Paul Buff
Registered: Oct 06, 2006
Total Posts: 2775
Country: United States
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kylegehmlich wrote:
Bacilonur's post about portable power in a pelican has rekindled my interest in building my own portable battery+inverter system, but I have a question regarding inverter ratings and strobe power.
Now, I'm not an electrical engineer, so please bear with me. I assume that using an inverter with X watts continuous would be sufficient for a strobe pack that is X watt-seconds (e.g. a 1000W inverter for a 1000Ws Dynalite pack).
Would using an inverter with a lower continuous wattage simply raise my recycle time or are there other factors to consider? The reason I ask is because I'd like to power a Dynalite M1000wi, but I'm not too concerned about slower recycle times and an inverter with a lower rating would be cheaper (and smaller).
And before anyone mentions it, yes I know the inverter has to be pure sine wave 
Thanks,
Kyle
It's not that simple. The WS of a pack has nothing to do with the current draw. First, there is apparent power and actual power - you don't need to understand this - just the implications. I'll try to be brief and provide some better insights.
A typical flash - monolite or pack has an efficiency in terms of apparent power of less than 50% and about 85% in real power efficiency. An inverter, of any type, only sees the apparent power, as do fuses. What this means is to charge the flash caps to 2000WS, it takes over 4000WS from the inverter. If it cycles in 2 seconds it takes 2000WS per second (2000W) from the inverter. That would seem to indicate a 2000W inverter would do.
But the charge current is not constant - it it much higher at the first part of the charge cycle and lower as the unit charges. This results in a much higher peak charging power than the 2000W indicated above. Here are the real world numbers:
Either of the packs mentioned, with a 120VAC input, draw about 50A at the beginning of the charge cycle. 50A at 120VAC is 6000 watts. Bottom line is, if the inverter or generator or power line is going to maintain it 120VAC output it must be capable of delivering 6000 watts peak current and 2000 watts average (RMS). Most inverters and generators can deliver about twice their rating as peak current. Based on this, it would suggest a 2000WS power pack that cycles in 2 seconds would require a 3000 watt inverter with a 6000 peak rating.
If you now use a 1000WS pack with a 1 second recycle time, the peak and average power don't change. You still need a 6000 Watt peak power rating. This is one huge and very expensive inverter and simply not practical for a Vagabond type system in terms of cost, weight and size.
This is where current limiting comes into play. By allowing an inverter with, say a 900W peak rating, when the light asks for 6000 watts, the inverter just lowers its output voltage so it can charge up the light at a longer recycle time.
But two caveats apply: 1. The light must be able to charge at less than 120VAC input during the initial part of the cycle and, 2. The inverter must be specifically designed to properly reduce its output voltage (current limit) according to the demands of a flash system. Some inverters can do this and some can't. Vagabond is specially designed to do this. Many off the shelf inverters don't current limit, and will simply shut down when their peak power capability is exceed. This includes both pure sine and non-pure sine inverters. (Non pure sine inverters are not recommended because they can damage flash units.
The specs for various off the shelf inverters rarely provide the necessary information regarding these parameters, and most flash systems do not specify how low the AC input voltage can fall before they crash or cease to charge. Most fully analog flash units tolerate low AC input voltages well and most digitally controlled flash units will crash if the input voltage falls below a certain critical voltage. Our next generation Einstein digital units are specifically designed to not crash under these conditions.
So the whole issue is very iffy for the customer to predict. This is why we make Vagabond and our lights consider all these potential issues so they work together. This is also why we can't say with accuracy whether or not a Vagabond will successfully power a particular competing light system
Can't write a book on this here, but these are the high points. Hope they helps. I will remind readers that, prior to Vagabond 1, there was no commercial available system to power AC flash units from a battery/inverter combination that I am aware of.
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