Inverter/Motor match

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  #1  
Old 10-03-12, 03:42 PM
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Inverter/Motor match

I want to determine what power requirements are needed for a 12 volt DC to 220 VAC inverter that will drive an AC single phase, 1 HP (SF 1.65), 220 V, 9.6 amp, 3450 rpm motor used to turn a water pump on a 200 foot deep water well. There is no start up capacitor visible on the pump motor. I donít know the power factor nor the efficiency of the motor. The data was gotten off the plate on the motor.
 
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Old 10-03-12, 04:21 PM
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Why do you want to use a battery instead of a generator. The real problem with your idea may be the battery if you want to run it for more then a few minutes. You looking at a ~200 amp input from a 12 volt battery or if you can get your hands on a 48 volt forklift battery ~50 amps.
 
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Old 10-03-12, 06:41 PM
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The pump motor would probably be on for no more than 5 minutes at a time. This set-up would only be used for emergency purposes, maybe twice a week. I will be using a solar panel to recharge the two or maybe three 12 volt batteries hooked in parallel. I plan also on using the system during the night for 4 or 5 hours to keep my refrigerator cool. I don't want to use a generator because I would be dependent on getting gas from a service station that has no electricity to pump gas in a crisis situation.
 
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Old 10-03-12, 07:45 PM
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I don't want to use a generator because I would be dependent on getting gas from a service station that has no electricity to pump gas in a crisis situation.
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Not if it used propane. Most grocery and convenience stores here sell propane. Is it that way where you are?

I'd be surprised if the 12 volt batteries lasted even 5 minutes but that is just a gut feeling. What is the amp/hour rating of the batteries you plan to use and what is the output of your solar charger under average not ideal conditions.

From: Battery ratings : BATTERIES AND POWER SYSTEMS
For example, an average automotive battery might have a capacity of about 70 amp-hours, specified at a current of 3.5 amps. This means that the amount of time this battery could continuously supply a current of 3.5 amps to a load would be 20 hours (70 amp-hours / 3.5 amps). But let's suppose that a lower-resistance load were connected to that battery, drawing 70 amps continuously. Our amp-hour equation tells us that the battery should hold out for exactly 1 hour (70 amp-hours / 70 amps), but this might not be true in real life. With higher currents, the battery will dissipate more heat across its internal resistance, which has the effect of altering the chemical reactions taking place within. Chances are, the battery would fully discharge some time before the calculated time of 1 hour under this greater load.
 
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Old 10-03-12, 09:18 PM
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I live out in the country and no stores around. I just don't like the idea of having to depend on someone else. The sun energy is free and I'm totally independent. I like that idea much better.

If I use a 50 AH instead of a 70 AH, just to be conservative, and assuming my motor draws 8 amps (conservative again) based on true power instead of reactive, I could draw for 50 divided by 8 equals equals 6.25 hours. That is no where near the 5 minutes, which is 1/100 of the 6.25 hours that I'll be using the motor. Why do you think the batteries won't last? The CCA of a battery is about 700 AH.
 
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Old 10-04-12, 01:48 AM
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Man, do you need an education on batteries! Much more than I am going to give you here.

The ampere-hour capacity of a battery is measured over a period of time, usually 20 hours but some batteries are rated at 10 hours and a few at 24 hours. A battery with a 50 AH capacity at a 20 hour rate means that it will deliver a total of 50 ampere hours over a 20 hour time span. That calculates out as a continuous current flow of 2.5 amperes for 20 hours. When you increase the current flow above this rate the total AH capacity goes down and the higher the rate the faster the AH rating goes down.

To run a 9 ampere 240 volt motor will require a 3,000 watt inverter just for the running. Since starting the motor may take three to six times the running power you would need an inverter with a peak output of at least 9,000 watts. Sucking 9kw from a 12 volt battery will kill it pretty fast unless it is designed for short, high loads. Such a battery is what is normally found in an automobile BUT these batteries are NOT good at low-to-moderate long-term loads. For that you need a deep cycle battery. The so-called "marine" battery is a compromise of a starting & ignition battery with a deep cycle battery.

But, it's your wallet. If you want to invest a fortune in batteries and an inverter then have at it.
 
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Old 10-04-12, 04:27 AM
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If I use a 50 AH instead of a 70 AH, just to be conservative, and assuming my motor draws 8 amps (conservative again) based on true power instead of reactive
Not even ball park of understanding how electric works. What is important is watts.
Watts is amps multiplied by volts so your motor draws 9.6 amps x 240 volts=2304 watts.

So the battery needs to put out at least 2034 watts of power. To determine amps needed for 2034 watts you divide the watts by the voltage. At 12 volts that is 2304 watts divided by 12 = 192 amps.

So how long will a 70 amp hour battery give 190 amps do you think? But starting the motor conservatively requires at least three times that so you need 3x192=576 so now you are trying to draw nearly 600 amps from a battery designed for 70 amps maximum.*

Simplest real world solution for your problem is a water tank high enough to gravity feed your house in an emergency and use electric when you have it to keep it filled. The solar cells can probably keep the refrigerator running. Best solution is a generator and a 250 gallon propane tank.

*Above explanation of electrical load is under perfect conditions with no resistance or impedance which is not real world. Actual load will be greater.
 

Last edited by ray2047; 10-04-12 at 05:02 AM.
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Old 10-04-12, 06:22 AM
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If you are really set on using batteries I would go huge on lead battery capacity or consider nicad, nimh or lithium cells which are capable delivering large bursts of current over short periods without damage to the battery. In the end I think a propane powered generator will be more reliable and functional in the long run. Any battery system is going to require charging and an inverter to convert the DC to AC. Every bit you add to the system (solar panels, regulator, charger, inverter, batteries...) is a failure point that can make the system useless. Batteries also have a limited life span whether they are used or not. You will have to plan for regular battery maintenance & replacement whether or not you use the system.

You have a similar application to starting turbine aircraft. You need a lot of juice from a battery for a short time. It's not easy or cheap to do. Aircraft do it with very expensive batteries specially made for the purpose. The batteries are short lived because of the abusive current draw and most/all operators go to great lengths to plug in to a generator (start cart) so you don't use the batteries for starting. It's one of those things that can be done but you really don't want to if you can help it. Most aircraft even carry a generator (auxiliary power unit) to avoid the battery problem. And, like at home when the power is out having a generator has other benefits like; lights, AC, running a refrigerator... and it can run as long as you have fuel.
 
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Old 10-04-12, 08:27 AM
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After considering all the negative aspects that you guys have presented concerning batteries, etc, I may have to reconsider and maybe buy a propane bottle and a generator. I'll have to look into that and see how much that will cost me. Thank You all for the insight.
 
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Old 10-04-12, 08:43 AM
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I think you will find a generator & propane tank to be about the same price or less than a properly sized bank of batteries, charger, inverter and solar panels. You can use a diesel or gasoline powered generator but you will have to properly rotate the fuel to insure that it does not go bad. Propane does not varnish, gum-up or harbor microbes like the usual liquid fuels so it's a perfect fuel for a generator that may not get used very often.
 
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Old 10-04-12, 08:59 AM
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Out in the country as you are propane can benefit you in other ways. If you heat with electric or even cook with electric it may be cheaper. You don't need to "get a propane bottle". You can get a site installed propane tank filled from a delivery truck. Very common practice in the country.

Of course if your intent on going outside the box you could use a wood fired steam engine to run a generator.
 
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