance. Switching supplies also can't handle 
surge currents without shutting down (and also are not cheap at this 
power level.

With a good DC current sensor, it turned out the starting surge is 420 
amps! It would take a couple of really hefty batteries to do that... and 
that kind of rapid discharge/recharge cycles are hard on them too.

Building a simple transformer/bridge/capacitor PS at that power level is 
not trivial, as I discovered. But I have a well-stocked junk box and 
spent a couple of hundred on a huge toroid transformer and other 
high-power components I didn't have already. Welding cable is fairly 
cheap on ebay if you find the right seller...

Now I have a 150 amp supply that can handle the inrush, weighs about 50 
lb, and will run any mil-surplus gear I can think of! :)

Charles WB3JOK/0
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