Non-regenerative electric braking

[Long John Rider]

Hi! I registered here since this seems as good a place to do EV research as any. I am not wise to the ways of the electrons, but I am trying out of necessity. Bear with me if you have the patience.

I am currently using an extensively modified, very old and very heavy two-wheeled cargobike as my primary mode of transport. About six months ago I went electric, got a brushless front wheel hub motor, gearless, 500w 36v. It came from a busted 28" wheel but is now mounted in a 20" rim. Both motor and ESC are run of the mill cheap Chinese stuff, so no regen capability there. The battery is 20Ah according to the seller, and I’ve never been even close to running out of charge the way I ride. I am very happy with the performance of the bike as it is but the upgrading is still not complete:
My mechanical brakes get hot to the point of potential brake fade under certain riding conditions. I would like to augment them with an electrical braking system. I am not particularly interested in charging the battery when braking, I would be perfectly satisfied to dissipate that energy of into the atmosphere with the least amount of fuss. I don’t need an additional emergency braking system, I want an undramatic way to gradually slow my speed for descents and planned stops that transports a meaningful fraction of the heat to somewhere other than the mechanical brakes. I also find the idea of braking without mechanical wear to be aesthetically pleasing.

The plan: Using a 40 amp three pole double throw relay to isolate the motor from the ESC and connect it to an array of electrical loads instead. The resistance could be made variable in two or more increments using additional relays, or something, should it prove useful.

Questions for those who know more than me:

-Is my plan at all feasible? If not, why not?

-Is it possible to get any meaningful amount of braking from a motor such as mine, at least at higher speed? How much braking force can I expect? (I want most of the kinetic energy to be converted to heat outside the motor rather than in it. I have been told that this type of "efficiency" affects how much braking can be done, and what type of resistors to choose.)

-what type of resistors; ohms and watts and so on? Delta or Y connected? My current plan is three wound resistors with heat sinks/cooling vanes, but I haven't quite given up on light bulbs, since that has the potential to look cool.

-fuses: assuming I go ahead with the plan and start experimenting with resistors, what kind of fuses would be prudent on the braking circuit to avoid burning up the motor should I mess up. Or am I being paranoid here? Would it be potentially bad to lose just one or two of the phases somehow?

From what I’ve read on these boards so far, I guess I could just get a fancier speed controller and be done with it, but this of mine plan seems more fun, more educational for me, cheaper and potentially good enough since I’m not really after the modest range increase regenerative braking might offer.

Any informed input would be greatly appreciated.

 

[Chalo]

Incandescent light bulbs in parallel would be by far the cheapest reliable way to get the kind of braking power you want, and you could tune the system by changing the number or power of the bulbs. The household kind won't light up; they'll discharge the energy as heat.

You'll need rough service bulbs to tolerate the vibration of cycling. Or you can use automotive bulbs, but you would need series strings of those, reducing system reliability and increasing cost.

 

[Long John Rider]

Yeah, automotive bulbs means I need several per phase. I guess I need to use them in series as well, since i hope I'll be making significantly more than 12 volts?
On the other hand, through the magic of Ali Express and a bit of patience I can get some nice looking resistors for cheap. These for example are less than 2 USD each, shipping included:

At that price I'll have to wait a few weeks to get them, but I'm planning on ordering the relay(s) from China anyways so no matter.

In my heart of hearts I kinda want an array of old-timey incandescent lights in brass fittings for that Tesla feel. It just doesn't seem practical. Resistors I could just tuck away out of sight under or even inside the cargo box, or something similar.

 

[bobc]

This is exactly what I'm planning for one of my projects. The control is not too tough, kV tells you the volts to expect V^2/R tells you the power you'll blow off. No electronics to go wrong. the downsides are 1) braking force is proportional to speed and 2) it's on or off.
Might be worth considering that plain water has more thermal capacity than most stuff & you get all that latent heat when it boils at a not inconvenient 100C - bolting the resistors to a small water container might be the lightest way to heatsink this.
(FWIW I once had to calculate the heat capacity of some lead acid batteries - lots of lead in there but its thermal mass was negligible compared to the water in the electrolyte)
I don't see the need for fuses - the motor will produce a predictable voltage and provided the resistors can cope with that voltage, what's the worry? The fuse is just another thing to stop your brakes working when you need them.........

 

[Long John Rider]

The control is not too tough, kV tells you the volts to expect V^2/R tells you the power you'll blow off. No electronics to go wrong. the downsides are 1) braking force is proportional to speed and 2) it's on or off.

I haven't thought about these things in about 20 years, and I was not the brightest kid in physics class back then either, but anyways...
I have no knowledge of the kV of my unit, so that will have to be a guess, unless I hook up a voltmeter and find out. I might just do that. All I really know right now is that I can power my front light even when the battery is out of the bike by spinning the front wheel, so some power generation is happening already.

As for the downsides; 1) is not really a problem I don't think. The faster I go the more braking I want/need, especially for hill descents with my kids in the box, which is really what I want this system for. 2) is not necessarily true I don't think: Let's assume that my motor generates 24 volts. I connect three resistors of 2 ohms each when the brake switch #1 is tripped. 24^2/2=288W - I assume this is divided over the three phases so three 100W resistors could handle it. Now I press the brake levers some more, trip brake switch #2, another relay throws which connects an additional three 2ohm resistors in parallel with the first three. IIRC this in effect means a 1 ohm resistor capable of handling 200W on each phase?

Am I missing something here?

 

[riba2233]

Those resistors cost much more than new controller with regen capability.

 

[Long John Rider]

Those resistors cost much more than new controller with regen capability.

I doubt it. If I order from China those resistors are about $2 each, 3 pole double throw relays are about $6. Even if I go all fancy with six resistors and two relays, that's just $24, half that for the simplest possible setup. If you can point me to a regen capable controller for that price including shipping I'd be much obliged. The only one I found on Ali Express was about 40 plus as much again for the LCD screen needed to run the regenerative function.

 

[riba2233]

This is just one example:

http://www.aliexpress.com/store/product/Greentime-36V-48V-500W-600W-30Amax-BLDC-Motor-Controller-Electric-Bike-Tricycle-Controller-Driver/313864_2012487273.html

Also, you will get few benefits, like reliability, more range, your battery will last longer and it will be much easier to use. And you'll save time on building and testing.

 

[silence]

Incandescent light bulbs in parallel would be by far the cheapest reliable way to get the kind of braking power you want, and you could tune the system by changing the number or power of the bulbs.

Would be cool to build a really functional brake light that way.

 

[Long John Rider]

This is just one example:

http://www.aliexpress.com/store/product/Greentime-36V-48V-500W-600W-30Amax-BLDC-Motor-Controller-Electric-Bike-Tricycle-Controller-Driver/313864_2012487273.html

Also, you will get few benefits, like reliability, more range, your battery will last longer and it will be much easier to use. And you'll save time on building and testing.

Yes, your suggestion is certainly the easy and reasonable option, if somewhat boring. It would be nice to have some use for those hall sensors too. With my current primitive ESC things can get a but loud and jerky at low speeds.

 

[cal3thousand]

Regen is quite easy to use. I know that you won't need the juice, but might was well use the regen capabilities, even if you couldn't care less about where the energy went.

 

[Long John Rider]

Regen is quite easy to use. I know that you won't need the juice, but might was well use the regen capabilities, even if you couldn't care less about where the energy went.

Sure. Might as well store some chemical energy rather than just making heat. Luckily all my rides with full batteries (I charge at home and at work) start at the bottom of hills, so overcharging won't be a problem either I guess.