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I want to add regenerative braking to my bike, not for energy recovery, but as a braking supplement/assist on long downhills. Is anyone doing this and how effective is it?

A 3 phase hub motor IS a generator when freewheeling. All that's needed to create braking resistance is a load that can be switched in on demand. In fact, a short circuit might be effective if heat buildup in the windings can be managed. As an electrical engineer, I understand this much.

My question is, has anyone done it and how effective is it as an assist, not the primary brake?

Joe

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    How will you dissipate the energy? Nov 27 '20 at 2:10
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    Electric brakes alone are generally not legal - laws take a long time to catch up with technology. You still need two separate friction-based brakes. Separately - is this an electric bike ? Or are you thinking of using a motor just as a brake ?
    – Criggie
    Nov 27 '20 at 7:24
  • Not regenerative braking exactly, but an AIRhub (airhub.com.au) could be used to provide extra resistance downhill
    – Andy P
    Nov 27 '20 at 8:55
  • Rather than "dissipate" the energy, break it down - how will you store the energy, and what will you do with it afterwards? How much braking power do you want? I'm thinking about the implications of overloading a dynamo into a power resistor strapped to a heatsink on the fork, as dynamos are approximately current sources. But as P=VI and I=0.5A, significant drag would get to excessive voltages too easily and I'm not sure the dynamo could take it. A drogue parachute would be an interesting project, but you'd have to stow it again after the descent
    – Chris H
    Nov 27 '20 at 9:46
  • What the OP is talking about is a dynamic electric brake, not a regenerative one. Nov 27 '20 at 12:45
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Dynamic brakes are used on electric trains (sometime true regenerative brakes are not used because dynamic brakes are far simpler). I think they may also be used on trucks to regulate speed on long downhill roads.

You’d need a generator built into a wheel, like a dynamo hub, but somewhat larger to provide greater braking force. Power is dissipated in large resistors with cooling fins much like the cooler on an PC CPU. Depending on how sophisticated you want to be you could include a controller circuit to vary the braking force.

The issue is that all this adds significant mass to the bicycle, and the generator will create a non-zero resistance even when off (unless you provide a clutch to disengage it). Given that by definition you need to ride this bike up a hill in order to descend it, is the trade off worth it?

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    With the latest information, I reckon the OP might have an e-bike. For very laden e-bike rides with long descents, e.g. touring in mountainous terrain, I can see the use
    – Chris H
    Nov 29 '20 at 8:38
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If you've already got a hub motor, and you've got electronics skills, a power resistor (rated to perhaps 100W) per phase on a heatsink (perhaps under the downtube) should do. You want the heat dissipated in moving air, rather in the windings, which will determine the resistance you can use. The difficult bit is likely to be getting motor specs in enough detail to avoid too much trial and error.

To switch it in, at least initially, I'd use relays (or one relay if you can find one with enough contacts. That would make it easy to disconnect the existing drive circuit at the same time, using changeover contacts. This would act as a drag brake with a fixed relationship to speed, so I'd start it at the top before building up much speed, otherwise the turn-on would be rather sudden.

I'd probably also add a little digital thermometer to the resistors - you can get cheap battery ones designed for panel mounting, with the sensor already fitted. This could be removed after testing, but I'd want to know if I was likely to melt the solder and lose contact, easily done with big load resistors unless they've got excellent heatsinking.

With well-chosen resistors, it should be possible to make this an effective drag brake to keep your speed down, but almost useless for stopping you.

There are also active load braking circuits, that dump the heat into MOSFET motor drivers (again with big heatsinks). I haven't used them myself in this application. They'd need more of a control circuit, but could be used to slow you down gently once you were going fast. However packaged versions may not be designed to provide long-term braking. Looking at the datasheets on these may give you a hint as to resistances to use.

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  • Seems to me you don't need a lead resistor. A PWM controlled short circuit could control average load current and therefore braking strength. Of course, the PWM MOSFETS have to be sized to withstand the peak currents.
    – joedarock
    Nov 29 '20 at 15:15
  • @joedarock MOSFET braking circuits don't (always) need load resistors, but dump a lot of heat in the MOSFETs themselves, meaning big heatsinks as well as big MOSFETs sized for peak current and average braking power, which could be a couple of hundred Watts. The resistors were for a simple on/off setup instead.
    – Chris H
    Nov 29 '20 at 15:24
  • Since the resistance of the windings is likely to be higher than the on-resistance of the fets, most of the heat will be generated there (assuming a short-circuit load). Still, all this is academic and we could,go on forever about it. Has anyone actually done it and what was the outcome?
    – joedarock
    Nov 30 '20 at 16:37
  • My reading so far disagrees with that assumption - the circuits I've seen use the MOSFET(s) in linear mode, not fully on, i.e. they're acting as controlled resistors. But we've got too far from my original simple suggestion for me to have much more to offer. Your latest edit says you're an EE - you're probably more qualified than most of us here to address this question
    – Chris H
    Nov 30 '20 at 16:43

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