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Old 07-22-2007, 08:31 AM   #1
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invertor theory 101 teach the new kid in cool skool

Ok the title basically say it all. I am new to rv/bus electrical teach me.
Yes I know the real basics I know that an invertor changes 12vlt or 24vlt dc to 110ac. I know that the bigger the wattage the more appliances it can handle. Is one high wattage invertor to run all of the 110 outlets better or seperate the circuts and run several smaller invertors for each circut.
and will you guys look at my post for ideas on electrical in conversion, had some questions about battieries alternators ect.
thanks Dale
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Old 07-22-2007, 11:14 AM   #2
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Re: invertor theory 101 teach the new kid in cool skool

A few more notes on inverters....

Sine wavefore

AC electricity follows a sinewave pattern as you probably know from peak to peak with a frequency of 60 hz. This is difficult to replicate. Old inverters actually used a square waveform which was okm for lights and motors, but electronics don't much care for it.

More modern cheap inverters use a modified sinewave. In this form a certain number of points on what would be the sine graph are selected and tangent lines are drawn from them until they intersect (mmm...calculus). In this way the sinewave still isn;'t a true sine wave, but is a close approximation. Almost all inverters you will see that don't have a screaming high price will be of this type and they work fine for almost all electronics. If you have uber sensitive equipment (laser printers come to mind) they will not work, but a computer will be fine. In fact, most UPS systems give a modified sine wave and they are designed specifically for computers.

The best inverters will produce a true sine wave. These are also the most expensive. As I stated before...most people do not require this, but depending on what you';re putting in it would be a good idea to look at one perhaps.


A bit on efficiency

Most inverter companies list their efficiency as >90%. Simply put that means that 10% or less of their input current is becoming heat, noise, and fan drive instead of 120vac. So why does it vary? Well they are at their least efficient when they aren't fully loaded. At full load they are working at that greater than 90% efficiency.

So is one inverter better than two? Well...it depends. I have two cheap ones (there was a thread about them last year) that I use to control my electrical needs. Often times I will only run 1 as it is capable of meeting my needs and will be at its most efficient since it will be loaded higher. More than the efficiency it saves wear and tear on an inverter and leaves me with a back up, but they are cheap so if I had to replace one so be it.

So what would be advantageous about a single inverter? If you get one with a built in transfer switch and converter then you won't want two. The one will control all functions and simplify wiring. Also, two inverters at 1000 watts are less flexible in some senses than a single inverter at 2000 watts. With my dual 750 watt setup I am limited to 7.5 amps on any single circuit even though I have a total of 15 amps available. A single 1500 watt inverter would be able to run a 15 amp load on a single circuit whereas I cannot.

When figuring for battery bank size I would multiply the AC current load by 10. Since our alternators put out 14.4 volts, but multiplying by 10 assumes 12 volts you should take into account efficiency and line loss all while making the math easier.

I need to get going now, but if you have more questions post up and I'm sure someone will be able to help.
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Old 07-22-2007, 10:59 PM   #3
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Re: invertor theory 101 teach the new kid in cool skool

is there any advantage or dis- advantage of using a 4000 watt inverter if a 2000 watt would do.
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Old 07-22-2007, 11:00 PM   #4
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Re: invertor theory 101 teach the new kid in cool skool

oh by the way are there any brand of inverters that I should stay away from
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Old 07-22-2007, 11:44 PM   #5
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Re: invertor theory 101 teach the new kid in cool skool

Well...using a 4000 watt inverter when a 2000 watt inverter goes back to that efficiency thing. They are most efficient when fully loaded so the 200 watt version would make most efficient use of your battery power. However, if inverter efficiency pushes you over the line in terms of battery capacity it might be time to look at adding more batteries. Size the inverter for the largest load you can ever imagine yourself needing. You don't wantto go shopping twice.

I'm not sure that I can distinctly name a good brand versus a bad brand because I have only used the el-cheapo Vector inverters. These have a reputation of being lower quality and I can see it. Mine are loud (part of that has to do with my poor mounting) and have gotten louder since I've owned them, but they still work great. They are a modified waveform in case you were wondering.

I think a lot of us run the Vector inverters, either under the Vector label or with another label slapped on them. They lack all but the most basic features, but I bought my 750 watt models for $40 a pop shipped off eBay so I'm not complaining. Les Lampman should be able to help you out more on the high end stuff, as should Steve as I believe they both are using more sophisticated stuff.
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Old 07-23-2007, 09:36 PM   #6
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Re: invertor theory 101 teach the new kid in cool skool

I don't know How sophisticated I really need to get, I want to run ac, the fridge and a microwave at the same time if I need too. THe only thing I'm really worried about Is my wifes monster hair dryer, curling irons, hot rollers, and clothes iron. Of which she will probably want to use all at the same time with the microwave cranking, and ac going and the tv on at the same time. !!!
I want to however run a lcd tv in the bedroom thought about a really small pure sinwave just for it and maybe surround sound. I thought I could use a cheap one to run the big stuff and a really small good one for the good stuff. i would just isolate an outlet for that.
I know about the law of degeneration, and realize that there is no such thing as a perpetual motion machine, but I had thought about adding a small wattage dc motor that would an alternator at 2500 to 3000 rpm to slow the depletion of the batteries. at that speed it should add about 15 volts to the batteries not sure about amps though, maybe I would not need to run a butt load of batteries in my bank.
As I was reading about deep cell batteries I have determined that one new battery should run about 120 minutes until it depletes to under 10.5volts. Is this info about right. older batteries about 100 minutes or less per battery.
I thought about 4 to 5 batteries would about right is this correct.
I found a 5000 watt constant 10000 peak 90% percent efficiancy for $325.00 new. modified sinwave. I am not sure what the amp draw is. about how many amps should I pull per hour, obviously a inverter tha uses less amps to do the job would be a better pick right?
the problem in trying to pick out an inverter now is the fact that I do not have the rest of my electric appliances yet to figure out how many watts I need to do the job.
The production rv world must have a standard number of watts for an inverter in a rv. I wonder what thier rule of thumb is.
I know a 5000 watt inverter is probably overkill but I am trying to look at the heavist load that I would possibly need, figure the the cost and then back off to a smaller unit an spend less money. I would drather come in under my maxium budget than over it
Dale
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Old 07-23-2007, 10:52 PM   #7
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Re: invertor theory 101 teach the new kid in cool skool

Ok, quite the post. I'll try and help you out as best as I can here.

First things first. I think you need reconsider your electrical needs.

Let's start with the inverter. You are looking at a MASSIVE inverter.

Watts are watts. If you have 5000 watts on the AC side, you need 5000 watts on the DC side.

Watts=volts * amps

So...5000 watts on the AC side means you have 41.6 amps available
5000/120=41.6

Now lets calculate the DC side of things.

5000/12=416.6 amps (!!!)

Your inverter won't often be at full load I don't think, but we're going to plan on 415 amps since it's close and a nice number.


Now let's look at batteries. The 120 minutes to 10.5 volts you were referring to is called reserve capacity. It is one of the ways of expressing the storage ability of a battery. It is calculated by taking a battery at 80* F and putting it under a 25 amp load. The number of minutes it can maintain that load down to a voltage of 10.5 volts is the reserve capacity. I hate using reserve capacity. It is complicated and no more accurate than amp hours.

Amp hours are just that...amps times hours. 1 amp for 5 hours is 5 amp hours. 5 amps for 1 hour is 5 amp hours. In reality they aren't equal, but more on that later.

A GC2 sized 6 volt golf cart battery is going to have about 225 amp hours of capacity. Since it is 6 volts you will need two of them wired in series to 12 volts, but this will not increase your capacity. You will still have 225 amp hours of capacity. Easy enough, right?

Well let's look back at your inverter load. It was 416.6 amps at full load and we're just going to call it 415 cause I like whole numbers.

So we have 225 amp hours available to us. We are drawing 415 amps. 225AH/415 amps=.54 hours of run time on a pair of golf cart batteries.

Oh but it gets worse...This little German fellow named Peukert determined that the higher the rate we discharge a battery at the lower it's amp hour capacity will be. Remember the 1 amp for 5 hours versus 5 amps for 1 hour? Both are 5 amp hours, but because you are discharging the battery more quickly in the second example you will actually need a higher capacity battery. You might often see golf cart batteries labeled with their reserve capacity at 25 amps (the standard) and at 75 amps. Thanks to Peukert the reserve capacity at 75 amps is less than 1/3 of that at 25 amps. Your 415 amp load is NOT going to last for half an hour like the simplistic equation of amp hours divided by amps would lead us to believe.

Thought it couldn't get any worse? Oh it does! All those ratings are to a battery voltage of 10.5 volts. A fully charged 12 volt battery is 12.6 volts (2.1 volts per cell). Fully discharged it is at 11.8 volts. Clearly 10.5 volts is far less than "fully discharged." To add insult to injury a deep cycle battery should NOT be discharged to less than 50% of capacity for the sake of lifespan. More than 50% discharge and you're doing permanent damage to the battery, deep cycle or not.

So...we don't want to draw our batteries down to less than 12.4 volts. That means our 225 amp hour pair of golf cart batteries really has only about 100 honest amp hours to give us. Ignoring our buddy Peukert for a minute that is going to only give you about 15 minutes of run time at full inverter load.

Pretty depressing, huh? Never fear. 12 volt living IS managable. The first and easiest solution is to just add more batteries. But there are limits as to what our pocketbooks can take and how much space we really want to lose. On top of that you are limited as to how fast you can recharge batteries. Remember our 15 minutes of run time? Well the general rule for recharging batteries is to keep the amperage at or below 25% of the amp hour capacity of the batteries. Since we have 225 amp hours we should keep our charge rate right around 50 amps max. Since we took out 100 amp hours we need to put 100 amp hours back in them. 100 amp hours divided by 50 amps=2 hours. Yep, we need to recharge those batteries for 2 hours after our 15 minute drain period.

So what's the next solution? Well, reduce the electrical load! Don't run the microwave unless the bus motor is running (alternator power) or you are plugged in. Try not to use the AC unless you have alternator power or shorepower available. Your wife's beauty stuff you were worried about? Look into camping versions. I have seen blow dryers and curling irons that ran on butane. Or just use a towel to dry the hair and rollers to curl it if she must do those things at all. Heat your coffee water with a propane stove and use those teabag style coffee pouches. It really is easy to do!

I don't mean to take the wind out of your sails. I just want your system to work well for you. It's best to plan realistically and then be surprised later at just how well it works.

If you're looking for more reading on the subject I suggest the poop sheets. http://www.phrannie.org/phredex.html

Oh yeah...as for the electric motor driving an alternator...you were right about no such thing as a perpetual motion machine. If the alternator produced 1200 watts (100 amps, 12 volts) the electric motor will inevitably require 1500 watts to turn it. It's just the way it is. You could, however, drive that alternator with a small gasoline engine.

I hope this helps.
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Old 07-24-2007, 09:01 PM   #8
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Re: invertor theory 101 teach the new kid in cool skool

In short inverters are very useful when your moving but as a long term use off road ect. they lack the punch to do the job. a generator is a must.
1. Only get an inverter as big as the job requires.
2. extra batteries are great but add more weight.
3. it takes more time to recharge batteries than it does to drain them
4. inverter are a short term fix for a lack of juice only.

teacher have I learned the implied lesson of the theory that was given in this assignment?

Ok then with generators how do I keep from spiking equipment such as computor equipment, lcd tvs ect.
are the regular surge protectors that i use at home adequate.
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Old 07-24-2007, 09:37 PM   #9
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Re: invertor theory 101 teach the new kid in cool skool

You're catching on. I think I may have scared you away from inverters though and that was never my intent. I use my inverter for up to 36 hours at a time without firing up the bus or charging my pair of golf cart batteries. It's all about balancing your loads. You just need to be realistic about what you can and can't do.

For instance...with that 415 amp peak load we were talking about you're still losing a battle with the engine running. You alternator is probably 200 amps or less and not rated for continuous duty at that output. Even if it was and the bus was drawing nothing electrically other than the inverter load you're still 215 amps short. Basically what I was trying to do is have you take a good look at what is really necessary on the road and what isn't.

Do you really plan on doing all that much boondocking or will you mostly stay in a place where you can hook up? If you're going to be hooking up then by all means run dual roof top AC's, a microwave, and every implement of beauty your wife owns. You get charged to hook up, not for how much you use. Just remember that when you lose that cord it's time to turn off the AC or use a propane stove, etc.

I think you're getting the jist of things. It's all about doing the math, really. Just remember that watts=volts * amps and all will be well. Watts are great because 2000 watts on the AC side is 2000 watts on the DC side. You can figure it from there.

If I were you I would get that inverter if you want. Get a set of 2, 4, 6, 8....whatever you can afford...golf cart batteries. Heck, maybe you can get a smoking deal on some Group 8D deep cycles. You just need to make sure you know the limitations of your system.


As for generator surge supression you have a few options. Some gennies like the Honda EU series make a true sine wave form and use an internal inverter. I think you would be safe. A cheap option would be just to get some small UPS units for your sensitive equipment. You can probably find them for $25 for a small unit.
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Old 07-25-2007, 05:58 PM   #10
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Re: invertor theory 101 teach the new kid in cool skool

well like I said I am new to this I took my uncles rv to the ocean and camped a few day now I'm hooked.
I'm just a dumb body and frame guy and a semi fair mechanic, but not a electrical engineer or a mathamatician either.
A nd for the most part have trouble with spelling and with grammar too.

as for the other post about bouncing, when you replace springs for airbags you dramatically change the ride aspects of a vehicle and for the most part you can increase the load capacity too. you are trading different types of mechanical operations. I can't explain all of the engeneering reasons but I can tell it does work. If you look at all of the newer trailers they are air ride only. it is just softer ride period particially because air provides a shock asorber effect as well as a resistance effect. and you can instantly change the aspects just by adding or removing air.
dale
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