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We currently use a three phase generator. The motor is controlled by a SEW VFD and the generator shows a current usage of between 2 and 7 amps in use. The motor can spend up to 50% of the time doing very little. Batteries lithium ion 8kw EV bank. If so what size inverters and battery bank for say 3 hour usage.Not sure how inverters react to VFD's. Thank you , John
First, you want to watch this from start-end: https://www.youtube.com/watch?v=4SXtGIx0x5w
You would need a 3P config of 3 x 8KVA inverters, but you might get away with 3 x 5KVA given your 50% idle characteristics and provided motor startup happens quickly, with no excessive load attached to the motor, not too hot ambient temp etc. etc.
Battery capacity for 3h runtime of that motor at 50% load ... 12KWh net + margin obviously. Minimum 16KWh would be my educated guess. The 50% load characteristics is not the important parameter here. Important is how the load changes of the motor are. Like immediate 5% -> 100%, then drop 100% -> 5% (rectangle) or are the changes more "sawtooth"-like?
I would like to add that if you were to keep your current generator, the required battery capacity could be much lower, actually 8KW might be enough. The battery/Victron inverters would simply serve as buffer/cache handling load changes/spikes and the genset could run at some optimum RPM constantly back-feeding the battery.
If you watched the video I linked, you will see that running the motor isn't the problem. Starting it is, where you are kind of lucky to have a 3p motor which needs "only" like 3 times its nominal power at startup.
Hi @John Birt
If you look at what your actual continuous power need is (must be able to read that from the VFD), you can get a feeling of the size of battery you need, and the size of the inverters.
I've been running a 30kW motor with a VFD, powered by 3x 8kVA quattro's, at about 20 kW max, I'd say when you don't need maximum power from your motor, 3x 3kVA inverters should do the trick.
