Regenerative Braking Circuit for Brushed Motor

[The7]

fechter wrote:
"After killing a few more brain cells, I figured out a way to make it work without a dc-dc converter and still work at low speeds.
For the brushed version, I don't see any way around the dc-dc'



Assume the power FET is between B- and M- in the brushed controller similar to Fechter's version for brushless motor. And there are internal diodes in the power FETs. The other assumptions and operation of the regenerative braking of the added controller are similar to Fectter's.

The regenerative braking control circuit for a brushed motor is as shown.

Excuse me for using free-hand sketch.

[fechter]

Ahh.. yes, grasshopper, I see you have mastered the concept.

There still needs to be some provision to prevent both controllers from being on at the same time (instant destruction), but that shouldn't be too hard. Something rigged to the brake switch would be one idea.

I have a nicer version of the drawing somewhere:

[The7]

Fechter. Thanks for the compliment.

This is an outstanding site for ebike in many areas.

[opqa]

Sorry for reviving this very old topic, but I'm interested in this schema.

I wanted to ask if you guys finally tried some of this circuits. And especially, about this The7 variation... In the original fechter version a dc-dc power supply with isolated output was needed to power the regen controller, theoretically this variation eliminates the need for such power supply, well, I'm not electronic engineer, but I don't see where the power circuit for the second controller is closing, so don't think this variation would really work. What do you think?

[fechter]

I don't think anybody actually ever tried it.

In the second variation, the power will feed to the regen controller through the body diodes in the main FETs. During regen, the power to the regen controller will be 'chopped', but the main capacitors will maintain voltage on the control circuit during the off part of the cycle. I think it should work.

Brushed controllers are pretty cheap these days. It may be worth a try.

You'd also probaby want to limit the throttle on the regen so it can't quite get up to 100%.

[amberwolf]

Alternately, you could go with something like the old 2QD from http://4QD.co.uk which already has regen capability.

It is a "demand-speed" controller, so it will automatically perform regen braking if you lower the throttle input below the speed you are actually at, to bring you down to the speed you have now set. (it was designed for things like golf carts, but it works on ebikes too--I used one on CrazyBike2 with a powerchair motor, though I had a freewheel in the drivetrain so couldn't use the regen).

There isn't any separate speed control for the regen on it, though, but the throttle controls the regen amount by simply releasing the throttle for full braking, or slowly reducing it to more gently brake.

(Some of the brushless controllers do this, too, like the EVAssemble 36V I am testing here:
viewtopic.php?f=4&t=21830

[opqa]

Thank you for you answers, I think I see it now, the power current of the regen controller is going to flow to B- in the part of the cycle when the mosfet of the regen controller is off and the motor is feeding the battery, the sense of the controller power current is the contrary of the sense of the motor current feeding the battery, so they will substract, but as long as the second is much higher, the sense of the total current is not changed and it can flow trough the body diode. Isn't it?

Amberwolf, thank you for your advise about 2QD controller, I will consider it, but I'm not sure if this kind of regen will be the best for this bicycle, it is for my girlfriend, and I observed that she was using the accelerator in "bursts" insteed of maintaining a steady position (altough I tried to tell her to do it the other way). Also we sometimes go on mountain paths, and in this kind of terrain it would be impossible to maintain the accelerator in a fixed position, so I guess you will be continously braking and accelerating, and would be even more inefficient than not having regen at all.

Do you know any other site where I can buy a brushed controller with "normal" regen? I've been searching for it but I have found them difficult to find.

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[fechter]

The only ones with nice regen that I've seen are very expensive/large.

My old MCIPC24 controller had that demand speed kind of regen and it was not very driveable. Coasting was not an option and you had to hold the throttle very steady to avoid over braking. I wound up building a regen current limiter for it that kept the regen current below a certain point that kept the drive train happy. This actually worked out quite well other than it still did not have a coasting mode, but that would have been possible if I tied in the brake switch.

[opqa]

Today I've tested this circuit with my motor, but unfortunately it hasn't worked for me. The only difference between the diagram and my setup is the diode, I haven't used it because I didn't have any one handy and also because I really don't see the need for it, Is it that necessary?

When I connect everything but the power to the regen controller the main controller works as expected, but when I power the regen controller some unexpected interaction happens between both controllers; when throttling the main controller even just a little bit, the wheel accelerates to top speed, being impossible to effectively control the throttle. I can't guess what's going on.

[fechter]

The diode is necessary. It's part of a 'bootstrap' circuit that gives the regen controller enough voltage to operate. I'm not sure why it would make the main controller runaway. The diode doesn't need to be huge, but has to handle the full pack voltage x2.

[opqa]

Thank you Fetcher, I realize that my electronics skills really suck, I've tried with a diode that I've taken from a broken voltmeter (1N4007, that should handle up to 100V IIRC), and now it works

I'm still not sure whether I will use it or not in real world, at this moment I've tried it just with the wheel spinning on air and two independent throttles, one for each controller. The greatest inconvenience I've found so far with this set-up is that the regen braking only takes place when I operate on the regen throttle immediately just after having released the main throttle, otherwise the regen controller voltage quickly drops below LVC and then it's not possible to regen any more until I accelerate again. Is this an expected behaviour?

This issue could be partially resolved by using a commutator as a brake switch, connected to both controllers at the same time but with opposite configuration, so that when the main controller is braked the regen one isn't, and the other way round. But there's still the problem that in real practise, if for some reason you release the brake you will not be able to regen again until having throttled (unless your speed is enough to provide a voltage higher than the LVC).

Aside from this, there's also the issue about how to limit the regen current, the simplest solution would be to attach a potentiometer in place of the regen throttle and give it a fixed setting. The coolest one would be to attach the same throttle in parallel to both controllers (with the avoiding both to operate at the same time), and introduce a limit for the regen controller, I can think two way of doing that:

a) Try to make a pull-down circuit that limits the maximum throttle signal that the regen controller will receive. (I would need help with this one)
b) To shave the shunt of the regen controller until the maximum current is acceptable.

Thanks again for your appreciations.

[fechter]

The regen controller should keep going to a very low speed without hitting LVC. Maybe you need to use a faster diode, as this pumps power to the regen controller during the PWM cycle, which is pretty fast. Diodes listed as 'high speed' or 'ultra fast' should be better. You may also find it works much better when actually riding as there will be a lot more current in the windings compared to having the wheel off the ground.

Another approach would be to use an isolated dc-dc converter to power the regen controller. This would keep the regen controller going no matter what. Most controllers will take around 100mA, so the converter doesn't need to be very big.

Lowering the current limit in the regen controller would probably be a good idea.

[opqa]

No, it works exactly the way you say, the regen controller is powered during the PWM cycle no matter the speed, the "problem" is that when you completely release both throttles then the PWM cycle completely stops in both controllers, is in this situation when the power to the regen controller is cutted and it stops working almost immediately until you throttle again a little bit.

What I say is this can be a little bit annoying while coasting down but I guess it's normal behaviour of this set-up. Right?

Another doubt that comes to my mind... Would it be possible to wire the same throttle to both controllers at the same time. I guess the answer is no because the ground of the regen controller is not the same as the ground for the main controller, isn't it?

[fechter]

Ah, I see now. I guess it might be helpful to lower the LVC as far as it can go (12V?).
So the dc-dc converter might be the only way to make sure the regen controller stays on. There are some cheap ones around.
Right about the throttle grounds. You could possilby use the same magnets and separate hall sensors in a single throttle.