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Cooling Dummy Load for Power Converters

07/16/2017 4:44 AM

I work in an engineering company, where we design and TEST power converters, for different DC and AC voltages and for different power levels. In summer temperature in the lab is above 30ºC, but when we start to test converters with the dummy loads (electric fan heaters) it get's too high for thermal comfort.

Why do we not use the output energy of 3 phase 400Vac inverters to power up Air Conditioners instead of Resistors?

I am asking this because someone must have thought of this, but I cannot say it is widely used, and I would like to know why. Possible reasons could be:

  1. Cost: Amortising a custom made air conditioner might be more expensive than a common air conditioner draining extra energy from the grid.
  2. Performance: If the energy I need to extract 20kW of heat from the lab is about 1kW, it might be better to just spend an extra kW from the grid, rather than bothering connecting it to our converter.
  3. Power levels: Do air conditioners require to operate at the optimum conditions? Lower power means bad performance?
  4. Noise levels: We have only fans, and at full power they are near 80dB loud. If HVAC was much noisier we could not deal with it.

I would like to apply it to routine tests, and maybe even for type tests, of 50Hz 3 phase 400Vac inverters. Is there something obvious I am missing that could go deadly wrong?

Some references to technical specifications of HVAC pumps discussing supply limitations would be helpful.

Regards.

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#1

Re: Cooling Dummy Load for Power Converters

07/16/2017 8:02 AM

Any reason those heaters can't be relocated to some other place, like outside the building where they would not affect the building's cooling system loads?

As engineering firm someone there should know how to calculate what size of extension cord to use.

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#2
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Re: Cooling Dummy Load for Power Converters

07/16/2017 9:54 AM

That could be a good idea.

However, I do not know how we would prevent burglars from stealing not only the heaters, but also the cables. As far as I know, fenced windows, the security guard, the alarms and the security cameras are barely effective for preventing people from getting inside the building.

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#4
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Re: Cooling Dummy Load for Power Converters

07/16/2017 10:36 AM

Put them on the roof where they are hard to see.

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#5
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Re: Cooling Dummy Load for Power Converters

07/16/2017 11:41 AM

Haha, boy you do live out in the sticks....I once had a 20 ton package unit stolen off the roof of a 5 story building....nobody saw or heard anything....we know they had to have a crane, and a big one at that...

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#9
In reply to #5

Re: Cooling Dummy Load for Power Converters

07/16/2017 7:56 PM

Yeah, happened to our neighbour here also, and that was two stories straight up to get to them.

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#19
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Re: Cooling Dummy Load for Power Converters

07/17/2017 11:08 AM

Sounds like you either have a crime problem, or a suspicion of crime problem. Hard to tell from where I sit.

Why not do both? How about putting the dummy loads up high and out of reach?

Have one or two dummy loads (resistive), and at least one A/C unit that only runs when the test is active on the dummy loads? Heat does affect the value of the dummy resistors, so that might be a ready justification for getting the A/C...

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#20
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Re: Cooling Dummy Load for Power Converters

07/17/2017 11:24 AM

instead of the resistors, use plain old water, with either salt or baking soda added to draw the needed amount of current.

Simple plain old metal electrodes ( Threaded rod) work just fine.

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#23
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Re: Cooling Dummy Load for Power Converters

07/17/2017 12:13 PM

20 kW electrolyzer - kind of a serious hydrogen-oxygen explosion hazard unless the bath is outdoors and fenced off?

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#26
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Re: Cooling Dummy Load for Power Converters

07/17/2017 4:17 PM

Then if that is the case, then these are awful dangerous,

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#29
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Re: Cooling Dummy Load for Power Converters

07/17/2017 4:49 PM

There is a difference between an actual heating element, and an electric current through salt solutions, where the potential is high enough to make hydrogen and oxygen. It appeared to me the suggestion was to electrolyze water as a test load.

One other weakness of that idea, the current-voltage characteristic of said electrolysis is never a constant load, but increases power draw (same potential more current) as it heats up, so it wants to heat up more.

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#30
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Re: Cooling Dummy Load for Power Converters

07/17/2017 6:14 PM

Yes the water in the tank serving as the load may even reach the boiling point after certain period time elapsed... and the higher the current drawn the hotter the water temperature will be!

Typically In such setup, output voltage is set then current drawn is regulated by either;

1. adjusting the water level in the tank

2. adjusting salt concentration

3. raising /lowering the height of the submerge electrodes..

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#36
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Re: Cooling Dummy Load for Power Converters

07/18/2017 8:43 AM

Maybe with 1/4"-20 all-thread electrodes, the thing will tend to self-limit, since electrode area is pretty small. I presume this set up is with AC applied, correct?

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#44
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Re: Cooling Dummy Load for Power Converters

07/18/2017 10:00 AM

It was an auto adjusting /reversing 3-phase motor coupled to the water bath load...

We used 3 pieces of rebars 1/2" X 60", separately mounted equidistant with one another on a 1/2" thick plastic slab. The plastic slab/ plate being hanged via a rope thru a pulley driven 3-phase motor for dipping level water bath adjustments that also adjust the amperage of current drawn by the water load..

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#3

Re: Cooling Dummy Load for Power Converters

07/16/2017 10:35 AM

How much inductive overload can your converters deliver? The LRC of a compressor can easily be 10x the FLA, and air conditioners can be sensitive to off-nominal voltages.

Better to follow TMC's advice and relocate the fans outside, or if you have a need for hot water, get some electric water heaters and use the elements as your dummy loads. Surely there's an engineer there that can figure out how to wire them up to match the converters' output.

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#12
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Re: Cooling Dummy Load for Power Converters

07/17/2017 12:44 AM

I "forgot" to mention one of our clients is an Air Conditioning manufacturer, and we do have IGBT inverters installed in Light Rail Trains powering their air conditioners. Therefore, my idea is very likely to work, but there must be something wrong with it since none is doing it nowadays in the industry. 10x times inrush current does not sound very nice, but if it works it should not be an issue.

BTW, I have not called them because they usually are not very eager to help their distributors. And I do not think I have a good deal to offer them (yet).

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#6

Re: Cooling Dummy Load for Power Converters

07/16/2017 11:53 AM

You could buy kilns and fire ceramics and sell in the gift shop, or bake bread and have loaves for employees to take home...Really you could use any purely resistive load....you could power a boiler to steam clean stuff.....or you could have an absorption system....

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#7

Re: Cooling Dummy Load for Power Converters

07/16/2017 1:05 PM
  1. Air conditioners start/stop according to air temperature - not when you want to have a load.
  2. They are notorious for being difficult to start 6 - 8 times running current.
  3. Running current will actually depend on entry/exit temperatures.

It seems to me far more sensible to put inverter output into the mains supply - only the rectifier + inverter losses will heat the room & be drawn as net amount from mains & you will reduce power demand from mains considerably - GREEN POINTS with knobs on.

Obviously, the units must be synchronised to mains & if you need a different output voltage, you will have to have a variable auto-transformer [VARIAC] between inverter & mains. Maybe rectifying output to a "DC bridge" and a "solar power" type inverter to mains would be more flexible - your inverter could then be 60 Hz if necessary. I suppose you could have some solar panels hooked-in as well.

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#13
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Re: Cooling Dummy Load for Power Converters

07/17/2017 3:48 AM

What I am thinking is that whenever I need heat, I would use resistive loads, and whenever I need to extract the heat, I would use the air conditioner (at fixed power, regardless of the temperature of the room).

Grid connected Power Converters: Using the AC mains as a load is very dangerous due to stability issues. Voltages go crazy, and so do currents, tripping the MCCBs. Back to back connections take no less than a month and it is very easy something to go wrong. For instance, bidirectional Electric Vehicle converters have been tested for a while but as far as I know, currently research papers is the best that has been achieved. However, since our electric bill would decrease, it is a possibility to take into account.

VARIAC performance: Regarding VARIACs, I have seen 2 types. The small ones, up to about 20kW, which are manual, can barely handle the ZERO LOAD currents of our converters. On the other hand, the big ones (hundreds of kW), use an induction motor to operate so they have wear and tear susceptibility, apart of being bulky, heavy and expensive. Autotransformers are helpful for preliminary tests, but when it comes to routine tests they lack productivity.

Product range: We have rectifiers, hooked to the 3ph AC mains, and 3ph inverters, hooked to DC power supplies. Inverters could be connected directly to the 3 phase load. Rectifiers cannot, so I would need to design an auxiliary "(2 stage) wide input 3 phase inverter".

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#22
In reply to #13

Re: Cooling Dummy Load for Power Converters

07/17/2017 11:56 AM

I've seen applications using a very simple inverter to take excess DC bus power provided by regenerating induction motors and return it to the supply (The induction motors are diesel engine driven, with very odd duty cycles, test stand work). If you coupled induction motors to your (new) test motors, maybe a more realistic test of end use, and provided them with IGBT converters, this US Drives inverter can take the excess dc bus power back into your power system. The inverter is pretty simple, so has lots of harmonic noise, even after the 3% line reactors on the inverter, but it works, and works well in an industrial production facility, without the resources you have available. The IGBTs in the drives on the motors used as generators work on dc bus voltage control, to reverse power flow.

http://www.usdrivesinc.com/index_files/PDFFiles/RegenModule.pdf

You could set up as many M-G (motor generator) sets as you needed, for the inverter sizes you need to test.

You can use your power savings to pay for the investment.

So the power system routes all this power through a separate (harmonic smoothing )transformer before connecting to the rest of the plant, you will only pay for the conversion efficiency for your testing, and you will be testing equipment similar to what your customers will have, maybe?

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#8

Re: Cooling Dummy Load for Power Converters

07/16/2017 5:03 PM

As others have said, ac's are not a good load, with high starting current and inductive pf. Can you use resistive heaters to heat a large tank of water? Water has a high heat capacity and can absorb a lot of energy. Just a thought...

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#14
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Re: Cooling Dummy Load for Power Converters

07/17/2017 3:53 AM

Keep talking.

Let's say I heat up tap water. What comes next? I pour it into the drain? There must be some legal issue preventing me from doing that in large quantities.

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#25
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Re: Cooling Dummy Load for Power Converters

07/17/2017 4:09 PM

What do you define as large quantities and why do you feel you would need such a flow rate?

If you are at an engineering firm I would assume that at least one person there is competent enough at their profession to do basic engineering calculations to know exactly what flow rate of water would be required to carry away 20 KW worth of thermal energy for a given temperature differential.

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#28
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Re: Cooling Dummy Load for Power Converters

07/17/2017 4:46 PM

And then use a fan to cool that water off (dry cooling) enough to send back around for another go, then there is no wastewater to dispose of.

It could be antifreeze or liquefied gold suspension, and the cost is about the same, if you never throw it away.

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#33
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Re: Cooling Dummy Load for Power Converters

07/18/2017 2:06 AM

That would be me?

Let's assume it's 100kW, average of what we have. At 4.184J/ºC/g, 70ºC (boiling point), we would have a flow of 0.3 L/s.

Not bad at all, but I still wonder for how long I could keep doing this without someone complaining. After all, it would not be the first project that does not work due to the poor capacity of the soil absorbing heat (WaterSeer debunk).

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#41
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Re: Cooling Dummy Load for Power Converters

07/18/2017 9:44 AM

Since when does water boil at 70 °C? Do you live in outer space or near it? Just kidding, I see now that you meant to say 70 °C rise in water temperature.

OK, 4.2 kJ/Kg-°C, so 100 kJ/s/(4.2 x 70) = 0.34 L/s (20.4 L/m, 1224 L/hr.

That is a considerable amount of boiling water to contend with, so you will need a substantial fan applied against a large coil, with a substantially larger pump than 20 L/m in order to keep the thermal rise of the water within bounds, and maintain a steady cool side temperature. When you are attempting to cool boiling water by fan, you will need a lot of air flow.

Other option: Heck just go ahead and make steam, and vent away the steam to the air. You will find the latency of converting to vapor a large heat absorber.

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#45
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Re: Cooling Dummy Load for Power Converters

07/18/2017 12:50 PM

I'm not seeing how any of that constitutes "substantial".

A small hydronic heating system circulator pump can easily circulate 5+ GPM (20+ LPM) through 50 feet of 1" PEX pipe. Double that at less pipe distance or a larger diameter.

Plus most any full size pickup truck radiator can easily handle 300+K BTU of heat transfer with any fair quality fan systems on it. Step up to a large commercial truck or heavy equipment radiator and you can do 2 -3X that easily.

But as I have said before, why bother when a substantial and passive electric load bank can be made from cheap common items like repurposed baseboard heaters and their parts solving the whole issue with minimal engineering and work to set up. Which for that matter they could be easily mounted on a cart that could be wheeled out to wherever it would work then put away when done thus eliminating the theft issue.

My point is, that to me all of this seems like what should be some very basic exercises in logic and basic engineering principles which makes me think the OP is not actually an engineer nor works with any real engineers or mechanically compliant people in general let alone at a legitimate business.

Too many basic details and the lack of justifiable reasons behind them don't add up in my books.

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#46
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Re: Cooling Dummy Load for Power Converters

07/18/2017 1:35 PM

Hot tub for the employees...?

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#47
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Re: Cooling Dummy Load for Power Converters

07/19/2017 2:00 AM

Well this would be a GREAT idea for my GF's boss. Last time she was supposed to get a pay raise she got a phone sticker with the logo of the company. Why paying people what they deserve when you can trick them with clever presents

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#10

Re: Cooling Dummy Load for Power Converters

07/16/2017 8:03 PM

As others have said they make poor load banks.

If you want to get rid of the heat use forced air cooling and vent it outside using a flexible HVAC (air conditioning) conduit or similar. This has the added advantage of being able to keep the expensive gear inside. You don't even have to modify the existing building, just open a window or door and feed the conduit outside.

A simple and cheap solution.

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#11

Re: Cooling Dummy Load for Power Converters

07/16/2017 11:32 PM

Instead of heating "AIR" make hot water for building use, and if the water gets too hot simply run the water some to cool the tanks down.

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#15

Re: Cooling Dummy Load for Power Converters

07/17/2017 8:20 AM

Here is a similar situation to yours, my cousin used to work as a glass blower in a house converted to a multi-studio. There were several high temperature kilns and each station had their own torch setup.

Yet despite all these heat sources and scorching summer heat from time to time, the artist loft stayed down near winter temperatures and I had to wear warm clothing while visiting.

The difference here is they had sealed every gap in the converted house and mounted 2 foot diameter, high speed, cowled fans, all pointing outside.

There were at least a dozen of these that created enough vacuum to slam the front door shut every time we entered and left.

However, the fans did produce around 60db of sound.

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#32
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Re: Cooling Dummy Load for Power Converters

07/18/2017 1:33 AM

I know that feel.

We already have extractors (fans) making about 80dB of noise. The worst part is that certified Personal Protective Equipment are not as effective to 50Hz (fans) as to frequencies above 1kHz (SMPS). I wonder if that would have something to do with WAVELENGTH of the sound.

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#16

Re: Cooling Dummy Load for Power Converters

07/17/2017 8:36 AM

and if you are worried about running the hot water down the drain, then get a cooling tower. and then no problems.

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#38
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Re: Cooling Dummy Load for Power Converters

07/18/2017 9:21 AM

For the scope of OP's project, they only need a loop with a fan blowing air over a set of coils of copper, in parallel flow. One of those floor type industrial fans on wheels should about do it.

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#17

Re: Cooling Dummy Load for Power Converters

07/17/2017 9:15 AM

I think the reason for that testing approach is just to verify the functionality of the product without breaking/destroying anything!

Any unexpected, unscheduled power surges, intermittent shutdowns will be harmful to both the test load(s) and the DUT...

In the early 70s, I used tanks of brine solution with 3 electrodes in it for testing UPS and power inverters similar to yours! The tank was located outside and fenced for convenience and safety..

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#34
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Re: Cooling Dummy Load for Power Converters

07/18/2017 2:14 AM

Liquid rheostats huh? Not bad!

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#18

Re: Cooling Dummy Load for Power Converters

07/17/2017 10:47 AM

At a navy reactor prototype they ran the power to a salt water tank and boiled water. If this is DC, you could run the power to Peltier junctions and use the output as a solid state heater or cooler depending on the season.

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#21

Re: Cooling Dummy Load for Power Converters

07/17/2017 11:34 AM

This is a potential investment decision that only the <...we...> can make, really.

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#24

Re: Cooling Dummy Load for Power Converters

07/17/2017 4:01 PM

At ~20 KW that could easily be dissipated through a window using a simple 'U' shaped ductwork or dedicated externally located wall mounted heater grid made from the guts of multiple smaller baseboard heater units.

For an engineering firm I have to wonder how good of engineers they are to not be able to figure out how to set up a simple 20 KW heater system in some fashion that is unobtrusive plus of so little concernable value and ina location of such to where no one would ever put forth the effort to try and steal it.

In terms of theft, things that are of low value located in hard to get to locations plus that are hard to remove tend to not be messed with simply due to the perceived ROI being not worth the effort and risk of taking them.

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#27
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Re: Cooling Dummy Load for Power Converters

07/17/2017 4:43 PM

Besides that, if the test loads are hot while being pilfered, they will let go in the morning, when they realize they just got through braising their finger tips, and that ain't no beef.

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#31
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Re: Cooling Dummy Load for Power Converters

07/17/2017 8:54 PM

I have doubts to the actual claim of needing to be theft proof to any great degree given at best his claim in post 2 says they have security countermeasures in place that are apparently working being 'barely working security' that does keep stuff from being stolen is still working security that is keeping things from being stolen.

So unless his security guards are in nightly gun battles to keep the properly secure I have doubts that theft is as much of an issue as he claims or want to imply it is. Especially so for low value items that could be placed in hard to access locations.

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#35
In reply to #31

Re: Cooling Dummy Load for Power Converters

07/18/2017 2:46 AM

Ok let me explain. All the measures are there, as I said. However, a few months ago someone tried to break in trough the air ducts. They have been found half broken. Luckily, the one responsible got stuck with something or just thought it is not worth it because they did not get in.

I do not think we have security issues. I am just saying that any proposal I make to the Board of Directors can be rejected if it contains clear security risks, as might be leaving equipment outside an industrial ship.

Call me crazy, maybe you are the one who is right and I am just talking nonsense.

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#37
In reply to #35

Re: Cooling Dummy Load for Power Converters

07/18/2017 9:14 AM

Now you are telling the forum this is aboard an industrial ship?

Really? Just drop the conducting lines over the side of the ship, and shock a couple of fish, and be done with it.

The other suggestion: place the dummy load outside, but always have some power active on the load. You will find out who the thief is - - posthumously.

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#39
In reply to #37

Re: Cooling Dummy Load for Power Converters

07/18/2017 9:33 AM

Industrial manufacturing building, no water nearby. I used Google Translate with the equivalent in Spanish (nave industrial) and I did not get a proper translation. My apologies, I did not have the luck of studying engineering in English.

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#42
In reply to #39

Re: Cooling Dummy Load for Power Converters

07/18/2017 9:54 AM

I studies Spanish two years in High School, and it was supposedly Castillian. Meaning that when I failed to speak it properly and swore, Mrs. Lattimore would wash out my mouth with Castille soap. LOL

Now my Spanish is more mutt-style version that comes out of Mexico. Not only that, I am more of less clueless how to engineer anything in Spanish.

No problemo my amigo, vamos a ver hoy a hoy!

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#40

Re: Cooling Dummy Load for Power Converters

07/18/2017 9:39 AM

Here ya go,, Take the power being generated, and use a solar grid tie in to use the grid to dump all the generated power into, Infinite capacity, and the "Load" never changes, Plus most elec companies will pay you for the power you put into the system. a win win for everyone.!

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#43
In reply to #40

Re: Cooling Dummy Load for Power Converters

07/18/2017 9:55 AM

Yes, that was the most beneficial answer to all concerned, unless they need room or district heating. 100 kW could heat a considerable office space, really.

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