capacator sizing

[dozernerd]

I've been building rc bulldozers for 30 years.decent at fabricating,adequate at wiring,excellent at determination.current build uses a gas engine spinning a 48v,200a DC brushed motor to power my DC drive motors.ive built similar ones in the past and have always used batteries to smooth the DC output to protect the high dollar,purple smoke filled MOSFET speed controllers.id like to use capacitors instead now because the current I'm generating destroys batteries after a while.i have no idea what size caps to get.thanks,ed

[Psi]

hm..  Are your DC drive motors also brushed, or are they brushless?

48V @ 200A is a lot so i'm assuming this is a pretty large RC bulldozer and does actual work.

My view is that PWM based electronic speed controllers are more likely do die from voltage spikes above their max mosfet voltage than from unstable input voltage that fluctuates a bit due to ripple from a generator on the input.

Some amount of capacitance is definably needed but your generator is DC not AC so it's less critical.
I would be more concerned about having a decent amount of voltage spike protection, probably TVS diodes, to ensure no unsafe voltages gets to those ESCs.

For capacitors you could use some super-caps in series to keep the size footprint small while getting lots of capacitance.
But if this is a largish RC vehicle then size might not be much of an issue. Normal caps tend to last longer than supercaps so normal caps are probably safer/better/cheaper.
(One caveat: If you go down the supercap route, make sure they are normal supercaps not hybrid ones which are part lithium battery and part capacitor. The hybrid ones have a minimum voltage before you damage the cap like a battery)

I don't really know much about 200A DC motors so I cant really make any firm suggestions as to the amount of capacitance needed. If it was me I would probably start with like 40,000uF of high temp electrolytic caps then scope it and see how it looks under all operating conditions while removing some of them.
200A/0.04F = 5000V drop/sec. So ~10ms from full cap to empty cap under 48V 200A load.
But as I said, I'm pulling numbers of of my butt that makes sense to me as a starting value, I've never used a 200A generator. You could also start with 10,000uF and buy more if needed I'm just not sure how safe that is for your other electronics, hence my idea to start bigger and reduce.

But I'd probably use a couple of 5kW TVS diodes on each ESC to protect them. The working voltage and clamping voltage of these TVS diode needs to be decided based on nominal voltage on the motors and the max input of the mosfets inside the ESC (which will be higher than the max ESC input voltage).

[Simmed]

sounds like supercaps 6x ?? ??F in series for 12v handling ?
then calculate for peak current handling etc

[jheissjr]

Do you have any videos or pictures of the bulldozers?

[Psi]

It does depend what exactly he's trying to buffer with the caps.
Just normal rotation of the DC generator as the brushes move between segments, or if the gas drive motor is hunting a lot and has unstable RPM.

My experience with the old black 2600F MAXWELL boost supercaps is that in high use automotive applications they tend to fail after 6 years.
I had some in my car for a while instead of a car battery. Failure mode is leaking a gel substance out of the cell terminals.
Modern supercaps maybe better.

Also as soon as you add supercaps in series you have to deal with them becoming unbalanced over time and causing some caps to be overcharged.  It's not a huge problem as it takes ages, but can be long term.
Maybe OP can find some supercaps that are full voltage from a single cell, which solves that issue. Ive not looked into supercaps in a while.
There are a few ways to deal with unbalancing that but it can be annoying.

[Psi]

sounds like supercaps 6x ?? ??F in series for 12v handling ?

I think he needs 48V

[dozernerd]

whoa thanks guys,I've been doing some research but lost this website for a bit...anyway,the generator rpm is steady,the DC generator head,and drive motors,are brushed.at max engine (3600)rpm the gen is around 4860 hz.i talked to the guy that built it,he said it has 81 commutator bars,its a beast.he thought the ripple would be 2 or 3 volts.my esc  is rated for 60v,and using the 50 year old Wisconsin engine that's currently installed,at 3500 rpm no load,I'm getting 42v.so over voltage shouldn't be a problem but I'd still like to hear a little more about this tvs diode.theres videos on YouTube if your bored at  rcd11n  thanks for the help,I'm upgrading to a bigger,newer engine and really want to get rid of the batteries!ed