Determining load of 240V appliances

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Old 08-15-09, 09:24 PM
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Determining load of 240V appliances

This is somewhat in conjunction of my gen hookup and i'm wanting to find out what my well pump and outside A/C-HP system requires to run and start up.

I have a good Fluke meter and a current clamp capable of reading 600A, how would I measure current with two hots?

My well pump is wired from the pressure switch inside (I assume) and I could clamp the wires from there or would this be an inaccurate reading?

Before I hook everything up, I would like to see what the start-up draw is and running current is for these two power hogs. Could I clamp the hot at the breaker if I cannot get the wires seperated at the pressure switch?

My electric grids have been disabled on my system (Lennox) which had the "even heaters" but in case of an emergency heat demand they would work, but I would leave them disabled unless the situation arose that I needed them...my 5KW gen would get it's arse kicked by the elec strips anyway

I can download the graph of the logging session to see what they pull, just want to accurately measure it.

Just one of these things that I want to have an idea if my gen would even work if needbe. I know I need to try it and see but want a ballpark guess on the current draw first.

To top things off, I don't know the requirements of a well pump but mine is 550' deep...i'm sure that requires some juice to get going.

Another thing for you experts.....is there a way....to install a 12V deep-cycle battery, to the panel to provide an AC voltage for high surge requirements....like some type of converter inline with the generator or the likes?

Kinda like how a car runs off the generator but can pull from the battery if needed. Always pondered the idea but not sure how to make it work....if it could.
 
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Old 08-15-09, 10:02 PM
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Concerning the starting surge of your 240 volt loads, yes, you can use the current clamp on one of the two leads coming directly from the circuit breaker. The problem is that I don't think your meter has a "peak hold" function which would be necessary to capture the peak current draw. Most dedicated clamp-on ammeters do have such a peak hold function.

Even knowing the peak current flow would not necessarily tell you if your particular generator would supply the necessary peak power. It would also depend upon what other loads were being serviced at the same time as when the well pump or the heat pump were to start.

The idea of a battery back-up for supplying peak loads is intriguing but I don't think it is going to be practical. Yo would need the battery to supply an inverter and finding an inverter that would parallel with the generator will likely be an exercise in futility. It is an idea that some small generators do address in that some manufacturers are building what are called inverter generators. My 2.8 kW Yamaha is an inverter generator and while the sustained output is 2800 watts it will hold an overload of 3000 watts for twenty minutes or an extreme overload of about 8000 watts for 3 seconds. The inverter generators (Honda also makes a few models) use a high cycle polyphase AC alternator feeding a rectifier bank to convert to a DC current. This DC is then inverted via a frequency controlled pure sine wave inverter to output a high quality sine wave AC power. Some of the inverters are able to take additional current from the starting battery to give a short time boost to the inverter output. Unfortunately, this technology is rather expensive and so far that I am aware is limited to smaller generator sets.
 
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Old 08-16-09, 06:30 AM
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The meter is an 89 IV and from the few times I checked automobile starter amps, it seemed to register the peak draw on the graphs, breaks down the time of the spike as well through the Flukeview forms software.

Do I measure one wire and double it (assuming it's equally divided)?

As far as the inverter...it's just a matter of finding a high current inverter I assume? Could something like this be pulled from one of these generators that are junked or even as a replacement part?

It's something that really has me wanting to pursue but if the expense far outweighs the fact I "just wanna see" if it works then I will ditch the plan.
 
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Old 08-16-09, 10:03 AM
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Do I measure one wire and double it (assuming it's equally divided)?
Both wires will be carrying equal amperage so all you need to do is measure one of them. No calculation necessary.

As far as the inverter...it's just a matter of finding a high current inverter I assume? Could something like this be pulled from one of these generators that are junked or even as a replacement part?
Not really. Separate AC sources need to be synchronized before they can be paralleled. This is beyond the capability of most home generators and inverters. There is also very little, if any, surplus or junked equipment available that might be capable of being used in this situation. Large scale inverters are available but you would need to run them in a "stand alone" configuration and then use a generator to recharge the batteries. The cost would be out of sight for anything but perhaps national security. Such a system as you envision would approach the cost of your entire home if not exceed it.
 
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Old 08-16-09, 11:35 AM
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Simply staggering the start of the blower fan, compressor fan and compressor with a delay contactor along with upsizing the compressor start capacitor would probably be more effective at getting the unit started than any inverter based solution.

Synchronizable high current inverters are commonly found in solar or wind power systems, and they are really expensive -- like $1 / watt. Somewhat cheaper are computer UPS inverters, however almost all of these are traded in at the manufacturers or sent to a recycling facility so I doubt you'll find one at any scrap dealers.
 
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Old 08-16-09, 07:55 PM
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Least now I don't need to buy a deep-cycle battery, was a long shot but figured I would inquire anyway.

Guess i'm not gonna crawl into a tube and replace a start capacitor on the well pump, i'll see if the gen can crank it up by itself and go from there. My A/C unit will start the compressor/cooling fan before the air handler fan so at least that's a plus.

What will I need to to get to delay the cooling fan to the compresor? This is something I can probably do myself. I will likely not need the A/C when the power goes out but it's good to know I can run it if needed, just mainly want the air handler to run to distribute the fireplace heat.

I'll get back with some data on these motors and we'll go from there, thanks alot.
 
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