Re: Power Inverters


"Dave Martindale" <davem@xxxxxxxxx> wrote in message
news:ggirq9$713$1@xxxxxxxxxxxxxxxxxx
"M.I.5¾" <no.one@xxxxxxxxxxxxxxxxxxxxxx> writes:

The typical modern, cheapo inverter works by using a high frequency
12vdc
to 160vdc switching supply. I'm in the USA. If you live where the
standard
is not 120V 60 Hz, scale appropriately.

If the inverter was to produce a DC output, then this would be a feasible
way to construct such a device (and as I have pointed out elswhere, there
are several advantages to doing so). But for an inverter to power a
standard computer power supply using an AC output, the output will be 50
or
60 Hz depending on which side of the great divide you live on. That
invariably requires a transformer to step up the switched 12 volt input
that
it gets. Although switch mode supplies will work off higher frequencies
than 60 Hz (we regularly use them from aircraft supplies (400 Hz), most
usr
would be reluctant to use them at anything other that the frequencies
stated
on the tally plate (50-60 Hz).

You *can* build an inverter this way, switching DC at 120 Hz and using a
transformer to step that up from 12 V to 120 V at 60 Hz. APC standby
supplies do this. But the transformer needed is large and heavy - fine
for stationary use in a box that also holds a big lead-acid battery, but
no good for a light portable inverter.

The previous poster is correct. Portable inverters switch the 12 VDC at
a high frequency, perhaps 10 kHz. There's still a transformer, but it's
much smaller and lighter (the amount of core iron needed to avoid
saturation is roughly inversely proportional to frequency). That is
rectified and filtered to provide about 140 VDC. Then there is an
H-bridge output switch that connects each side of the "AC" output to 0V
and to 140 V alternately. That's why these inverters have floating
neutral outputs - because the neutral is sometimes at ground, and
sometimes at 140 V relative to ground.

Now it is you that is providing the amusement. There would be no
advantage
to building an AC output inverter that way. Examples abound here in both
straight inverter and in UPS form (essentialy the same thing). All use an
output transformer, though granted the modified square wave output
requires
a smaller transformer than a true sine wave would.

I have seen one inverter that used a 60 Hz transformer. It was surplus
when I bought it a couple of decades ago, and appeared to have been
built in 1970 or so. *All* more modern portable inverters that I've
seen use high-frequency oscillator and a small transformer. The
difference is not small - one with a 60 Hz transformer weighs perhaps 5
times as much for the same output power.

I have never encountered
an inverter that employs a variable pulse width to adjust the RMS output.

The pulse width doesn't vary - it's designed to be a particular value
that depends on the peak to RMS voltage. The Xantrex/Statpower
inverters, for example, use a 140 V internal DC bus, and about a 63%
duty cycle. This gives 140 V peak (enough for switching supplies), 120
V RMS, and also almost cancels the third-harmonic component.


As I have said, it is quite possible to build them that way, but so far I
have never encountered one in a portable unit. For a few hundred watts the
output transformer won't be that large, and will be smaller than the
equivalent transformer required to handle a sine wave. If weight was the
primary consideration then it could indeed be a good way to go. How do they
regulate the output (assuming the output is regulated).

For an AC output inverter thus would be the 160vDC capacitor that doesn't
exist except in your imagination. I rather fancy, that you have assumed
that inverters are built the same way as switch mode power supplies.
Although DC output types may be, 50 and 60 Hz AC output types most
certainly
are not. Just opened one up that turned up that I aquired a coupe,k of
months ago.

What make and model?


Oh come on, it's made in China. As a £15 unit, its a no name unit.

I own a number of Xantrex and Statpower inverters, and have opened most
of them up at one time or another. They all have relatively small
high-frequency transformers and high-voltage electrolytic capacitors for
the high voltage DC bus. They're often laid out so you can pretty
easily identify the oscillator, the step-up transformer, the rectifier,
HV filtering, and the H-bridge output.


Fairy nuff. Maybe I should spend more than £15.


.

Relevant Pages

  • Re: Power Inverters
    ... Although switch mode supplies will work off higher frequencies ... You *can* build an inverter this way, switching DC at 120 Hz and using a ... transformer to step that up from 12 V to 120 V at 60 Hz. ...
    (comp.sys.laptops)
  • Re: Inverters & Laptops
    ... Today's power supplies, like the one running your desktop PC as you read ... It even has all kinds of crazy pulses ... that switch at the transmitter's radio frequency, ... cheapest Radio Shack 1KW inverter, ...
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  • Re: Inverters & Laptops
    ... Do you think it's OK for a square wave inverter to feed a UPS that feeds a ... Today's power supplies, like the one running your desktop PC as you read ... It even has all kinds of crazy pulses ... that switch at the transmitter's radio frequency, ...
    (rec.boats.electronics)
  • Re: Inverters & Laptops
    ... There's no excuse for using a square wave inverter these days. ... Today's power supplies, like the one running your desktop PC as you read ... that switch at the transmitter's radio frequency, ...
    (rec.boats.electronics)
  • Re: Is there a 2-way manual switch to go between inverter and shore power for the TV ?
    ... hesitate toggle switch. ... straight through off which will damage the inverter if motors are still turning. ... when shore power is available and switches to that. ... I had just such an automatic system set up in my rig. ...
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