random casual discussion on dynamic brakes
 

So I was bored out of my mind today and I was thinking of how to build an extremely simple regenerative braking system for racing application. Alternators happen to be very light and can actually generate a pretty large amount of power when the field coils are fully energized at higher rpms. Unfortunately the output voltage would be really high and a pain in the ass to convert and store. Doh should've thought of that.

But then I realized, if we can use an alternator to provide say, 100hp of braking force, that could significantly reduce the amount of work the brakes have to do... and then forget a resistor bank, can't we just send all the current through the metal chassis? The chassis has a ton of mass so the heat wouldn't be a problem at all. More specifically, 70kW for like 3 seconds (power drops as speed drops) is only 210kJ, which shouldn't raise the temperature of the whole body by more than a few C. IIRC alternators run at close to max speed can generate something like 120V or more, and since this is only intermittent use we can boost the field current, and replace rectifiers with higher inverse peak voltage ones. With a 200A alternator (and of course solid gears or a chain drive instead of belt and perhaps oil cooling) we could have something like a 40hp brake (power tapers off as the inverse square of speed though) at the cost of <25 pounds of weight (including wiring), which can remove 10% of the braking load in a 100mph to 40mph "stop" (rough rough calculations).

Additionally, in a race application, there is a lot of braking so something else to consider would be using an alternator that has a bit of excess capacity (for higher charging current), and cut it off completely when accelerating (say by a switch attached to the gas pedal) to reduce any sort of loss, and let it generate energy only when the throttle is completely released and the brake pedal is depressed. This alternator on the motor could be wired in series with the other alternator under said braking conditions to provide the extra field current and bleed off even more power.

I imagine it's cheaper to find junkyard alternators than to buy expensive big brakes, wheels, and the more expensive tires (altogether which weigh more anyways), and this doesn't wear down the pads. Oh by the way this extra alternator can attach to the driveshaft with 2.5:1 gearing or so. Sound economics?

You couldn't use the chassis to dissipate the energy. For one thing, it's not nearly resistive enough, you'd basically be shorting the alternator. And it would play holy hell with any electrical device using a chassis ground.

Where would be the alternator connected to? The driving shaft? The wheel? In order to provide stopping force? And why would you want to send 200A current to the chassis?

If the alternator is connected directly to the drive shaft, I wonder what kind of load is on that shaft: you have the engine driving the shaft and then alternator applying the opposite force to it to provide stopping power. How long will the drive shaft sustain this abuse before it breaks?

Darn, I thought the chassis would have enough resistance being some high strength steel...I guess that wouldn't work then.

But yea chassis ground would need a suitable alternative.

this thread got my attention for battery/alternator concerns. I check in jcwhitney site for battery replacement... this is where I get my alternators too... im curious, will the alternator adjust its load if the battery is already charged? I've heard that reducing the amount of work of alternator is a good thing.