why this regenerative braking can't be used in conventional cars?

The simple answer to your question is “Because conventional cars do not have the parts necessary, and if they did, they’d be a hybrid by definition.” Longer answer, in order to take advantage of regenerative braking (my personal reason for purchasing a hybrid.), a vehicle would first need a generator. (motor in reverse as the article phrases it.) Second, it would need a place to put this newly generated energy; a battery seems to be the method of choice right now. Third, how to uses this newly stored energy? Electrical motors seem to be the way to go. So, we now have a generator/electrical motor and a battery; add that to the other parts of a conventional car and short of a few pieces I omitted for simplicity, you’d have a hybrid. How’s that for an answer?

I find myself surprised by the claim ( in the original article ) that, in order to achieve regenerative of dynamic braking, the direction of rotation of the motor is reversed. This sounds very extreme! I think the claim perhaps needs checking out. I think you may well find that all that is involved is some reconnection or change in control of the windings of the motor so that its normal ' back emf ' in motor mode is increased to above battery voltage. In this way the direction of both current and power flow are reversed, the motor continuing to turn in the same direction, but now acting as a generator. This is the way it USED to be done - I'll be very happy to be proved wrong, but, as I have said, the sudden reversal of direction of a rotary machine does sound extreme, probably unnecessary and thus somewhat unlikely.

BMW have implemented regenerative braking on a range of their cars. Instead of using a separate capacitor/larger battery, i.e the prius and civic hybrid, the car's main battery is charged through regenerative braking and hence reduces load on the alternator.

Because everyone in this discussion is Anonymous except me, it makes it hard to reference your comments. So we’ll do it this way, Anonymous #2, it’s not that they are putting the motor in reverse, but rather simply switching polarity, in turn, making it a generator. The motors in a hybrid are AC powered, but for the sake of simplicity, I’ll make it DC in this explanation. A motor has a + and a – terminal, when it’s a motor. When you want to make it a generator, you simply make the + the – and the – the +. This in turn makes it put energy into the battery, rather than taking it out. So, when you LIGHTLY brake, you tell the vehicle to regeneratively brake by switching the polarity of the motor.

The reason they have chosen to only allow minor regenerative braking is because if they allowed full regenerative braking, it would lock the wheels. Think of it this way, the “motor” can be just as powerful a generator as it is a motor. It would brake with all the force it has when it’s a motor. (Entering a spot of uncertainty) I guess it’s hard to allow for variable regenerative braking, one where the further on the brake you press the more load put on the generator and more it’ll slow you down. So they just went with a moderately set level of regeneration. (Entering certainty again.) So when you press harder on the brakes, the vehicle employs its traditional brakes. (For the sake of ability of the regenerative technology right now, and to put those people’s minds at ease that don’t trust not having “actual brakes”.) Someday, your vehicle probably won’t have “actual brakes”, just dynamic regenerative brakes that can brake lightly (like now for the most part.) up to all of the braking ability of the system with the help of ABS (2.0?)

One last thought. To give you an idea of the power I’m talking about, a DeWalt Heavy-Duty 3/8" (10mm) 12V Cordless Compact Drill produces 230 watts of power (and on a full charge, I can’t stop it from spinning.) the Electric Motor of a 2007 Prius is 50 kW, or 217.4 times the power of the drill. So respect the motor/generator.

BJONES you are terribly wrong bro.. it doesn't matter if you change the polarity.. that only makes the motor run the other way.. (reverse).. a motor creates rotational power when electricity is added.. so it "spends" the electricity.. to transform a motor into a generator you just do the opposite you apply rotational motion and you'll get electricity.. its that easy.. if you don't believe me grab a small motor stick the two wires to your tongue and rapidly turn the axle.. you can feel the small current..

Thanks

So what you’re saying is that the polarity is never switched, but rather when one takes their foot off the accelerator, the once motor turns into a generator because the wheels are still spinning?

BJones - the way the motor behaves depends on how it's connected. If you just take your foot off the accelerator, the motor will spin freely because it's effectively disconnected. To get the generator effect, you need to reconnect the motor to an electrical load. In regenerative braking that load is charging the batteries.

I agree with BJones. I'm in college, and just learned about Regenerative braking. The polarity changes inside the motor. This makes it a generator.

hi there, i was wondering if any of you guys would be able to help.I am designing a truck for my final year project at uni and plan on using regenerative braking to capture some of the energy lost. What i am struggling with is how to work out how large to make the storage unit, and also how to work out how long the energy stored will last once in use. Any advice or guidance would me greatly appreciated,

Jon.

Sweet, a chance to influence the future of automotive design! I would suggest placing the battery beneath the bed of the truck. Have it take up the whole area of the bed, but thin enough not to make the bed too shallow. High voltage, so that amperage can be relatively low. I would think in the truck realm, a high torque motor to get the truck started from a stop would be the way to go. It would not need to go fast on battery, but be powerful. I actually drive a truck and one thing that I hate is braking. I spend money to get going and then “pay” to break (break pads). Another thing that I hate is driving a truck in stop and go traffic. If my truck worked like a Prius and practically ran silent in that situation, it’d be a major plus for me. I’d love to help you any way I can.

Bjones, thank you. Do you know the braking force created from a truck with a 40ft trailer on full load? or know of anywhere i can find such information? I need this to establish how much can actually be saved. Id agree with a thin motor to stretch across the bed.

regards,

Jon.

Sorry for taking so long to respond. Anonymous, what type of truck are you designing? When I wrote my first response, I thought you were talking about a pickup truck, like a Dodge Ram. However, your last response makes me think you’re talking about a semi, like a Peterbilt 387. Mind clarifying?

01/13/08 - I'm thinking about trying to add hybrid electric to my 1982 Toyota pickup truck and am interested in this thread. A few preliminary questions: I can't see any post dates from my view of this conversation. How can you tell if the conversation has ended or very old? If this is posted as anonymous it is because the login procedure doesn't seem to allow me to do it any other way.

To BJones: My thoughts exactly, I want to try and recover some of that energy that I have paid for to go up hill, on my way back down. And I've been noticing that it doesn't take much throttle to maintain a speed on a level surface, so it shouldn't require a huge electric motor to be able to use it then too. If energy recovery works good, I would later try increasing the battery pack size, and experiment with it being a plugin hybrid, much like people are currently doing to convert Prius's.

01/20/07 - This is a semi-active thread. I seem to be the only active “regular” and while I have a pretty strong idea on how regenerative braking works, I don’t profess to be an expert. Anonymous, the project you describe sounds to be quite the undertaking. Good luck! One piece of advice that comes to mind is, make your system high voltage. The higher your voltage, the less amperage you need for a given amount of power. One area of knowledge I do profess to have an above average knowledge in is UPSs. I have participated in all of APC’s Data Center University’s classes. But enough blowing my own horn, back to business. A white paper that might be a smidge of stretch for this topic, but I personally think it would help is Use of 208V vs 120V for Servers in case the link doesn’t work, it can be found on APCs site. It talks about how 208V is overall a better choice in the data center over 120V. If formulas are more your style, P = I × V also proves my point.

Let me know if I can be of further help.

"Anonymous" is partially correct, but the problem is that the wheels are turning in the same direction. Typically, a motor and generator can be the exact same thing (polarity and all) if the shaft input is reversed from the motor-driven direction. You can picture this as physically turning a water pump backward to move the water back into the tank from where it was pumped to - the reverse turning of the shaft induces a current in the coil that puts the charge back into the battery.

As stated, though, the problem with regenerative braking is that the wheels turn in the same direction for driven and braking operations. So instead of putting charge back into the battery, the wheels want to drive the charge in the direction it was already going. But since the charge is driven by the wheels now, and not the battery, the torque required to drive it inside the coil must put a reaction-torque on the shaft (known as "electromagnetic drag"), which slows down the drive wheels. This is very similar to leaving your clutch engaged when not accelerating - the friction in the motor slows the car down. As a solution, a clutch or torque converter is implemented to effectively disconnect the motor from the wheels, preventing such drag.

Now, as BJones stated - the solution to the generator question is simply to switch the polarity on the motor while braking. Obviously, the electric clutch must be re-engaged, as well. As stated, the charge will still flow in the same direction in the motor coil, but since the polarity is switched, it goes to the opposite pole on the battery, recharging it.

The problem with variable regenerative braking is that you must figure out a way to vary the electromagnetic drag. The easy way to do this is to meter it with another external motor or other power source, but this negatively affects the efficiency since some energy is spent in order to get some back (sound like economics?) - this goes against your purpose. The other way to do it is by variable coiling, which significantly raises the complexity of the design and the amount of work to achieve it - raising your cost of entry.

Welcome to engineering. :-)

Is that possible for me to know how much the energy will be generated through the regenerative braking? Is there any formula? Thanks.

Not a clue. Anyone else?

Since it mention in the above article that, 'this heat energy dissipates into the air, wasting up to 30% of the car's generated power.'

so can we assume that the regenerative braking can restore 1% from the 30% loss generated power?

3/20/2008 7:06 PM EST

Perhaps more. It would all depend on how efficiently you can turn the kinetic energy into potential energy, such as mechanical energy into electrical energy.

Not meaning to post anonymously on purpose, but here goes.

The only way to change the braking torque on a motor used for braking is to use a motor with stator windings instead of a permanent magnet. Typically a brake pedal only has an on/off type button arrangement and so you would need to add a torque sensor. As a motor torque is proportional to the current applied to the motor, you would be able to work out a control system for working out how much torque to apply using the motor but applying current to the stator windings.

The next problem I see is, the output voltage created by the motor is proportional to the rotational velocity of the motor. I think the prius has a 500v motor however has an array of batteries that makes about 220v. However the motor could applying a large range of different voltages. You would need a boost/buck converter to control the charging voltage on your battery. Ie. I think a lithium cell likes to be charged at about 4.0v for a 3.7v cell obviously raising the voltage a little more as the battery is more charges as a typical 3 phase battery charger does. The duty cycle of a buck converter which change the output voltage. Lastly converting ~220v to 500v to charge the motor.

I'd be very keen to know if anyone else has more information on this topic, I'm currently developing a system for regenerative braking in dc control robotics for my thesis project and there is not a lot of material out there.

David

Welcome David. Thanks for your contribution. (I’m not quite sure why I’m playing moderator, but I am.)