I have an alternator that charges at ~58 V at 100 A. If its load (a lithium bank) cuts out suddenly (bms cutoff), I need a way to shunt power for 200 ms to a dummy load to prevent the current from falling to 0 A and voltage spiking to 230~280 V (after about 100 V it begins burning out the alternator diodes and destroying everything else connected in parallel, so it doesn't rise to infinity).
Ideally the voltage would be kept from going beyond 75 VDC. 200 ms is how long it takes the alternator field to dissipate after the regulator quickly cuts power to field, so that's how long I need to shunt energy away from the main circuit.
For 12 V and 24 V alternators, this is an $89 device made by a company called Sterling Power, but nothing exists for 48 V. I've been told the physics of 48 V make it much more difficult.
A manufacturer of a 48 V alternator regulator gave me the following data for a 200 V alternator "load dump" scenario. My alternator is 100 A max, so I only need that much protection:
"You will want to see about suppressing the voltage to below 100 V or so max, else will be out of the range of viable to most devices to survive. The detail here is a multi-joule 200 ms or so event, not just a HV/LC event – most devices already have protection for that in them. Suppressing a 200 A 48 V alternator for 200 ms at 100 V comes out to 3,000 joules, if I have done my math correctly. That is the goal here. 100 V suppressing, with a 3 kJ rating."
He cites 100 V but, just to be clear, that's not where the spike begins. It begins at around 58 V x 200 A in his example, and voltage then rises as current falls, so by the time it reaches 100 V, current will only be 116 A. He likely calculated 60 x 200 x 0.2 = 2400 and then rounded it up to 3000 joules for headroom.
For my situation, it begins at 58 V x 100 A and lasts 0.2 s, so 58 x 100 x 0.2 = 1160 joules. And maybe we round up to 1500 joules to give some headroom. That's how much energy I need to shunt to a dummy load once the voltage crosses 75 VDC.
I found many voltage suppression devices for 48 V (Transtector was great about looking into their diverse 48 V suppression offerings) but their devices are for microseconds events, not 200 ms. So it's really getting into the area of sustained, albeit brief overvoltage, and not so much a transient.
Any ideas? I thought of maybe a 72 VDC converter with a 65 VDC minimum input (90 Vmax) but wasn't sure if it would be fast enough to begin shunting current to a dummy load attached to its output.
There must be a way to build a large inductor or an RC network that could absorb that kind of energy after the voltage crosses a set threshold.