MEP-831 testing question

[Dwnorton1]

I recently acquired this 831 and have been playing with load tests. I have noticed that this unit will take 3000 watt instantaneous load with no problem, but does not like to come off of full load to -0- Governer goes to hunting with motor surging back and forth trying to find its happy place. Sometimes you can dump load back on and it will stabilize, sometimes you just have to kill it. It really likes load to be stepped down. I suspect it's governor is just barely out of adjustment as occasionally I have to pop the hood and just touch linkage and magnet will grab. I am going to go through adjustment procedure in morning. Just wanted to see if chasing the rabbit was normal on these. Any experiences with these units would be much appreciated. Love the unit just want to tune it and understand it's quirks and capabilities.

 

[DieselAddict]

Reduce the gain on the controller a touch. See how that changes the behavior.

 

[Dwnorton1]

Well I went through TM steps. Speed was a little slow -5vac from recommended speed. Also adjusted stability and gain pots.
DieselAddict was correct on diagnosis. To me the gain seems to slow the electronic governor movement deceleration. It appears if this actuation to to quick the magnet grabs the target further slowing, causing the governor to try Grab back control and then it overshoots and tries to slow and magnet grabs in a negative feedback cycle. Switches between load and no load pretty well as shown.

http://youtu.be/Gjq07sPQaZE

i went back out to try and make sure it was still reacting the same as was in video. It's back doing same thing. I measured target-magnet relationship and it was .3125. I am thinking of adjusting gap a tad wider. Thoughts???

 

[DieselAddict]

5/16 to 3/8 is the standard gap. Looks like that is about where it is. You can try widening the gap a tiny bit at a time. Don't let it get too wide. Stay under 1/2".

These things are sensitive to small adjustments.

 

[Dwnorton1]

Well had it all tuned where it was loading and unloading like a champ. Decided to see if it would start my little 2.5hp compressor, which it failed to do. After failed test went back to water heater elements and was still functioning nicely.
Was in cool down when noticed smell. Opened top to see what was up. That's when I noticed I was in process of letting all of the smoke out of the inverter. I didn't have a container to catch it it so it all escaped, so I wont be able to re-install it.
Actually could see visible flame inside of inverter and could hear it as well.
I was so smug about getting these units and my ability to tune them up, well needless to say that kick in groin brought me back down, painfully.
Guess I have a parts unit now unless I can find a reasonable inverter, not sure those words even go together on these units.
As the saying goes, if your going to be stupid better be tough, and in this have case deep pockets. Ouch ouch
If I failed to read something about these units not being designed for that kind of load please point out my ignorance, it would be some pittance if my stupidity saves someone else my mistake.
I am picking up another one in OKC on Wednesday so I hope I am more successful with it.
I'm going to keep jar close by the next time so I can try and save the smoke.

 

[DieselAddict]

You should have been fine with a 3kw resistive load. No way it is ever going to start an electric motor that sized. To the generator that was like putting it into a dead short.

What heater elements are you using? Are you sure it is only 3kw load? You can't go by the load meter on the units. They are calibrated for a 0.8pf.

 

[DieselAddict]

You can find inverters on Ebay.

 

[Dwnorton1]

After I posted, went to flea bay. There are two listed for $300 each. Guess I may go that route, I'll decide when I pick up next one. It may need parts don't know yet. This one stings. Thought I might get generators and fix /tune and sell to support my addiction. May be better to quit cold turkey. Time will tell.....

 

[DieselAddict]

Take the inverter out and see if the vents were clogged or there were any issues that could have contributed to the failure.

 

[MtnSnow]

Depending on the LRA rating of your 2.5 HP air compressor your Mep-831 is probably a bit small for your compressor as a 120v 2.5hp motor would usually be around 21.5amps RLA (Running Load Amps) @ 120V with a .90 Efficiency rating on the motor (good quality motor) & a .8 power factor. Unless you have a very poorly designed motor (the formula to calculate the amps needed when HP is know is as follows Then you have to account for Startup load (usually known as LRA or Locked Rotor Amps aka Startup Load which usually substantially much higher (in refridgation it might be up to 4 or 5 times the RLA value!)

Amps = HP x 746 Then divide that number by the sum of (Volts x Efficiency x Power factor) for example as show above: 2.5hp x 746 = 1865 divided by (120volts x .90efficency x .8pf = 86.4) so 1865/86.4 = 21.585amps but change the efficiency to .6 (57.6) and it becomes 32.378 amps! and that is just the RLA (Running Load Amps) then take into account the LRA or Startup load!!

 

[Dwnorton1]

The name plate on compressor says 15fla. Think it's 2.5 peak hp. I knew inrush would be much more. Figured I was right on threshold of generators output. Just wanted to really test out gen. Boy did I. Destructive testing in hindsight.
I will check and see if inverter vents are plugged. In talking to dieseladdict it, I grossly punished the machine by not taking into account of power factor. Was running combinations of 3000w inductive loads.

Ouch. 31a @ 120v is 3720w. 3kw is 25a. That is the limit except for a brief time. I've run them at 3200w to make sure the engine had enough excess power to handle it but you should not run one at 31a.

In my mind these unit are like the 802s which can handle 125% of rated load without missing a beat.
That thought process was a $300 screw up. If you are reading this please do not use 3000w elements for testing for any duration or you stand chance of needing new inverter.

 

[DieselAddict]

Sorry to see this happened. I hope you can find an inverter to get it going again.

Let us know if you find any air flow issues for the inverter once you pull it apart.

 

[Dwnorton1]

After a intensive root cause analysis it has been determined to be caused by human error on part of dumb a__ operator. Me. I so wanted it to be cooling fan. Tested operational, could be D.C.-D.C. Converter on board but doubt it. Blew a couple of capacitors. If they had not melted adjacent board then in would have been plausible to do just board repair.



Caution pictures are graphic in nature. Not for faint of heart. I can see Guyfang shaking head upon reading this.



I propose that we collaborate and write a sticky for load testing. I hate to see anyone else make such a dumb/expensive mistake. It way already exist buried in a thread.

 

[Dwnorton1]

Further investigation of this failure is ongoing. I'm no component level electronics guy but I believe that these were some form of snubber, resistor-capacitor, both the blown one were on L2 which happens to be the phase that I had compressor hooked to. I tried to verify loads on the inductive heater elements, but my old fluke that I used first day testing was inconsistent, so took back off meter and noticed corrosion, spent much of life in salt air. So I do not trust its readings. Getting certified meter tomorrow to verify true load I was putting on unit. I'm inclined to believe that the inductive load was not what took out inverter. I had been testing with a 240v load so you would think a single phase wouldn't have blown. I also had felt of inverter while it was smoldering and it was not overly hot except in arc area. Was trying to decide at time how I was going to put fire out if it had continued.
I hindsight should not exposed unit to kind of load compressor may have put on unit. I never thought a 15fla units inrush would do damage. Just wanted to see if it would run compressor in remote location if needed. I know these were more used for radio/electronic constant load type uses in military. Clean power consumers. 802s 803s etc more for hotel loads.
I will post readings with true loads after I gather the data.

Picture of type of component that failed on L2 phase.

 

[dav5]

Hi,
I am planning on using my 831a to power my cottage and I am hoping it will run the 40 gallon water heater with everything else turned off. The rating is 3850 watts so it looks like I will be a bit over the 831 capacity but it will be a resistive load. My 803 wouldn't have any problem whatsoever being over that much over capacity but it sounds like the 831a would have a problem. Any advice?
Also why wouldn't the overload cut out when the inverter was overheating?

 

[DieselAddict]

My advice is to swap the elements for lower power ones. You can't overload these as much as the other MEPs and have them survive.

 

[Dwnorton1]

My advice is to swap the elements for lower power ones. You can't overload these as much as the other MEPs and have them survive.

I can fully attest to that. Have rev 2 unit. Troubleshooting speed controller on it right now. Will have precise data once I get it running on different wattage water heater elements. No voltage spiking device on this machine.

 

[Guyfang]

Stop beating yourself up. Crap happens. Important, is that now that it has happened, you all learn from it. I did something like this to a $255,000.00 turbine engine. I felt like shi* for weeks. Didn't help. It did help when I found out why.

I posted several weeks ago, that I had found 2 or 3 of these inverters for sale through Iron Planet, or what ever the heck its called, the Gov sales web site. That might be a good place to look also, rather then just fleabay.

Maybe you can repair it? Or get it repaired.

You know, if some smart guy started repairing them, he might just be able to make a little money at it. If it were me, I would get a defective one and repair it. Then put a notice in SS and other places. If someone wanted to buy one, you could sell it to him for what ever a new one would cost. Or they could ship you the bad one they had, and you send them your good one, at a discount, versis a new price. Then fix up the unserviceable core, to start again.

 

[dav5]

I would appreciate seeing the water heater element results.

 

[Dwnorton1]

240vac 120vac 1500watt L1=11.39a
240vac 120vac 1500watt L2=10.7a



240vac 240vac 3000watt L1&L2=10.6a

http://www.youtube.com/watch?v=6f4qyrCRxZQ


Had to borrow speed control from inverter challenged rev 1 unit. Rev 2 unit had constant 24vdc to actuator. Plugged it it and was already tuned from my testing of failed unit.
My hypothisis is that either my air compressor damaged rev 1 unit inverter or just finished off an ailing inverter. These inductive loads should have had not effect in its demise. Within specs

Thank you for encouragement Guyfang, believe me this is not the first electronic component I have let smoke out of, and doubtful
it will be last.