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Old 05-19-2017, 09:17 AM   #1
Bus Nut
 
Join Date: Jun 2011
Location: KANSAS CITY
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Question AC conversion?

I have read:

An inverter supplying 1800 watts [15 amps @ 120V] will draw 180 amps at 12 volts.



There is 145 amps of 12v DC electrical power available from the engine driven alternator. A 145 amps of 12V would translate to 14.5 amps of 120V.

I have a 120 volt AC that draws 4 amps. My alternator is 145 amps. My bus uses no electricity to run the engine after start up. Can i run my rpm's at about 1800 rpm while stationary and use an inverter to run the ac ? I understand the fuel costs.
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Old 05-19-2017, 09:55 AM   #2
Skoolie
 
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Quote:
Originally Posted by Versatile View Post
I have read:

An inverter supplying 1800 watts [15 amps @ 120V] will draw 180 amps at 12 volts.



There is 145 amps of 12v DC electrical power available from the engine driven alternator. A 145 amps of 12V would translate to 14.5 amps of 120V.

I have a 120 volt AC that draws 4 amps. My alternator is 145 amps. My bus uses no electricity to run the engine after start up. Can i run my rpm's at about 1800 rpm while stationary and use an inverter to run the ac ? I understand the fuel costs.
You're missing conversion losses. 145A @ 12v does not equal 14.5A @ 120v. At best the inverter is 90%-95% efficient (probably more like 75%-80%). Therefore, your 1800W of DC power becomes more like 1500W once its converted to AC.

What you should be doing is starting with the load and working the other way to see what you need to power for it, factoring in conversion losses along the way....
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Old 05-19-2017, 12:26 PM   #3
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It gets kind of murky when we use nominal vs actual numbers..

An alternator's output isn't really 12 volts. Not if it's functioning correctly. It should be higher, around 13.2-13.9, in order to put any charge into a lead battery. The voltage will probably sag under heavy load (I don't know how much, and it probably varies from one alternator design to another) so let's call it 13 volts.

MarkyDee's point about inverter efficiency is spot on, but there's already so much wiggle room in the numbers originally posted that we might want to tighten those up a bit before we try to worry about efficiency.

Since OP's question was really about running a 4 amp air conditioner, not running an 1800 watt inverter at full capacity, I'll focus on that. It'd be 4*120=480 watt load. Allowing for 85% inverter efficiency, the input to the inverter will draw 480/0.85=565 watts. Supposing the alternator was loaded down and making only 13 volts output, that'll be about 43 amps drawn from the alternator.

Keep in mind that when an alternator nameplate says "145 amp" it rarely means 100% duty cycle -- the 100% rating might be just half of the peak current rating.

Supposing we're talking about a diesel engine, 1800 RPM is pretty high. One would think the belt and pulleys would be set up so the alternator can reach full performance at more like 1100 RPM.
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Old 05-19-2017, 12:34 PM   #4
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So will it work?
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Old 05-19-2017, 01:37 PM   #5
Skoolie
 
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I guess I should have gone through the numbers instead of posting what I did. I was trying to do the whole "give a man a fish / teach a man to fish" thing. I apologize....

Anyway, I agree with family_wagon that there is a lot of wiggle room, and I agree with the statements about the alternator, but with a caveat that I will note in just a sec.

So, working backwards from the load, which is a 4A @ 120V load (or 480W), through the inverter (say, at 80% efficiency just to be conservative), would make 480W/0.8, or 600W on the DC side. At 12V, 600W is 50A. Therefore, if the DC source is able to put out 50A @ 12V, then the load should run fine.

Since the alternator is able to put out 145A at full load, and the AC only needs 50A through the inverter, then everything should run just fine - at full load....

But if the engine is running at idle (and this is the caveat), then the alternator's output may very will be less than full load. Therefore, the OP should check the alternator's specs to make sure it can generate a minimum of 50A at whatever the engine's idle speed is.

Just remember that these numbers are conservative, but they are also rough. This means that they should be nearly "worst-case" but not at "worst-case".
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Old 05-19-2017, 02:59 PM   #6
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Thank you. I like that.
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