From: John De Armond
Newsgroups: rec.outdoors.rv-travel
Subject: Inverter Alert
Date: Tue, 12 Dec 2000 04:55:43 -0500

I learned something very interesting this weekend.  I grabbed one of
my inexpensive Universal Chinese Inverters, in this case, a Vector
HP140 150 watt unit and ripped it apart.  I was investigating
converting the high voltage inverter part of the unit into an
induction heater.  I was poking around on the printed circuit board
with my Flukescope digital oscilloscope.  I was on the stepped
square wave output switching MOS-FETs when my eye caught the
frequency display - 50 hz!  Punched the scope over to the meter
function to get a better fix on the frequency.  49.9 hz!  Just to
check, I got my Fluke 88 DVM that has a high resolution frequency
counter onboard.  49.99 hz.  How 'bout that.  This damn thing is
running at 50 hz.  I traced out the oscillator circuit and computed
the frequency given the component values to make sure the thing
wasn't malfunctioning.  50 hz it was, intentionally.

If this is common, if many of these inverters are running at 50 hz
instead of 60, that could explain a lot of the problems people have
reported using cheap inverters.  Motors will run slower, as will
clocks.  Transformers will run hotter.  And any equipment that uses
the line as a frequency reference (digital clocks and some cheap
TVs, phonographs, etc) will definitely malfunction.

Since most of the rest of the world is on 50 hz, I'd not be at all
surprised to find that some of these import companies are peddling
50hz equipment on the American market, hoping that nobody notices.
It might be interesting for anyone else who has the capability to
measure frequency to check their cheap inverter and see what
frequency it is running at.

John

From: John De Armond
Newsgroups: rec.outdoors.rv-travel
Subject: Re: Inverter Alert
Date: Tue, 12 Dec 2000 19:05:38 -0500

Donald Sampson wrote:

> Your finding that your Vector inverter is running at 50 Hz is disturbing.  I
> just bought a 1000 watt Vector but haven't hooked it up yet so don't know if
> it runs correctly or not.
>
> Any ideas on how to check it out without a scope?  Maybe hook up a clock for
> 24 hours and see if it runs 20% slow or something?

That'll work. If you have something with a synchronous motor in it
(line operated analog clock, old style turntable, 120 volt muffin
fans, etc), you can operate the fan on the inverter while
illuminating it with a neon lamp operated on shore power.  If the
frequencies are close, the motion of the motor will be strobed
almost to a halt (not quite unless both are phase locked AND the
motor has absolutely no slip).  If it's 50 hz, there will be many
mixed up strobed images.  Works best in the dark.  An electrical
tester or neon night light is a good source for neon light.

John