Wednesday, September 28, 2016

SSPA Power Supply - Load Test #6

Success!  Load test #6 is probably the final load test that I'll need to do with this power supply.  

I had no issues with the power supply.  The final protection diodes didn't get much hotter than 95F during this test of 62 amps @ 53 vdc.  In the last test I measured peaks around 180F.  So my junk-pile heat sinks and muffin fans cut their temps about in half.  

This test proved that my fixes for:

  • The high temps on the final diodes reduced to 95F which is 50% lower than what they were at, and only 33% of their max temp spec.  Plenty of headroom there now.
  • The new contactor and time-delay resolved sporadic initial startup and immediate power down of random supplies (safe mode due to surge).
  • Replacement of the 400A shunt with a 150A shunt resolved the low amp reading on the 150A meter (doh!).  I'd read that I should overrate the shunt for safety, that proved to be incorrect.  The ammeter now reads accurately.
  • The voltage monitors work in-circuit now.  If voltage is below or above range, the final contactor is forced open.
The only issues I had with this test was that the wires (#14 ga) on the Load Tester elements (and not the power supply being tested) started to get too hot after about 6 minutes of testing.  THAT is acceptable for now :-)   But I'll get some #10 for the future I think.  Maybe even #8 for safety.  If those wires had melted through I'd have had a fire hazard.

Fairly LONG demo video (not real stable if you get sea sick take your meds now LOL).

SSPA Power Supply - Fixes Post Load Test #5

Here are the 'fixes' (or changes) post Load Test #5.  I've added a 200A final contactor so that I can use the voltage monitors to control if we allow power output from the power supply based on voltage being in the proper range.  I can also now delay allowing voltage output until all the supplies have stabilized on initial startup.

I've also added fans and heat sinks for the final dual diodes.

Here's a short video demo of the smoke-test after these changes were made.

And now on to the full load test #6 next.

Monday, September 26, 2016

SSPA Power Supply - Heatsink/Fan for Final Diodes

As mentioned previously on this blog one of the issues noticed during the most recent load testing of this power supply was the final protection diodes got quite hot during a 62 amp load test on a single bank for 4 each 13.5 vdc supplies providing 53 vdc (after the diode drop) in series.  I measured at least 150F on them possibly as high as 180F.  These diodes have a max temp of 150C which equals 302F.  So it wasn't catastrophically hot.  But hot enough to want to try to manage it better than doing nothing.  

I had initially thought I would put some thermal paste under them and bolt them onto the case bottom.  However, since I had some spare parts to do better than that I decided to opt for something a lot more robust.

I had some spare heat sink material and I have a lot of spare computer fans around so I decided to try to mate them up and build to identical heat sink setups for each set of diodes.  Each bank of 4 supplies in this (there are 2 banks) has a dual diode at the end before both banks merge into a single output.  This is to keep their output separated from each bank.

So here are some photo's of what I came up with.  I still have to apply the new thermal paste under them, and make some new cables to run to the output terminals, but I was able to get things migrated around in the case, and the completed heat sink / fan combos built and installed this past weekend.

These fans blow INTO the heat sink instead of sucking out of them.  This forces most of the air UP the fins and out of the case through exhaust fans at the top of the case.

Friday, September 23, 2016

SSPA Power Supply - Final CASE cleanup and prep

Here are some pictures of some of the final prep work on the power supply case along with it's air filters and exhaust fans.

Yeah it's not super pretty but it's plenty functional :-)

Note the strap that retains the supplies firmly in place using a case screw.

Air filters built in the front doors are washable/removable.

Front doors closed position

SSPA Power Supply - Final Diode Cooling

I discovered through some recent load testing that at just a bit above 1/2 max load on the power supply the dual protection diodes I'm using get quite hot, at least 150F degrees.  So to help control that I'm going to cut up a large heat sink I have tomorrow and mount these 200 amp protection diodes on them.  I'm probably also going to add fans to the heat sinks as well.

Here are some pictures. 

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'.

Thursday, September 22, 2016

SSPA Power Supply - Load Test #5

The first attempt failed to start all 4 supplies in bank 0.

This afternoon I drilled and filed out the new holes for the 6 addition water heating elements being added to double the load test on the power supply from 33 amps to 62 amps.  And also added both a water inlet valve and outlet value (3/4").  This will allow for water flow into and out of the tank.  Cold water comes in the bottom and hot water drains out the top.

For this test I didn't run water flow through it.  I simply filled the can up and ran the test since it was basically a short 'smoke test'.

I ran into two small issues this time around.    I was only running "bank 0" so 4 each 13.5vdc supplies in series.  Instead of bank 0 and bank 1 which would be 8 supplies.  The idea was to put a good strain on the system.

So what I found is that the 'final' dual diode warms up pretty good.  I measured about 150F on it which is too high. I need to heat sink those final diodes and possibly even add a fan, or TEC to them to really keep them cool when there's load.  This test was just between 1/2 and a 3/4 load test.  75 amps would = 3/4 and this was drawing 62 amps.  

These diodes are rated for 150v @ 100 amps.  The power supply bank that feeds them is capable of 100 amps.  However I am running two wires from the positive side out which split the load one wire goes to one of the dual diodes sides the other wire to other side.  I suspect this is sharing the load somewhat but I haven't tested that.  I might not be too :-)  Won't know for sure until I test it out next time.

I also found that the protection diodes on the power supplies where about 90F.  This could just be heating up from the power supply they're connected to...and not because they're taking any sort of load (because they shouldn't be).

I found that with this much load on the system instantly at start up...all of the power supplies didn't want to come up.  The first attempt I had about 26v the second was 53v the third try was 40v.  So what's happening is that these power supplies don't like having that heavy load placed on them instantly at start up and they'll power down (what appeared to be randomly).

This didn't happen when I was pulling 33 amps.  This is the first I've seen it happen, so the load test process is helping me find things I need to address.

I have been thinking for a while that I need a FINAL relay that can be controlled from a few places.  Like when the output voltage isn't where it should be for one of the banks I would want to OPEN the FINAL RELAY.  

But also to solve this 'instant load at startup' instability (which won't actually happen when I'm using this with the SSPA anyway, but I'm just trying to make this be robust and survive me being stupid in the future.  So to solve that I just need to add a timer to control a final relay and tell it to wait 10 seconds while the supplies startup and stabilize, and then close the relay and draw power as needed.  

I've thought I needed something like that for a while, but I've been putting it off.  

I'm pretty sure I know the relay I want to use, but I need to find one for sale.  It may take a while to find one that can be controlled by 12vdc and be able to handle 54vdc @ 200 amps :-)  But I'm sure I can find one.