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mike
Guest
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 Posted:
Fri Mar 11, 2005 2:22 pm Post subject:
Re: YOU THINK YOU KNOW, BUT DO YOU? |
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Dave wrote:
| Quote: | 12V vehicle Inverter
200watt AC device @ 120v (do vehicle inverters push 110 or 120v?).
Is there an equation that will convert what an AC device draws on the
inverter to what in turn the inverter will draw on the car? Using the
above example..?
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I haven't started a firestorm of controversy in a couple of days, so
here goes...
There ain't no such thing as a 200W AC device. There ain't no such
thing as a vehicle inverter.
What you can find is a device that happens to draw 200W under a very
specific set of conditions...120V RMS 60 Hz. sine wave is typical.
But unless the power factor happens to be 1, rms volts x rms amps won't
equal watts.
You can also find a device that converts 12VDC to 120VAC under a VERY
specific set of conditions that probably aren't anywhere near the
conditions specified for the hypothetical 200W AC device.
While you can find sinewave or squarewave inverters, the state of the
market is modified sinewave. That means alternate rectangular pulses
who's amplitude is the peak value of the desired sinewave being
approximated.
That's typically 120V X 1.4. The pulse width is adjusted so that the
RMS value of the waveform matches a sinewave.
If you're driving a resistor, or a light bulb, you can get 90%
efficiency give or take.
So, your output power is 90% of the input power. And you can do the
simple RMS voltage ratios to figger that out.
But what if you're not driving a light bulb? Then it depends on what
you're driving. Power factor can go all to hell...way further toward
hell than if you were driving the same device with a sinewave.
If you need a very accurate number, you have to model everything.
Or you could just use input current = 1/.9 X 120/12 X output current and
wing it. But a vehicle electrical system can vary quite far from 12V.
And don't get me started on the fact that the 200W number on the device
nameplate may or may not bear much relationship to the actual wattage
consumed in your particular application.
My personal favorite is to use an ammeter and measure it. (almost)Never
calculate anything you can easily measure.
mike
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Robert Monsen
Guest
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| Posted:
Sat Mar 12, 2005 1:34 am Post subject:
Re: YOU THINK YOU KNOW, BUT DO YOU? |
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mike wrote:
| Quote: | There ain't no such thing as a 200W AC device. There ain't no such
thing as a vehicle inverter.
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And there ain't no such thing as a solid object. Turns out that they are
mostly empty space. What a shock that was when I found out. ;)
| Quote: | What you can find is a device that happens to draw 200W under a very
specific set of conditions...120V RMS 60 Hz. sine wave is typical.
But unless the power factor happens to be 1, rms volts x rms amps won't
equal watts.
|
I always thought that for those (mythical) inverters, the 200W tag was a
limit, which meant they didn't want to supply more than 200W out the
device, and that if you did, they wouldn't be responsible if your
voltage sagged.
And I'm still waiting for Don Lancaster to use his magic sinewaves to
come up with a perfect sine wave inverter. What about it, Don?
http://www.tinaja.com/magsn01.asp
--
Regards,
Robert Monsen
"Your Highness, I have no need of this hypothesis."
- Pierre Laplace (1749-1827), to Napoleon,
on why his works on celestial mechanics make no mention of God. |
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Fred Abse
Guest
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| Posted:
Sat Mar 12, 2005 2:23 am Post subject:
Re: YOU THINK YOU KNOW, BUT DO YOU? |
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On Thu, 10 Mar 2005 14:49:33 -0800, Dave wrote:
| Quote: | This should be an object lesson contributing something meaningful to a
discussion instead of an obviously nerdy, smartass, socially oblivious,
human contact lacking, left brain comment.
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Hey, who's the one asking for help in this thread?
--
Then there's duct tape ...
(Garrison Keillor)
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Rich Grise
Guest
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| Posted:
Sun Mar 13, 2005 2:56 am Post subject:
Re: YOU THINK YOU KNOW, BUT DO YOU? |
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On Thu, 10 Mar 2005 15:07:47 -0800, Dave wrote:
| Quote: | Ah, true Usenet nerd police. I was waiting for your arrogant comments;
they always come, here goes the volley.
Clearly a case of a nerd who's little (although I'm sure intelligent)
one track mind puts meaning on what he thinks is fact.
What is actually so, is that I use a newsreader at home, but I can't at
work, so I use Google. I did scroll up son, but the equation was not
the one I was seeking. I always get what I need here.
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Yeah. Too bad that's so far from what you want.
'bye. |
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Sean Leistico
Guest
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| Posted:
Sun Mar 13, 2005 6:10 am Post subject:
Re: YOU THINK YOU KNOW, BUT DO YOU? |
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I learned it in Stagecraft class decades ago.
Ol' Mr. Peerbolte called it the West Virginia formula
The abbreviation for West Virginia (before two-letter postal codes) was W.Va
W VA
Watts = Volts x Amps
Algebra will give you the corrolaries for finding Volts or Amps
I know this formula is accurate for AC, and I'm assuming DC--can anyone give
confirmation on that?
"Robert Monsen" <rcsurname@comcast.net> wrote in message
news:QeKdnbP6o7uFU63fRVn-sA@comcast.com...
| Quote: | Dave wrote:
Getting the 17 amps is what I was asking. What is this equation?
The forumula is Power = DC current x DC voltage.
So,
200W/12V = 16.66A
However, that assumes your converter is 100% efficient, which it is not.
So, say it's 70% efficient, then
200W / 0.70 = 286W, so 286/12V = 23.8 A
--
Regards,
Robert Monsen
"Your Highness, I have no need of this hypothesis."
- Pierre Laplace (1749-1827), to Napoleon,
on why his works on celestial mechanics make no mention of God. |
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Fred Abse
Guest
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| Posted:
Sun Mar 13, 2005 5:43 pm Post subject:
Re: YOU THINK YOU KNOW, BUT DO YOU? |
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On Sat, 12 Mar 2005 20:56:32 +0000, Rich Grise wrote:
| Quote: | On Thu, 10 Mar 2005 15:07:47 -0800, Dave wrote:
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| Quote: | I always get what I need here.
Yeah. Too bad that's so far from what you want.
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ROFL!
--
Then there's duct tape ...
(Garrison Keillor) |
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John Fields
Guest
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| Posted:
Sun Mar 13, 2005 10:32 pm Post subject:
Re: YOU THINK YOU KNOW, BUT DO YOU? |
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On Sat, 12 Mar 2005 19:50:17 -0600, "Sean Leistico"
<sleistico@kc.rr.com> wrote:
| Quote: | I learned it in Stagecraft class decades ago.
Ol' Mr. Peerbolte called it the West Virginia formula
The abbreviation for West Virginia (before two-letter postal codes) was W.Va
W VA
Watts = Volts x Amps
Algebra will give you the corrolaries for finding Volts or Amps
I know this formula is accurate for AC, and I'm assuming DC--can anyone give
confirmation on that?
|
---
It's accurate for DC, not AC.
For AC, it's:
W = VA cos(phi)
where phi is the the difference in phase between voltage and current
in the load.
--
John Fields |
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Dave
Guest
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| Posted:
Wed Mar 16, 2005 10:55 pm Post subject:
Re: YOU THINK YOU KNOW, BUT DO YOU? |
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| As usual, just needed a little time for more posts to trickle in. Done. |
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