Home of Professional Cinematography since 1996
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 scalable.
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 power adaptors.
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 correct phasing?
>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 it.
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" units.
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