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Old 03-19-2011, 05:57 PM   #4496 (permalink)
isaac_alaska
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Quote:
Originally Posted by adamj12b View Post
Isaac,

Running 400V and reducing to 170 would be pointless unless you are using batteries that sag by 60%. You would most likely not need to go over 250V to always get 170 at the motor and nothing lower.

As for going higher then 250V, you will need to redo the power board at the minimum to increase the isolation gaps or else you will have problems with the high voltage humping the gaps.

If you want high voltage, you will be better off going with IGBT modules.

Are you riding the sled long range or doing something like grass drags?

-Adam
i was planning igbt modules, as i like having a few large components onboard instead of 30 to 50 small transistors. we are riding the sled both long range, and doing drags with it. we're competing in the sae clean snowmobile challenge, and one of the events is longest range (therefore efficiency is important.)

is there any reason you can think of that lower voltage would be more efficient? (maybe igbt's are more efficient at lower voltages?) but from evereything i've done, higher voltage is always more efficient. i haven't gotten into power electronics in very much depth in my learning career, but i'm almost done with my bachelors in electrical engineering (studied power and controls, but not a lot of power electronics)

running more cells in series would also mean having fewer cells tied together in parallel. i'm thinking this would be a good thing, as a cell that develops an internal short is going to bring down all of the other cells in parallel with it. the other option is to simply parallel the strings and leave the individual cells alone, but this leads to bms nightmares.

the sled also competes in a maximum pulling power event, where it's attached to a rolling contraption and the brakes on the contraption are applied until the machine won't pull it anymore. another event is acceleration under load, where you run 500 feet as fast as possible, pulling a 500 lb load.

The batteries we're using have very low internal resistance and a very high discharge rate, so voltage sag isn't an issue, higher voltage would simply be for higher efficiency and the ability to run fewer cells in parallel with each other.

what are your thoughts?
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