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Published : 19th April 2004
Jim Sofranko commented that :
>I'm curious how the inverters keep the phase correct?
Here's an overly-simplified explanation :
What inverters do is simply to reverse (invert) the polarity
of a DC power source, X number of times per second -- i.e.,
120 times per second on this side of the Atlantic. This produces
an approximation of 60 Hz alternating current that's crudely
squarish in its waveform, but very precise in terms of frequency.
If the original DC source is low-voltage, the inverter section
is followed by a transformer that kicks the output voltage
up to 120 or whatever is needed.
The frequency of the inversion is established by an oscillating
circuit. If you connect two or more oscillating circuits together
in the right way, they'll naturally fall into unison.
So theoretically you could connect any number of inverters
(or inverter-equipped generators) together, and they'd all
be in phase. Many inverters designed for solar house-power
can be ganged this way, so the installation can be infinitely
Square-wave AC isn't a great idea, for various reasons. So
most modern inverters modulate the width of their "positive"
and "negative" DC pulses to create a better approximation
of a sine wave -- and also to regulate their output voltage.
"True-sinewave" inverters use higher-frequency pulses
that are modulated to approximate sinewave power very closely.
Very small, compact power inverters sometimes use a higher
inversion frequency (to allow the use of smaller transformers)
followed by a frequency-dividing scheme that brings the frequency
down to 50 or 60 Hz.
BTW, I keep a small 100-watt inverter in my car's glove box
at all times. It's great for charging camera batteries, and
for running electronic devices for which I don't have 12-volt
Marin County, CA
> Square-wave AC isn't a great
idea, for various reasons.
The big "no no" is to connect any piece of equipment
that servo locks or references to the line freq. Especially
video VTR’s. Cheap inverters put out a very abrupt square
wave that causes motors to overheat and burnout. It will also
cause comparator circuitry to burn out and don't even think
about dimmers. That's why inverters that output a sinewave
cost so much. They have better regulation, filtering and a
bigger transformer for upconverting to 120VAC. A good sinewave
inverter is going to cost, big time.
Could you get HMI type flicker from a incandescent bulb running
off a square-wave inverter?
New Orleans, La
>Could you get HMI type flicker
from a incandescent bulb running off a >square-wave inverter?
I doubt it. Filaments inherently have thermal inertia. Any
wattage over about 100w responds too slowly to be a problem.
But you might hear it 'sing'. Consider the decay in illumination
after switching off the power...
"Electronic" or "solid state" theatrical
and TV studio dimmers have been using modified waveforms for
many years without noticeable flicker. E.g. At 50% they are
switching the power from zero to maximum very abruptly, right
at the peak of the AC cycle.
Perth, Western Australia.
>The frequency of the inversion
is established by an oscillating circuit. If >you connect two
or more oscillating circuits together in the right way, >they'll
naturally fall into unison.
Great explanation on inverters!
But I still don't understand how the oscillating circuit will
naturally fall into unison with another circuit if you simply
connect two generators.
My impression of the original post, which I may have misunderstood,
was that all was needed was the correct, external, plug configuration
to achieve 240v using two generators. Is it necessary to wire
the oscillators together internally for them to achieve a
>But I still don't understand
how the oscillating circuit will naturally fall into >unison
with another circuit if you simply connect two generators.
Is it >necessary to wire the oscillators together internally
for them to achieve a >correct phasing?
Sort of a generator genlock. The power companies due this
so there must be a way. I think if you are using incandescent
fixtures it wouldn't matter. Now fluorescent or pulsed light
sources could be problematic. How are you using the inverters?
For lighting or other uses?
I'm curious, Can you power small HMI's off a square-wave inverter?
Start up current draw would be the killer.
New Orleans, La
>Sort of a generator genlock.
The power companies due this so there >must be a way. I think
if you are using incandescent fixtures it wouldn't >matter.
I have to disagree! The power companies have 'fancy' equipment
to do this.
Sure, once two alternators [I think we need to use the right
name here] are running and connected in parallel they will
stay locked, because if one tries to stray it will find its
current increases. But one has to get them in phase before
'slaving' or KABOOM! [and it really is 'spectacular'!]. The
device used to be called a synchroscope - a meter with a needle
that can rotate 360 degrees. The powerhouse operators would
make fine adjustments to the unlocked alternator's speed until
its synchroscope indicated it was within a few degrees before
putting the alternator on-line.
I agree that generators [DC devices] can be paralleled easily
but if they are driving independent inverters, the inverters
would need some sort of link cable to carry synchronising
information before they could be used together.
BTW, this is all related to why the world's TV systems started
out as either 50 or 60 hertz derived - as the receiver's spinning
disc was mounted on a synchronous motor connected to the power
mains, in phase with the motor on the 'camera' at the TV station.
Time for the other Clive to chime in?
Perth, Western Australia.
>I agree that generators [DC
devices] can be paralleled easily but if they >are driving
independent inverters, the inverters would need some sort
of >link cable to carry synchronising information before they
could be used >together.
The sync signal could be as simple as a pulse at the start
of the positive going inverter phase. One generator could
be set as a master that supplied the pulse, and the others
could be slaves that received the pulse and then synced to
Sorry I didn't explain this properly.....
Honda make a cable that connects their inverter type generators
together. A five core cable with the appropriate plugs connects
into and out of a box on which is mounted a power outlet.
"Some unscrupulous people will probably copy this inter-connecting
cable, as it is very simple but relatively expensive."
If you parallel two 2kva inverter generators together...two
wires of the five core cable plug into one generator's electronics
package, via sockets on the generator, and the other end of
the same two wires plug to the other generator's electronics
package. This then allows the inverters to run in unison.
The A.C. outputs of both generators are also paralled using
the remaining three wires in the inter-connecting cable, active,
neutral and earth.
The A.C. output is then drawn from the power outlet mounted
on the box in the middle of the inter-connecting cable. This
outlet (in this case) is rated at 4kw. It is possible to gang
three or more of these units simply by plugging the electronics
together. This is only possible because the output A.C. is
derived by electronic inverter.
Phasing A.C. generators(you don't want to even think about
getting them 180 degrees out of phase) to synchronize their
outputs has nothing to do with this process.
A further note....In Oz.(Australia) These small generators
which are light and easy to rig, reliable, stable and quiet...
have virtually taken over from sine wave inverters to run
small HMI's for car travelling shots.
Jim Sofranko writes :
>I still don't understand how
the oscillating circuit will naturally fall into >unison with
another circuit if you simply connect two generators.
You don't connect the generators. You connect the two inverters
that are being fed by those two generators.
>My impression of the original
post, which I may have misunderstood, >was that all was needed
was the correct, external, plug configuration to >achieve 240v
using two generators.
If you can synchronize the inverters you can either double
the voltage (by wiring their outputs in series) or their current
capacity (by wiring their outputs in parallel).
>it necessary to wire the oscillators
together internally for them to achieve >a correct phasing?
Not all inverters are designed to be synched in this way.
Those that are so designed are provided with appropriate wiring
terminals for this purpose. In some cases these connections
may mutually synchronize the multiple oscillators. With other
systems it may be that one oscillator is serving as a "master"
sync source, bypassing the oscillators in the "slave"
Which ones do it which way is beyond my meagre knowledge!
Marin County, CA
Tom McDonnell writes :
>A good sinewave inverter is going to cost, big time.
Sinewave inverters have been dropping in price for the past
few years because they've become so popular, there's more
competition, and the technology is no longer considered particularly
exotic. They're a bit less efficient than squarewave and modified-squarewave
types, but I think that gap is also closing.
If you're looking for a good heavy-duty sinewave inverter
for field use (say, Xantrex or Exeltech), check out boating-supply
houses -- their prices may be lower than elsewhere. And please
heed the installation instructions, particularly with regard
to overcurrent protection (fuses or breakers) and safety shut-offs
in the DC supply.
Most of all, treat the inputs and outputs with respect.
The input source (i.e., a 12-volt car battery) won't give
you a shock, but can deliver huge amounts of current, which,
if short-circuited, means huge amounts of heat and a possible
battery meltdown or explosion that could ruin your whole day
by spewing hot sulphuric acid all over your nice, clean jeans.
The output is the same as what comes out of your wall outlet
(power point) at home.
Dan "safety first" Drasin
Marin County, CA