Friday, September 23, 2016

SSPA Power Supply - Lessons from Load Testing

  • Creating a DC load testing device is pretty simple should you ever need to do this.  It was also very helpful to point out some short comings in my DC power supply design.
    • Use a cheap 10 gallon (or more) METAL barrel.  
    • Drill 1"+ holes in the barrel to accommodate the amount of "AC" water heating elements you intend to use. (I used a step-bit to get this very tight, in order to avoid leakage)
    • Clean the metal around the whole at least 1/2" with a wire brush or sand paper.
    • Use high temp silicon like (RTV "Red" which is good to 650F, or SIL-100-GP which is good to 450F) to help seal inside and out of the hole you drilled.
    • Use high temp thread sealer between the water heater element and the retaining nut.
    • Use Stainless Steel 1" NPS nuts (probably will cost more than the heating elements!) mine where $7.50 for the nuts, and the heating elements were $5.79.
    • Heating elements I used are 120v/1500w @ 10 ohms.  Which makes the calculations fairly simple.  It's basically just a 10 ohm resistor.  two in parallel = 5 ohm, three = 3.33 ohm, four = 2.5 Ohm, six = 1.6 Ohm etc...  Once you know the ohms of the heating elements you're using it's just (ohm/#ele)=resulting ohm.  Then using ohm's law you can calculate the amps @ volts you'll be pulling through it.
    • WARNING...Be sure to use a wire-gauge that can handle the amps you've calculated!  Or you've basically created a fire-starter.
    • That's basically it.  Wire the heating elements in parallel and fill the barrel with water, connect power and test.  BE SURE TO Monitor the water temp as well as the heating elements and wires used.
    • WARNING...if you don't understand what I'm talking about, don't attempt to do this!
  • I discovered that at 33 amps I saw virtually no stress on any components I was using.  It wasn't until I doubled that by adding 6 more heating elements to the load and drew 62 amps @ 53vdc that I noticed my final protection diodes were hot and needed to be heat-sinked and possibly a small fan added.  (more to come on that in the next set of tests).
  • I discovered at 62 amps at TURN-ON time (instant load on the power supply when 240vac was applied) that sometimes individual 13.5vdc power supplies would startup and immediately go into 'safe-mode' and shut back down.  That's a safety feature built into the dps-1200fb power supplies.  
    • This was happening because I have no disconnect-relay or 'contactor' device at the final of the power supply.  So any load attached at start up would cause an instant surge before the bank of supplies had a chance to stabilize, and they would go into safe-mode.  Which basically means I would have to power it all down and try again and hope that the next time they'd start up (and they usually would).
    • This isn't acceptable behavior.
    • I also already have the ability to 'act' if a bank of supplies isn't putting out the voltage I expect.  It's already built into this system, but because I have no final relay/contactor to energize I can't really 'act'.  Even tho I have the monitor and relay in place to do so.
  • Resolution of the last two items in this list, I've found and ordered a 12vdc coil, 220 Amp @ 55vdc 'Contactor'.  below are some pix of it.  I found this one on Ebay "New" for about $43.00.  I'll add a 10 second startup timer which will fire at power on time.  After 10 seconds the power supplies should be stable enough for a good sized surge.
    • I can also wire this same contactor up to the bank voltage monitors so that if something fails and I don't have the voltage I expect output power will not be available because I can force this contactor to go 'open'.