# Regenerative Braking(Trying to Understand)



## Aaron (Jan 21, 2009)

Hi all.
I've looked at some information about regenerative braking, but there are some things I don't quite understand.

Does it come into effect just by taking your foot off of the accelerator while in gear of only when you push the brakes?

If it's only when you apply the brakes, how do you get the motor to slow the wheels?

If it's just by letting off the accelerator, would that mean you couldn't coast?

Maybe I'm complicating things too much in my mind, oh well, I'm sure someone will set me straight.


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## major (Apr 4, 2008)

Aaron said:


> I've looked at some information about regenerative braking, but there are some things I don't quite understand.
> 
> Does it come into effect just by taking your foot off of the accelerator while in gear of only when you push the brakes?


Hi Aaron,

Regenerative braking (regen) can come in either variety. It depends on how the control strategy is implemented.



> If it's only when you apply the brakes, how do you get the motor to slow the wheels?


Usually there is a sensor on the brake pedal. Then controller excites the motor differently turning it into a generator, thereby producing negative torque (which slows the vehicle) and delivers current into the battery (charges).



> If it's just by letting off the accelerator, would that mean you couldn't coast?


Here again, it depends on the control logic implemented for your particular system. Sometimes, there can be a throttle position which allows a pure coast. Sometimes not.

Regards,

major


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## bobgratonii (Jan 10, 2009)

It depends of your configuration and how you set it.

-A DC motor isn't ''made'' for generating electricity, it is designed to consume it (well, for most of DC motors, some can). So this manner you would have to couple a generator (i.a. alternator) to the tail shaft of the motor. It will have to be used wiwh a clutch-like system so it will engagne only when demanded. This demand could be done by a switch on dashboard, on break pedal or when releasing the power pedal.

-A AC motor is capable of both way (consuming and generating). So when releasing the pedal, the controler of your motor will allow an electric flow to get back into your battery pack, and helping in breaking that way. If the controler doesn't allow the current to get back, it will simply stop generating electricity and let you go by your actual swing. This option is set to the controler via a serial cable by a computer. You can also have a switch to allow or not this fonction (depending of the controler model)

Anyways, for coasting, you have to push a lil the pedal to allow the controler to put some juice. Regen cost a lot to implement and is efficient only when stop N going. It depends of your needs!


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## Aaron (Jan 21, 2009)

Ok thanks.
I like the idea of only activating regen when the brake peddle is pressed.
Does anyone have a blog about how they set up their braking in this way?


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## Qer (May 7, 2008)

bobgratonii said:


> -A DC motor isn't ''made'' for generating electricity, it is designed to consume it (well, for most of DC motors, some can).


Actually, as far as I know every DC-motor can regen if you want to. The general misunderstanding is that series wound DC-motors can't regen, but they actually can! The problems are:


It's tricky as hell (and you need a controller that can handle it).
If the motor is advanced regen will eat brushes and most likely blow up if you start to regen hard.
But if you have a controller that can manage to handle the regen and you have a motor with the brushes in neutral a series wound motor can regen. Of course, that means that you have to be easy on the Voltages though...

Then you have the Peukert effect (if you run lead), losses in general and the fact that most (all?) batteries can't handle even close as much power when they charge as when they discharge. It all sums up to if it'll be worth it and I'd say that if you live in a relatively flat area and don't do a lot of stop and go, don't bother with it.

I've been considering regen as well but I've decided to give such extravagance up and instead go for LiFePO4 since I have to commute relatively far and at some speed plus that where I live the nature is FLAT. Sure, LiFePO4 are expensive but the advantages are:


No horrible Peukert effect
The weight of the car will probably be rather close to the original weight which makes it much easier to get street legal in Sweden
No problem to get the desired range (I'm aiming 100 miles but will be very happy with 50)
I can use an advanced series wound motor (cheap and powerful)
No need (or at least less need) to heat the batteries in the winter
Cheaper in the long run (this is, however, debated but I think that they are)
Just my 2 cents.


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## major (Apr 4, 2008)

Aaron said:


> Ok thanks.
> I like the idea of only activating regen when the brake peddle is pressed.
> Does anyone have a blog about how they set up their braking in this way?


Check out 

http://www.metricmind.com/ac_honda/main2.htm

Click on "Throttle and Regen Controls".

I usually put the pot on the brake pedal. The first 1/2 to 1 inch of pedal depression activates the pot and then the hydraulics start to work as you depress the pedal further. Most brake pedals have enough free play to do this.

I work with AC drive systems and have them programmed to use a 0-10 volt input for torque command. They have 2 inputs, one for positive torque (motoring) and one for negative torque (generating). The negative input has priority. Using torque control, you get a natural blend of regen torque and friction brake torque if you need to stop faster. I also have used an adjustment pot on the dash so that the driver can fine tune the amount of regen, or turn it off if it proves uncomfortable for the driving conditions. The result in completely intuitive to the driver, and does allow for a pure coast when neither pedal is depressed.

Regards,

major


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## Aaron (Jan 21, 2009)

I'd like regen, but I probably won't be able to afford it.
It sounds like I'll be getting that job I was looking at, I go in today to discuss it. So maybe I'll actually be able to afford converting an EV.

I'm hoping I don't run into the age old problem of having enough time but not enough money turning into enough money but not enough time. We shall see though.


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## Aaron (Jan 21, 2009)

Hey thanks for the info major. That sounds exactly like the setup I'd like.
I'll be sure to try something similar on down the road whenever I'm more financially stable.


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## tj4fa (May 25, 2008)

Couldn't you just use the existing 12V brake light power when the brake pedal/switch is depressed to operate an electro-magnetic clutch (like from an AC compressor) connected to a small alternator/PM motor?

If the PM motor/alternator is wound to generate High voltage at low rpm(higher than battery pack voltage) couldn't it give back some power to the battery pack while putting some drag on the main drive motor?

The way I'm thinking, is that this setup could be coupled directly on the main drive motor tailshaft and when the 12V brake light voltage is not present to activate the clutch/motor combo, the only loss would be from a small amount of bearing loss of the clutch/alternator combo while freewheeling.


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## Aaron (Jan 21, 2009)

Here's another question about regen.
Does it act like a regular braking system where how far you depress the pedal determines your rate of deceleration?
Or is it just like an on/off switch where deceleration is the same until the brake pads come into play?


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## major (Apr 4, 2008)

Aaron said:


> Here's another question about regen.
> Does it act like a regular braking system where how far you depress the pedal determines your rate of deceleration?


Hi Aaron,

That is how I do it. It gives the feel of power brakes. The further you push down the pedal, the stronger the regen torque. Can be done with a pot on the pedal linkage or a pressure transducer in the brake hydraulic line.



> Or is it just like an on/off switch where deceleration is the same until the brake pads come into play?


Could be done this way as well.

major


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## Aaron (Jan 21, 2009)

Thanks for the info.

It looks like I'm definitely getting that job I was looking into, so I should have the funds to get me a donor car and start working on a conversion.


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## sailfish11 (Sep 15, 2007)

Qer said:


> Sure, LiFePO4 are expensive but the advantages are:
> 
> 
> No horrible Peukert effect




 Is this true? I hadn't heard this before.


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## Qer (May 7, 2008)

sailfish11 said:


> [/list]
> Is this true? I hadn't heard this before.


Well, depends on how you look at it. All batteries have a Peukert effect, but as far as I've understood it it's very much better (ie lower) in a Lithium based battery than in a lousy lead acid battery. So you still have a Peukert effect, but it's not horrible or at least not as horrible as lead acid.

Yeah, I could've been clearer about that. Sorry.


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## joamanya89 (Feb 13, 2012)

I've got a quiestion here, what if you use a microcontroller to che on a tacometer and you do all the needed stuff so when you push your break or just leave your accelerator pedal it just conect a direct circuit between your batteryes and your DC motor (been them close), so you can charge your batteryes with no problem an with out an other thing else than your DC motor.


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## major (Apr 4, 2008)

joamanya89 said:


> I've got a quiestion here, what if you use a microcontroller to che on a tacometer and you do all the needed stuff so when you push your break or just leave your accelerator pedal it just conect a direct circuit between your batteryes and your DC motor (been them close), so you can charge your batteryes with no problem an with out an other thing else than your DC motor.


Nope, you need a regeneration capable motor controller.


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## joamanya89 (Feb 13, 2012)

major said:


> Nope, you need a regeneration capable motor controller.


Yea sure, but, what's a regeneration capable motor controller? is a controller that does exactly what I have said, I'm an electric engineer student, so Im doing my own controller, and I'm in that situation now


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## major (Apr 4, 2008)

joamanya89 said:


> Yea sure, but, what's a regeneration capable motor controller? is a controller that does exactly what I have said, I'm an electric engineer student, so Im doing my own controller, and I'm in that situation now


Let us know how that works out for you  Circuit diagrams and video would be nice along with the battery current trace.


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## joamanya89 (Feb 13, 2012)

Sure! that's not a problem, but I have just one question, supose that I have got a motor of 120V, how do I know about how much voltage is it going to give me when it will be rolling? the courve of the motor worck for the enegry generation too?


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## PStechPaul (May 1, 2012)

I would say the first order of business (or design) is to determine what motor you will be using, which also depends somewhat on vehicle size and specifications. Once you know the required power, torque, and RPM range, then choose the type of motor. I greatly prefer three phase induction motors, but most first-time EVers seem to choose series wound DC traction motors such as those in fork lifts. And a third option, especially for smaller projects like e-bikes, is the BLDC. 

Since you are a student, take time to learn all you can about all three major motor types, and I would suggest actually building simple versions of these motors. There are lots of examples on youtube. And build your own controller for these experimental motors, using something like a PIC or Arduino. You should also obtain some small motors which can often be gotten for free from discarded fans, toys, printers, drills, and other equipment. 

As you work with these motors, you will gain a feel for their capabilities and limitations, and you'll be able to see how they can be used for dynamic braking and regeneration. THEN you can choose a full-size motor for your EV and you'll be able to design a controller that has a chance of working and meeting your needs. But at higher power you will also have to consider other factors such as high current connections, inductance, transients, and mechanical issues. Good luck!


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## joamanya89 (Feb 13, 2012)

do you know what? that's a maravilous idea, siriusly, I'll do hat first, with a regular 24V brush type motor so then I'll try all the stuf I want on it, after that is just change the scale, an be carefull abot de inductance of corse! thanks!!


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## Aelwero (Aug 24, 2010)

major said:


> That is how I do it. It gives the feel of power brakes. The further you push down the pedal, the stronger the regen torque. Can be done with a pot on the pedal linkage or a pressure transducer in the brake hydraulic line.
> 
> major


Could that be set up as a 4th pedal? as in a regen pedal next to the clutch (intended for left foot use, the "clutch" foot), one that can effectively maximise the use of regen, but separate, so that when you NEED brakes, it's still the same setup you've been using your whole life... (in an emergency, you don't really think, you just stand on the big brake pedal). Generally you use the clutch right before hitting the brakes, and I think it wouldn't be much of a stretch to use a "pre-brake" pedal instead, and it would make EV stick driving a little more "natural" to someone who drives an IC stick...

Does that compute?


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## WarpedOne (Jun 26, 2009)

> in an emergency, you don't really think, you just stand on the big brake pedal


Actualy, that doesn't compute. If you don't think, you will do what you normaly do, to slow down the car - step on regen pedal. Only after you realize that what you are doing is not what you need to do, you will maybe remeber to step on brake pedal.

having 4 pedals is just wrong. Go/slow and panic! is optimal.


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## jeremyjs (Sep 22, 2010)

There's also another option for how to set up the regen. You could use a properly rated pressure transducer and hook it into the brake master cylinder if there's a port of t it off of a brake line. The more pressure you apply to the brakes the more regen you get. Just like Jack on EVTV did. Seems like it'd be pretty well perfect as you'd get a very linear application of breaking power and it should be very simple to implement.


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## PStechPaul (May 1, 2012)

Not having driven an electric car I can't say from experience, but I would think that having regen would be similar to ICE compression braking, whereas without it, the car would tend to coast with little slowing effect except for the motor and drivetrain friction and windage. I think some hybrids and EVs have a regen mode you can enable and disable as desired. I would think it most natural to expect some regen braking effect as the throttle is released, and the accelerator is all that is needed for speed control except on steep descents and heavy traffic and controlled intersections (stop signs and lights).

I'm not sure how powerful the regen effect is. It probably depends on the type of motor (and controller), but I think motors can be driven to several times their normal torque, but for regen I would expect maybe a negative torque of not much more than 1/2 rating. 

I have heard of actually driving the motor in reverse to get maximum dynamic braking action, which may well exceed the nominal torque, but that would no longer be regen. But it may be useful as additional stopping power especially if the car is heavier after EV conversion.


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## major (Apr 4, 2008)

PStechPaul said:


> Not having driven an electric car.....


Do us and yourself a favor and go drive one. If nothing else, visit a Nissan dealer and test drive a Leaf, or maybe a Tesla dealership 



> .....I can't say from experience, but I would think that having regen would be similar to ICE compression braking, whereas without it, the car would tend to coast....


Regeneration in an EV (or HEV) can be implemented in a number of ways, one of which is "pedal-off". This applies a moderate amount of negative torque (regen) when the driver lifts his right foot. Often this simulates engine compression braking. My current EV has this feature, along with proportional regen braking from the brake pedal. I find it nice to drive, but still prefer the pure coast mode where there is no regen until the brake pedal is pushed. 

My primary ride for years was a 5 speed manual. Most of my commute is country roads, flat and straight with little traffic and stop signs every couple miles. I became accustomed to knocking it out of gear and coasting like 1/4 mile to the next stop. Seemed like a good way to save fuel. Might take a little longer to get to where I was going, but WTH. It is difficult to get a pure coast with my EV. I can see where the regen goes into the negative power meaning battery charge, but I feel I could do better (use less energy) by coasting.

For the normal driver in a hurry or in traffic, the regen is preferable over coasting, or in hilly terrain.




> I'm not sure how powerful the regen effect is. It probably depends on the type of motor (and controller), but I think motors can be driven to several times their normal torque, but for regen I would expect maybe a negative torque of not much more than 1/2 rating.


The "regen effect" is negative torque. Ignoring the series wound motor, motor and control systems have practically symmetrical 4 quadrant speed torque characteristics, meaning as much negative (braking) torque as positive torque (motoring) and in either direction of rotation. Of course with the system in the EV, the battery will also enter into the torque equation in the special case where it would be overcharged, but few EV users charge their batteries at the top of large hills or mountains.




> I have heard of actually driving the motor in reverse to get maximum dynamic braking action, which may well exceed the nominal torque, but that would no longer be regen. But it may be useful as additional stopping power especially if the car is heavier after EV conversion.


Reversing the motor while in motion is plugging, or plug reversing. The most common way is to simply throw the reversing contactor with a series wound motor. And this is, or was, common practice on forklifts. An uncontrolled plug at full speed will draw twice stall current so is to be avoided. To control the plug reversal with PWM, a plugging diode is used. These are still in the Curtis controllers at the A2 terminal. However plugging is discouraged in high speed EVs because all the kinetic energy is dissipated in the motor and controller.

Dynamic braking uses a resistor to dissipate the energy. This is common practice in diesel electric locomotives and large trucks.


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## gojo (Feb 1, 2011)

Had regen, and I got rid of it.

I had a BLDC motor with regen. It was nothing but trouble. It saved on brake pads, and gave a little more range, but I had a lot of stuff going wrong with it. The hall sensors finally went out on the BLDC motor, and I couldn't get replacements from China. I replaced the BLDC with a brushed motor (Netgain) and DC controller (Curtis), and I couldn't be happier with the simplicity, and dependability.


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## major (Apr 4, 2008)

gojo said:


> Had regen, and I got rid of it.
> 
> I had a BLDC motor with regen. It was nothing but trouble. It saved on brake pads, and gave a little more range, but I had a lot of stuff going wrong with it. The hall sensors finally went out on the BLDC motor, and I couldn't get replacements from China. I replaced the BLDC with a brushed motor (Netgain) and DC controller (Curtis), and I couldn't be happier with the simplicity, and dependability.


Sounds like you had a crappy motor and got rid of it. What does that have to do with regen


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## Aelwero (Aug 24, 2010)

WarpedOne said:


> Actualy, that doesn't compute. If you don't think, you will do what you normaly do, to slow down the car - step on regen pedal. Only after you realize that what you are doing is not what you need to do, you will maybe remeber to step on brake pedal.
> 
> having 4 pedals is just wrong. Go/slow and panic! is optimal.


I understand exactly what you are saying, but it's on the clutch side... If you drive a stick, you're cruising along, you see a stop light coming up, and without really putting any thought into it, you take your left foot and push in the clutch to disengage the tranny before taking your right foot off the gas... your feet are well trained to operate a stick that way, because if you don't, the whole car does the herkajerk dance and stalls out in the middle of the road to let you know you forgot. I'm not saying it should be one brake or the other, I'm saying regen with one foot, brake with the other, in the exact same manner people have been driving two footed with clutch/brake for a century or so...

The only difference between the unthinking/texting/eating a big mac guy with that setup and a typical clutch is that the EV guy won't do the herkajerk dance and drop his phone/hamburger...


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