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  #26  
Old 06-02-2005
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im sorry i must not have made my wiring plan clear. i plan to run power to the inverter right off the battery, but id like to have a switching wire from the radio circuit.

what id like to know is, first can that be done, second, how large of an inverter would i need, what gauge wire should i run for the power, etc. do you have a part number or something for the relay, ive never been good with them.
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  #27  
Old 06-02-2005
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Originally Posted by n3elz
Okay, but I asked where you got that volts and amps for the first one? I'm trying to confirm what you're saying. It sounds like you could derate the first one to 100 watts effectively, and consider the whole load within the range of a 300 watt power inverter.

And, Colin, I don't like "fudge factors" when there are real calculations you can run based on real world data. Although, in this case, not having any spec for inverter efficiency can be a pain.

In the commercial world, where money is monitored, OVER designing is as bad as UNDER designing! Determining exactly what one NEEDS, and not using fudge factors pulled out of a hat, seems more rigorous and it's how I proceed as best I can with the data provided.

Everyone objects to the numbers I came up with but they would be valid if the volt/amp rating of the first one was a REAL power consumption, and not as I suspect just the fuse rating.

If his numbers had been right, and one of you told him, from the seat of your pants, "just use a 300 watt that should be fine" -- you would be doing him a disservice.

If you want a good answer, you need good data, and a method that works.

If you want a lousy answer, don't solicit an engineer for one! They will give you an analysis every time.
John I know you know what you are talking about and what you came up with will do the job. We are saying the same thing I don't object to what you said, in your first post you said 500 or 700 watts will do it in my last post I said the same thing and just now realized we are saying the same number. We are getting to the same answer different way. You calculated it out down to the watt and I used my experiance and some data I found on inverters. and there we are saying the same thing.

Edit: at first it did not sound right to me because it sounds like alot of power to feed 3 little battery chargers, I was doubting his number at first, but the numbers don't lie if that is the right data then thats it.

Last edited by Ranger1; 06-02-2005 at 01:56 PM.
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  #28  
Old 06-02-2005
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Originally Posted by SoundPer4mance
im sorry i must not have made my wiring plan clear. i plan to run power to the inverter right off the battery, but id like to have a switching wire from the radio circuit.

what id like to know is, first can that be done, second, how large of an inverter would i need, what gauge wire should i run for the power, etc. do you have a part number or something for the relay, ive never been good with them.
I think I already answered that but here it is differently: battery to relay, relay to inverter. Relay activated by switching wire from radio. Do you want an actual schematic with relay pin numbers?

Use 8 or 10 gauge wire probably. Matt (fireranger) posted a gauge chart for 12 volt systems of what gauge to use over what distance.

You can get 40 amp 12 volt relays cheap here: http://www.allelectronics.com but they charge $6 to ship ANYTHING just about, so buy some other stuff you need while you're there. You can pick up a bunch of stuff for the $6 shipping and then the place is dirt cheap.

Last edited by n3elz; 06-02-2005 at 01:50 PM.
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  #29  
Old 06-02-2005
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nevermind question answered before i posted thanx a/w
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  #30  
Old 06-02-2005
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Quote:
Originally Posted by n3elz
And, Colin, I don't like "fudge factors" when there are real calculations you can run based on real world data. Although, in this case, not having any spec for inverter efficiency can be a pain.

In the commercial world, where money is monitored, OVER designing is as bad as UNDER designing! Determining exactly what one NEEDS, and not using fudge factors pulled out of a hat, seems more rigorous and it's how I proceed as best I can with the data provided.

Everyone objects to the numbers I came up with but they would be valid if the volt/amp rating of the first one was a REAL power consumption, and not as I suspect just the fuse rating.

If his numbers had been right, and one of you told him, from the seat of your pants, "just use a 300 watt that should be fine" -- you would be doing him a disservice.

If you want a good answer, you need good data, and a method that works.

If you want a lousy answer, don't solicit an engineer for one! They will give you an analysis every time.
Hey man, I'm not some teenager w/ a soldering iron and a box full of wiretaps either. I just don't know where you can come up w/ these figures w/o having a power inverter in mind. .. Unless your 70% efficiency figures are worst case, 'fudged' figures anyhow. .. But then like I said, I'm no expert on inverters. Maybe you know something I don't (and you aren't sharing w/ the rest of the class).

Even in your original calculations you went from 394W to 500W.. Sounds like fudge to me.

Besides, this isn't a comercial steel mill here. Budget is of decidedly less concern. Also a concern is (in my opinion) future capacity and the ability to overload the system. Why buy a 400W inverter that will just barely cover your needs for $35 when you can get a 900W for $65 and be covered for life.

I never suggested he could get by w/ a 300W inverter.. given the specs he listed.

And do you seriously mean to tell me that you don't design in some headroom in most of your projects?!
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  #31  
Old 06-02-2005
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Actually, most inverters I've seen are 300 watts, and then they jump to 500. If there's a 400 that's fine.

I made that clear: I don't have certain information. But my knowledge of switched power supplies informs the 70% number as a WORST CASE scenario. In light of the fact that I didn't have the numbers I designed for a worst case BASED ON REAL EXPERIENCE.

What I disagreed with was your casual reference to some percentage, without basing it on ANYTHING that I could see. Maybe it was how you communicated.

You can design in some headroom, but not much. I'm looking a 3 million dollar DC drive conversion here for two mills -- one with a 5000 HP, and one with a 6000 HP, 700 volt motor.

I would love to buy MUCH MORE than I need to help ensure no failure. But then you're talking about 10% or more difference in cost. At that level it's $300K and that's real change, my friend.

So, I rely on historical data from the mills, and rating standards which specify 150% overload for 1 minute, with 1 minute recovery (50% duty cycle) and 250% overload for 1 minute, 4 minute recovery (20% duty cycle). Using those drive ratings and the historical trends of mill currents I can buy almost exactly what I will require without designing in excess headroom. You only do that when you don't know what you're doing or you go broke -- fast.

You don't necessarily relax your approach just because it's a small job. The principles remain the same and you generate as "hard" a number as you can, then refine it as the data gets better (such as the actual inverter ratings).

Just "winging" it as you did is not sound engineering practice and that's just all there is to it. Argue what you want, but just throwing percentages around as "fudges" isn't engineering -- it's called "guessing".
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  #32  
Old 06-02-2005
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Sounds like apples to oranges to me. You've got years of data about how your mill works. I don't know how you'd get data anything like that to make accurate design decisions based on the info Bob gave in his posts.

The 400 vs 900 W spec jump was based on the site I linked in earlier. I didn't do a helluva lot of research though, just a quick google search. I'm sure you can get inverters in nearly any wattage range you like. The retailer I just happened to find offered units at the specs I mentioned at the price points I mentioned.

I still think your 70% efficiency figure sounds like a guess to me given you don't even know the brand, let alone the model of inverter he's eying. How that guess is so much better or more significant than a gross overestimate or fudge-factor like I suggested is beyond me. But whatever, here's to you.

Point of the matter seems clear, he needs more than 400W to comfortably power what he listed. I happen to think he should go big while he's at it. Not unlike you w/ your recent axle refit. You never know when you're going to want to power the electrical equivalent of a set of 35's..
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  #33  
Old 06-02-2005
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You've completely missed the point. You didn't address your "fudge factor" basis. My "guess" was a "safe" supposition based on REAL experience with power converters, and actual numbers always trump "add maybe 25% to be safe" statements like yours.

Point: I had a design methodology and applied it. You just winged it.

Your "guess" was not based on anything at all -- mine was based on experiential data with power converters.

But, if you don't see the difference between doing a calculation with a substituted safe value, and just saying "add so many percent because I think so" -- you really have missed the point. As I said, what you did AIN'T engineering.

By the way, take a look at real power converter specs. You'll find a 90% efficiency unit can only hold that spec at certain places in the power curve. Most real power converters have a "curve" that varies throughout the range. The average efficiency is usually considerably lower with varying loads.

As I said, EXPERIENCE with power converters gave me the 70% number as a "conservative" design extimate. Yours was a TRUE guess based on nothing you've described.

I thought they taught you guys better!
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  #34  
Old 06-02-2005
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...... ok guys, i think i got what i need, lets not let this get out of hand
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  #35  
Old 06-02-2005
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You misunderstand. Colin and I ENJOY these arguments. Let us have our fun!
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  #36  
Old 06-02-2005
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but i have to read through all your crap to see if you have more info for me. sheesh!!
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  #37  
Old 06-02-2005
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PHHHHBBBBLLLLLLTTTTT!!!!
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  #38  
Old 06-02-2005
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John as I mentioned the sites themselves list ~~90% eff. for a modified
sine wave inverter. with .4amp no load current(the 900watt model)

I would really just get that 65$ that "BUBBA" linked to its actually quite stout
with 3 30amp fuses 900cont. 1800peak rating. You could use it for running just about anything. for 30$ more

Most of the cheaper ones... are very misleading.. for instance
the harbor freight "700" watt model is 700watt cont. 1400peak..
BUT it only has 1 30amp fuse... 30x 13.8 x.9 (fuse ratingxvoltage xeff)= 373watt.. a far cry from the 700 its "rated" at.

The 65$ 900 watt one 90x13.8x.9= 1118 so the fuses can handle the rated load.

I'm sorry john but I do think you are full of bs on the whole steel mill thing.
You cant compare 300000 capital investment to an inverter..
Its one thing to not spend the extra 300000 on equipment.. its another to skimp on an inverter... I mean I spend 20$ on GAS in 2 days.... I'd rather
just get a beefy inverter and be done with it.
Also listed was a "400 watt" inverter.. which puts out 3.3amp
Its 35$ but I felt that might be too small.
Even that one has a 40amp fuse 40x13.8x.9=496
The cheapo brand again has ... gasp a 20amp fuse in their "400" watt inverter.

Edit: this is just a friendly argument... If it comes off as anything else..
:(
Rand

Last edited by Rand; 06-02-2005 at 11:38 PM.
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  #39  
Old 06-03-2005
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Sure I know you guys are unhappy with what I said (although BS is a bit strong, Jason, lol). The only thing is: how much do you design? I'll stand on it. Designing INTELLIGENTLY with both cost and performance in mind is my life. All your criticisms fall flat because you don't really have a method -- just opinions.

In fact, if there is any BS here, it belongs to people who challenge the attempt to use a more rigorous methodology by substituting only opinion. But hey, that's what I expect from non-engineers and liberals...

Edit: Oh, and one last thing: the inverter's quoted efficiency (the one Rand quoted above) is "90%" at ALMOST NO LOAD. Isn't that interesting that they don't quote the efficiency at full power. Hmmmmm...maybe there's some things about power converters that you need to know when sizing them....

Last edited by n3elz; 06-03-2005 at 08:26 AM.
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  #40  
Old 06-03-2005
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Sorry John, didn't know you were such an expert on inverters..
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  #41  
Old 06-03-2005
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Quote:
Originally Posted by n3elz
(although BS is a bit strong, Jason, lol)


dangit Rand, you need to put your name up there, almost thought John was talkin to me...

Dont stop now, I'm learning alot from you guys bickering.. lol..
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  #42  
Old 06-03-2005
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Originally Posted by NHBubba
Sorry John, didn't know you were such an expert on inverters..
An inverter is nothing but a subclass of power converters, of which I know quite a bit. We have some BIG ones here. Small ones are based on the same principles, just smaller switches and magnetics.
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  #43  
Old 06-03-2005
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Wow all this for a simple question..... I agree with Jason...one way or another Im learning from you two
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  #44  
Old 06-03-2005
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Originally Posted by rngprerunner
Dont stop now, I'm learning alot from you guys bickering.. lol..
Meh.. okay..

I still think you should go w/ a the biggest damn inverter you can get your paws around, within reason. Don't go spending $1000 on something, but I would think something in the ~900W range would be awefuly attractive.

Why? I imagine efficiency and actuall (as opposed to marketed or advertised) capacity is certianly an issue. I don't know enough about inverters per-say to contest what John is saying, so I say do as he says.

However I do feel like I know a few things about specifying for capacity. And this situation strikes me as a situation where scalabilty and trying to predict future use is important. In my buisness we often specify systems w/ scalability and expansion in mind. It has pretty much never served us or our customers to specify EXACTLY what they need and nothing more. They always invariably end up upgrading to a newer (higher performance) software package, adding another video, audio or lighting source, upgrading to a higher wattage lighting system, or adding audio channels, etc etc. Some of what I'm saying here pertains only to specifying equipment for theaters, which is the end of the pool I'm coming from. Some of it is probably as irrelevent as the budget constraints of a steel mill in PA. But some of it isn't. In this case I think getting a larger capacity inverter, or overdesigning, is perfectly warranted considering the costs. Even if just in case..

Bob, if you ever think you may want to power something else off this inverter, ie a laptop, a low-wattage work lamp, another charger, whatever. Then you might want to consider going w/ a higher wattage inverter, especially when the cost difference is so small. All the devices we've discussed here can be had for under $100. Unless you're looking for portability, or some other factor not mentioned, I'd go for the gold..

But that's me!
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  #45  
Old 06-03-2005
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Yes BS was strong john however it was 1am.. and I was tired lol.
are your inverters modified sine or pure sine wave john?
modified normally run 85-90% pure around 70%....
I will agree if you are Very close to their limit their effeciency goes down..
all the more reason to get a bigger one :)

I see your point but I dont feel its relevant.. because we are talking 30$ not 204583843$$$ and its more future proof. Ie Laptop, 4th 5th chargers... etc

Rand
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  #46  
Old 06-04-2005
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Originally Posted by n3elz
It's okay. I'm not a muslim cleric who is considered too holy to be questioned. No fatwah's will be issued for Ben's death -- torture, yes -- death, no...
Hey I heard someone was issuing fahitas and thought I'd... Oh. You.



Um, while John is like any man and thus not ALL knowing, I can speak from 38 years of experience and say that in these maters he most assuredly knows his ****.


That being said, where are my fahitas?

Last edited by N2NLQ; 06-04-2005 at 06:54 AM.
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  #47  
Old 06-04-2005
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did someone say fajitas???? man, now im hungry
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