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Old 09-28-2012, 10:36 AM   #21
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On AC you need to consider power factor as well. Power factor is the phase relationship between the voltage and the current. It is possible, with a purely capacitive load to have lots of voltage and current, but no power.

120 VAC x 1 A x 0.8 PF = 96 W
Interesting, Dave. What type of camper applications would this tend to apply to?
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Old 09-28-2012, 10:42 AM   #22
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Originally Posted by David Tilston View Post
On AC you need to consider power factor as well. Power factor is the phase relationship between the voltage and the current. It is possible, with a purely capacitive load to have lots of voltage and current, but no power.

120 VAC x 1 A x 0.8 PF = 96 W
In today's electronic and electrical stuff you're more likely to have an inductive load. Purely inductive or capacitive is NOT going to happen. The effect is very small, therefore for practical use can be ignored. If you wanted to get really picky you could through in power line wire IR loss, wire inductance, capacitance, Q, and etc., etc.
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Old 09-28-2012, 11:48 AM   #23
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Capacitive loads have a leading power factor, inductive loads have a lagging power factor and resistive loads have the voltage and current in phase. For an example take the full load amperage x the voltage of a one horsepower AC single phase motor and you get the apparent power but when you compare that number to the true power to obtain one horsepower (746 watts) the numbers do not match That is because the voltage and current are out of phase. A normal home is assumed to be at 80% PF . I do not believe in a trailer the PF is a major concern because trailers do not have large inductive loads such as a well motor but if your worried about being absolutely accurate in your calculations you should consider it. Unity PF is hard to obtain
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Old 09-28-2012, 03:04 PM   #24
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On a trailer, all the DC loads are at unity PF. Just the nature of DC.

I did not mean to start a large discussion about power factor, I was just saying why, with AC, Volts x Amps is not always Power. Typically the largest inductive load on a trailer would likely be an old converter. Mine has a large transformer, (coil) in it.

I used to work for one of the power companies in Alberta. I was a lineman and a substation designer. We used large banks of capacitors, which are nearly pure capacitive loads, to provide power factor correction for large factories witch have lots of motors. Motors can have very low inductive, power factors, but since they also have resistance, they must have a large restive, (real) power element.

I am about to launch into some AC theory to tell what I thought was one of the strangest moments in my electrical education.

Reactive VARs, and Capacitive VARs cancel each other out. Resistance is what limits current flow in a typical DC circuit. It also limits it in an AC circuit, but there is additionally inductance, (coils) and capacitance, (capacitors). They are not referred to as resistance, but instead, impedance. These also limit current flow. Inductive impedance, and Capacitive impedance cancel each other out. That is part of the reason that you can use capacitors to correct the power factor of motors. It is also called a tuned tank circuit. Used in old radios for tuning in stations.

Our teacher in high school electricity was demonstrating this by putting a large coil in series with a large capacitor, which was selected to have the same impedance as the coil. This left just the resistance of the coil to limit current. After the demonstration, he unplugged the circuit and was discussing it while holding the plug in one hand and fiddling with the terminals on the bench with the other. I guess he touched the bench ground and just started swearing up a storm. Capacitors, it turns out have another property. They store a charge.

Ohms law states that current = voltage / resistance

This can be transposed to: voltage = current x resistance

This is also true for: voltage = current x reactance

The reactance was around 1000 ohms, and at slightly less then one amp, the capacitor was seeing voltages of nearly 1000 VAC. Pretty impressive for something plugged into the wall. ~1000V is what was discharged across my teacher, to ground. He said he nearly dropped.
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Old 09-28-2012, 03:18 PM   #25
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..........I did not mean to start a large discussion about power factor, I was just saying why, with AC, Volts x Amps is not always Power. .........

So, is this why a hard start capacitor is useful to allow a small generator to start an air conditioner?
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Old 09-28-2012, 03:31 PM   #26
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So, is this why a hard start capacitor is useful to allow a small generator to start an air conditioner?
This is just a guess, but the capacitor would help overcome the inductive reactance, in the air conditioner motor. This leaves the generator to overcome the resistance. If the motor has a power factor of .8, then the generator is relived of 20% of the burden.
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Old 09-29-2012, 05:39 AM   #27
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After sitting all day, the battery measured 12.8 v. I will run another test today with the appliances and report back the results. Thanks for all the advice. Lots of things I never considered.
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Old 09-29-2012, 06:40 AM   #28
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FWIW: I think you need to go back to the original readings and find out what your true current draw is. CRT TV's typically draw more like 60+ watts, that's about 5 times what you were measuring and, in itself, would explain why your battery went down so quickly. In addition, you need to know if your battery was fully charged in the first place.
How are you measuring the 1 amp draw?
How are you measuring battery voltage?
Do you have a second meter to verify any of the above?
What does it say on the back of your 13" TV for current/wattage requirements?



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Old 09-29-2012, 08:06 AM   #29
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So, is this why a hard start capacitor is useful to allow a small generator to start an air conditioner?
The capacitor causes a phase shift between the start and run windings of a single phase AC motor and is used to increase starting torque Typical uses are well motors, air compressors, table saws and air conditioners . Three phase motors do not need the torque boost so capacitors are not needed. Capacitors in three phase applications are for PF correction. Improperly applied capacitance can do as much harm as good
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Old 09-29-2012, 08:28 AM   #30
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I think it is called a "soft start" capacitor for hard starting a/c units??
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Old 09-29-2012, 08:52 AM   #31
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I think it is called a "soft start" capacitor for hard starting a/c units??
Dunno. The places that sell them call them hard start capacitors.

Supco SPP6E E Series Electronic Potential Relay Style Hard Start Kit,1/2 - 3 Horsepower Range,88-106F Capacitor Size: Amazon.com: Industrial & Scientific
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Old 09-29-2012, 09:21 AM   #32
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You might also consider a much lower drawing LCD TV.
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Old 09-29-2012, 09:28 AM   #33
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Whoooops, already mentioned. Should have read the whole thread.
;o}
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Old 09-29-2012, 09:37 AM   #34
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The link below is a real good starting point to understanding 12 Volt & battery issues the the orignal poster may find helpful.

The 12 Volt Side of Life
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Old 09-30-2012, 07:01 AM   #35
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Thanks everyone! After running a 2 hour test again yesterday, I thinks its best to take the advice given and buy a modern 12v/ac tv. Naxa seems to get good reviews and have a model with a built in dvd player.
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Old 09-30-2012, 08:00 AM   #36
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You might watch WOOT.COM and see if the Haire 24", edge lit, 1080p, flat screen w/dvd player comes up again. It works on 120 VAC or 12 VDC, has a ton of inputs, and was less than $200 including tax. Ours arrived 2 days after ordering.

After a short learning curve on set up, it works great as a TV (85 channels with "Rabbit Ears") DVD Player and a computer monitor for Netflix.

Anyway, I always check my daily WOOT offerings.



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Old 09-30-2012, 02:32 PM   #37
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You might watch WOOT.COM and see if the Haire 24", edge lit, 1080p, flat screen w/dvd player comes up again. It works on 120 VAC or 12 VDC, has a ton of inputs, and was less than $200 including tax. Ours arrived 2 days after ordering.

After a short learning curve on set up, it works great as a TV (85 channels with "Rabbit Ears") DVD Player and a computer monitor for Netflix.

Anyway, I always check my daily WOOT offerings.

Thanks Bob. I will do that. Now I need to figure out what gauge wire to run from the front of the camper where the battery is to the sink side of the counter. That's a pretty long stretch and Im not sure how much of a power pull a 12v tv will have on the wiring.
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Old 09-30-2012, 02:39 PM   #38
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#14 or better should be fine. Flat screens that run on 12 VDC pull less than 3 amps.



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Old 10-01-2012, 04:34 AM   #39
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Thanks again Bob!!
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Old 10-01-2012, 05:09 AM   #40
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Thanks Bob. I will do that. Now I need to figure out what gauge wire to run from the front of the camper where the battery is to the sink side of the counter. That's a pretty long stretch and Im not sure how much of a power pull a 12v tv will have on the wiring.
You may want to run additional devices in that location, so it might be prudent to use 12 gauge wire. I've found it quite cheaply on eBay sold as a Siamesed speaker wire.
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