Power Inverter Calculations

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Old 06-13-12, 04:43 PM
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Power Inverter Calculations

I want to install a power inverter to power my sump pump in case of emergency when I'm traveling. I'm having a hard time sizing the inverter. I don't want to install something oversized, when a smaller power inverter would do it.

The problem is different sources all give very different numbers. The pump is a Westinghouse; type T; 1/3 HP; 6.2 A. I first tried with a MSW (modified sine wave) 400W-12V inverter, then a 750W-12V. Neither solution starts the pump. My multimeter reads 6.9A pump as the average value absorbed by the pump when connected to the outlet. However, at start the multimeter showed the maximum current absorbed exceeded its 10 A rating.
  • An information web page from one manufacturer, suggests a safety factor of 3 for a sump pump, starting from the pump rating. And that's why I initially tackled the project with a 400W inverter: 1/3 HP is about 250W, and times three gives 750W. The 400 W inverter supposedly has a peak power of 800W.
  • One site by a distributor has a table of recommended inverter power for different appliances. Almost at the bottom, the power recommended for a 1/2 HP pump was 1100 W, and that's why I tried with a 750W inverter for my 1/3 HP pump, since the rated peak of the inverter is 1500W.
  • A customer service rep from a different distributor suggested a 2300W. I just don't want to fork $250 when perhaps $150 will.
 
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Old 06-13-12, 04:58 PM
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I think the problem is your inverter being square wave, rather than pure sine wave.
 
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Old 06-13-12, 04:58 PM
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First off your pump does not absorb anything. Current is the flow of electrons, or amps.

Your pump rated running current is 6.2 amps or 744 watts (6.2 x 120) or 828 watts by your multimeter.
As you noted, A motor can draw up to three times the running current, so your looking at somewhere around 2200 - 2480 watts at startup.

Since the 750/1500w inverter did not do the job I would say your stuck getting the 2300 watt rated or better.

For that cost, you may want to look at a sump pump that has battery backup option and skip the inverter. You will still have to set up a charger for the battery with the inverter, and there is no real way to switch between wall power and battery power while running on the inverter. Google "sump pump battery backup"
 
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Old 06-13-12, 05:12 PM
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There are also water powered sump pumps.
 
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Old 06-13-12, 06:25 PM
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Thanks for you reply Tolyn Ironhand. The battery charger portion of the project is already solved. I bought a $10 trickle charger, which does the job, and keeps the battery topped up. I've been testing it for 5 days and as the top up voltage increases, the current decreases (from 500 mA to 90 mA)

Since there are intermediate solutions between 750W and 2200W, for instance 900W, 1100W and 1500W, I wanted to size the inverter properly. What I'm trying to understand is if it is enough that the peak value of the inverter tops the peak value of the pump. So with your calculation of 2480 W perhaps it's enough to use an inverter rated at 1500W with peak 3000W.

As Justin Smith rightly observed my inverter is a modified square wave or MSW (by the way MSW is different from a square wave). However, many appliances run on MSW. I understand there is an efficiency factor, i.e. 1000 W MSW will not be as powerful as 1000 W sinusoidal power. So perhaps I have to use an additional multiplier to calculate the power of my inverter.

Any further help or sugestion are appreciated.
 
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Old 06-13-12, 06:29 PM
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Why are you trying to re-invent the wheel? Battery powered standby sump pumps are a proven technology and will be less expensive than the inverter, deep cycle battery and charger you are attempting to build.
 
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Old 06-13-12, 06:44 PM
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A trickle charger is good to keep a battery topped off when in storage or unused but might not really designed to charge an battery that has need some heavy use. Plus, like I mentioned, when you go out of town the pump will be on "inverter only" power. So, if your pump runs a fair amount of of the time anyway, the charger might not be able of keep up. With a battery powered stand by unit it will only use the battery if you lose power.
 
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Old 06-13-12, 07:42 PM
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You should also consider if your inverter goes into "Fault mode", your pump will have no power.
 
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Old 06-13-12, 08:09 PM
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I had my sump pump fail me not too long ago and it was a major problem. I've always had it in the back of my mind to get a backup system. I have to get one installed, but after looking at battery back up systems. I decided they weren't for me. The batteries don't last that long when the pump needs to use it. You also never know when the battery is going to quit on you. I plan on getting one of the water powered back ups. You need to have city water for them to work. If you have a well, forget it. This is the unit I plan on getting: Amazon.com: Basepump RB 750 Water Powered Backup Sump Pump: Home Improvement All the other brands seem to have very poor reviews. As soon as I get paid for the side work I have been doing, I'm going to get it put in.
 
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Old 06-13-12, 08:42 PM
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Trolyn Ironhand, the trickle charge is all it will need to operate. The sump pump is rarely activated, 3-4 times a year, for 1-2 days, 30 seconds every 5 minutes on average. The system will be in service 2-3 times a year, with the battery already charged, and the trickle charge will keep topped up. In the rare occasions in which, while I'm travelling, the sump pump needs to be activated, it will start operating from a fully charged state.

Furd,
I have priced powered standby pumps, and I had the impression they were more expensive. Besides I will have an inverter available for other emergency uses.
 
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Old 06-14-12, 04:56 AM
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Are you sure you have the recommended sized battery cables........it matters!

You might want to consider an inverter/charger rather than just a straight inverter.
An inverter /charger works the same way as an uninterpretable power supply (UPS).
They have a built in switching mechanism to allow the load to operate on regular power while it is available and will keep the battery charged.
When the power goes out it switches to inverter operation.
I went this way rather than a 12 volt pump as the inverter is available for other things in a power failure if the pump is not needed.

I also had a 1/3 hp pump but found it was over sized for the flow I normally get.
I was wanting to replace my pump anyway and it was cheaper to buy a smaller pump that up-size an inverter for a pump capacity I didn't need.
A 1000 watt Zantrex inverter charger comfortably powers my 1/4 hp pump and moves more than enough water for my needs.
Their 1800 watt model should power a 1/3 hp pump.

One consideration with having too a large pump is less run time on a given battery and the size of cable needed to connect the battery.
The 1000 watt model calls for #2 cable and the 1800 watt 2/0 which is pretty thick wire.

The Zantrex inverters are fairly decent quality units and the ratings are somewhat understated.
If you are using a somewhat inexpensive inverter you likely would need to go to the 2300 watt model.
 
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Old 06-14-12, 07:23 AM
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GregH, I think you are making a good point. I had noticed, while measuring amp draw with my multimeter, that using the smaller connection cable of the multimeter limited the power output (a drill, which although 1/3 HP like my sump pump target application, works without a glitch because it has less surge). However, I assume that the cable provided by the manufacturer of the inverter is good enough for all the inverter has to give.

I'm sure you are right about the pump being oversized.

The 1000 W Xantrex inverter you suggested is nice, but I don't need the built-in powerful 20A charger that is part of the package. My trickle charger handles the situation, and it's only a 0.6 A charger.
 
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Old 06-14-12, 12:05 PM
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Right.
The charger that is built into the Zantrex units are the least expensive parts for them to include.
The whole idea behind these units is that they operate on normal power while it is available and switch to battery when the power goes out.
The advantage is that you don't have to run all the time on inverter power which on an inexpensive unit would likely shorten its already short life expectancy.

Thinking that a .6 amp charger is good enough assumes that you will have a power failure and then the power will be back on for the several days it will take to bring your battery back to full charge.

The wire that is included with many inverters is usually not as large as what is suggested in the manual.
A way to check is to connect a dc voltmeter to the 12 volt connections at the inverter and compare the static voltage and the voltage under load.
You should be withing 10% - 12 % of the battery voltage.

What size and type of battery are you using?
 
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Old 06-14-12, 12:34 PM
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GregH, I'm using a regular automotive battery. Must be around 60 AH.

Regarding my trickle charger, it will be enough for the use profile I have. If you read the details I wrote about the use, you'll realize that a quick charge is not necessary, as emergency situations may be spaced one year apart.
 
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Old 06-14-12, 07:45 PM
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I think Greg has a winner!

I would also like to point out, of the inverters I have used, they also had onboard fans that would run regardless of the load on it. This will also drain down the battery.
 
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Old 06-14-12, 08:36 PM
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The small inverters you are trying to make work are not meant for extended motor loads and when you plug in a pump to the inverter connected to a battery is actively working even though the pump is not running.
As Tolyn says, your battery will be draining causing the charger to be continuously charging.

Six months down the road you may need it and it might not operate.
 
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Old 06-14-12, 08:45 PM
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Trolyn Ironhand,
The fans of the unit I tried do not run continuously, but only when temperature is high enough to warrant them.
 
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Old 06-14-12, 08:49 PM
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Yes GregH,
If you look at the demand of the system, when the system will be in waiting mode, the trickle charger will be enough to keep things topped up, so no drain of the battery.

Is there anybody in this forum who knows how to make the calculations for the power requested on a MSW?
 
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Old 06-14-12, 09:41 PM
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Calculations will not give you the capabilities of a particular inverter.
You would do well to refer to the manufacturers literature for whatever inverter you are interested in.
 
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Old 08-09-12, 12:00 PM
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I eventually settled on a 1500 W modified square wave power inverter and everything worked.
 
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Old 08-09-12, 06:12 PM
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Thanks for the update!
 
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Old 10-31-12, 03:05 PM
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Update:
In preparation for hurricane Sandy, I tried another inverter, and it works with a smaller inverter rated at 1100 W (modified square wave, same brand, peak 2200 W).
 
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Old 02-18-15, 10:45 PM
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Confirmation of pump power rating

Hi Gciriani,

I am at a similar position where I have to choose an inverter to power my sump pump. I found this discussion helpful. However, I couldn't help notice that the pump specs given by you don't seem to add up.

You stated the pump specs as 1/3 HP 6.2 A. However, at 120 V outlet voltage and 6.2 A current, the power rating of the pump comes out to be 744 W, which is 1 HP, not 1/3. In fact, based on your multimeter reading, you are using more than 1 HP of power.

Based on that, using a 1100 W inverter for a 744 W rated pump is pretty reasonable.

Can you please help resolve this discrepancy for the pump rating? I have been looking through a lot of information online and there is a lot of speculation. Yours is the first post I came across where someone actually tried and succeeded in implementing an inverter powered sump pump.
 
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Old 02-18-15, 10:55 PM
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746 watts is one ELECTRICAL horsepower. There is very little correlation between electrical horsepower and mechanical horsepower in smaller size motors. Once you get above about 100 horsepower motors the electrical horsepower and the mechanical horsepower become closer.
 
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Old 02-19-15, 05:21 AM
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Hello all.. old thread... Please start new...

Thanks and welcome to the forums...
 
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