Can anyone explain how the sudden and large jolts of electrical power from braking can be stored in a battery. In order for meaningful amounts of energy to be converted to actually stopping a heavy car the electrical jolt must be pretty big. I have visions of boiling batteries and smoke wires, but of course that doesn't happen so how does the system handle it.
rq
Dynamic breaking, or what I know it about does not produce large amounts of current and voltage. The motor poles are reversed and the motor acts like a generator. Since it requires power to drive a generator it is pretty effective breaking. A small amount of generated power is put back into the batteries. Sustained breaking, (minutes) might deliver a charge big enough to boil the batteries. Not sure where you going to find a hill like that or how your going to get the electric car up there.
RQ schrieb:
Hello,
a good brake is able to stop a car much faster than the motor would need to accelerate to the same speed before braking. In other words, the brake is able to consume much more power than the motor is able to generate.
But it is impossible to store the energy from an emergency braking back to the battery, the battery and the electrical brake ( in fact, an electrical generator ) should be too big, heavy and expensive.
An electrical and a mechanical brake must be combined, if you brake soft, the energy can be stored in the battery, if you have to brake hard, the mechanical brake is used and the energy is lost.
The trick is to avoid hard braking and to brake early and soft instead. But if you can avoid braking, the energy conservation is even better. Every loading and unloading of the battery is causing a loss of energy.
Bye
The electric motor(s) is(are) used as a generator(s). The forward momentum of the vehicle powers the generator(s), and then in turn charge the battery. Braking power is limited to the generating capacity, just like acceleration is limited to motor power. In other words, you can use the regenerative braking to slow down gradually. Also you can't accelerate from 0-60mph instantaneously, therefore you cannot brake instantaneously. In order to panic stop you will need friction brakes. If I recall correctly, the theoretical efficiency of regenerative braking is 33%.
Dwayne
stop and consider, the braking can approximate the accelerating current. that may also be controlled. they usually use a pulse-width modulator and it controls the current when it's regenerating (charging) just like as when i's trying to accelerate the auto. it does, however, take a bit more charging to restore an amount of power when regenerating than it delivers when powering the auto.(discharging) NIMH cells take about 1.5 times charging power for the amount of delivered power. sammmm
I would bet that "dynamic braking" could give a subjective feeling of 'medium braking" if not "hard" braking.
Theoretically, at least some of the excess braking current could be stored in a capacitor until the battery could catch up. Above a certain limit, excess energy would need to be dumped by means of a mechanical brake or resistive load.
Dwayne schrieb:
Hello,
if we assume an efficiency of 75 % each for the electric motor, the generator, the charged battery, the discharged battery, we get a total efficiency from kinetic energy before braking back to kinetic energy after acclerating again of 0.75*0.75*0.75*0.75 which is 0.31 or 31 %. Very close to the 33 % you recalled.
Bye
LISTEN: You guys are in a nightmarish cycle on breaking an electrical vehicle.,
Somehow feeding it a back current or reversing the voltage is not a good braking method even if the motor is designed for reversing.
what someone seems to recommend is like switching a standard motor into reverse to slow it down or hard brake it., though the motor won't drop off as a transmission would I seriously doubt that it is the best method to brake an electric motor.#1 the strain is not something you'd be comfortable in a vehicle. #2 there has to be an outside mechanical influence on the drive system to brake the motor #3 precision braking of en electrical motor using electrical or electronic signaling is probably more of a dream than it is a reality for EV's. [yet possible]
you're better off just designing a power transfer/cut off to the motor & diverting the transit power to the batteries recharge circuitry as suggested, while employing conventional braking., if you want to get fancy to boost your specs & prices; then fine, mess around with motors & generators and see if you can build one that would screech to a halt on a digital fed reverse voltage/power application, (consider variable ev weight) would you need a clutch ? A State of the Art Clutch? I think it would be too dynamic for current performance needs and it'd heat up & consume too much of the sparse energy in the vehicle.The Energy being the Issue. Clutch/Transfer + Generated Power to Batteries ~ Dynamic Braking =3D all power disconnect not too good., besides what good would an idle generator do connected to a battery pack.
nevertheless a few seconds or minutes of generated power backfed into your system is better than none, even if a minor consideration., one could coast on a downhill strip and recharge };-) could be the difference in a fun ride or pushing it to an outlet.
=AEoy
give me a few more years :-)
Regenerative braking does not involve an energy input from the batteries or power supply. Rather it uses the forward momentum of the vehicle to generate electricity from the electric motor(s). Regenerative braking is well know and currently in use on some production electric & hybrid vehicles.
"Know where all that braking energy goes when you stop in a normal car? Nowhere, it's wasted. So when Toyota created the Prius, they came up with a better idea.A regenerative braking system operates the electric motor as a generator. So when you put your foot on the brakes, the kinetic energy is converted into electrical energy and stored in the battery.The system is particularly effective in recovering energy during city driving where patterns of repeated acceleration and deceleration are common.What's the point? This system increases overall efficiency, and thus fuel economy."
There are several 'braking' methods with electrical motors. So far people have responded with a mix of terms, not all of them correct. There are three principle methods used to 'brake' a system driven by electric motors, 'dynamic braking', 'regenerative braking', and 'reversing or "plugging"'
'Dynamic braking' is the method of using the motor as a generator and disipating the electrical energy produced in some form of dummy load such as a resistor bank. When using DC machinery, this is very easy to do. The armature is disconnected from the power supply and connected to a resistor bank. The field current can be regulated to control the amount of braking affect. This method of braking is widely used on diesel-electric railroad locomotives. The resistor bank is a huge bank mounted inside the roof of the engine compartment at the back end of the locomotive. Large radiator-sized fans blow air across the resistor bank to disipate the heat. Engineers use them when descending long grades rather than burn up the friction brakes on the wheels.
'Regenerative braking' is very similar, except the electrical energy is not just dumped to a dummy load. Rather, it is sent back to the power source. Obviously, the type of power source has to be able to accept the energy. Secondary cell batteries are one such source, but there are others. Traction motor railway systems (electric trains) often use regenerative braking and send the electrical energy back into the grid. Contrary to some of the ideas posted, DC motors can be shifted from motoring to generating by simply increasing the field current. No special contactors, or switching is required, just set the field rheostat to maximum current and regenerative braking will quickly slow the motor to minimum.
Both of these methods have the disadvantage that no braking force can be developed until the motor is actually being spun by the load. Once the load comes to rest, the braking force goes to zero (the motor/generator cannot develop an electrical output at zero rpm).
'Reversing' (also sometimes known as 'plugging') is a method that can be used to bring a spinning motor to a dead stop very quickly. This method is to disconnect the normal supply and reverse the motor connections. Then briefly reconnect the power supply. Often, the reconnection is controlled by a small shaft-mounted switch that is driven by a drag-clutch. The moment the motor speed drops below the switch's threshold or begins to reverse direction, this 'plug switch' opens and causes the motor controller to disconnect the power supply. This type of 'braking' can stop the motor almost as fast as 'starting' can bring it up to speed. I think this is the type that Roy QT was thinking of when he mentions special equipment requirements. The current surge when 'plugging' is *higher* than the starting current. Limitations on duty-cycle and power supply requirements are usually needed. And even the shaft components can be a problem. I've seen shaft keys sheared by in-advertant 'plugging'.
Most hybrid vehicles in the news these days use 'regenerative braking', where the energy is put back into the battery system. Some manufacturers try to make it sound like a 'major advancement'. But a 1000 kg car, traveling about 26.8 m/s (about 60 mph) has a total kinetic energy of 359 kJoules of energy (K.E. = 1/2 m* v^2). Thats about 100 kWh. May sound like a lot, but one gallon of gasoline has about 30480 kWh. So if regenerative braking is perfect, for each stop from ~60 mph you reclaim the energy of about 1/2 of a fluid ounce of gasoline (1 1/2 tablespoons).
Hardly seems worth it.
daestrom
For some interesting comments about energy storage:
There's still no free lunch!
It's nice to know EV's are so advanced
Though: it would seem that on short runs & stops like in city traffic it would use up' more than generate' energy.
I think the ony terrain where a truely helpful charge can occur is when the motor/generator is coasting off the batteries as on a nice & long down hill plain.... if & when you have to hit the accelerator, just how much generation can you obtain from the toyota motor/gen. and plaese don't tell me they are using an overunity principal and get more generated energy than torque pervolt.
it's a great step if they have somehow made it able to maintain it's orgiinal or close to starting point charge, then again this can be enhanced and done with solar panels. correct?
=AEoy
Somehow feeding it a back current or reversing the voltage is not a good braking method even if the motor is designed for reversing.
what someone seems to recommend is like switching a standard motor into reverse to slow it down or hard brake it., though the motor won't drop off as a transmission would I seriously doubt that it is the best method to brake an electric motor.#1 the strain is not something you'd be comfortable in a vehicle. #2 there has to be an outside mechanical influence on the drive system to brake the motor #3 precision braking of en electrical motor using electrical or electronic signaling is probably more of a dream than it is a reality for EV's. [yet possible]
you're better off just designing a power transfer/cut off to the motor & diverting the transit power to the batteries recharge circuitry as suggested, while employing conventional braking., if you want to get fancy to boost your specs & prices; then fine, mess around with motors & generators and see if you can build one that would screech to a halt on a digital fed reverse voltage/power application, (consider variable ev weight) would you need a clutch ? A State of the Art Clutch? I think it would be too dynamic for current performance needs and it'd heat up & consume too much of the sparse energy in the vehicle.The Energy being the Issue. Clutch/Transfer + Generated Power to Batteries ~ Dynamic Braking = all power disconnect not too good., besides what good would an idle generator do connected to a battery pack.
nevertheless a few seconds or minutes of generated power backfed into your system is better than none, even if a minor consideration., one could coast on a downhill strip and recharge };-) could be the difference in a fun ride or pushing it to an outlet.
®oygive me a few more years :-)
---------
Regenerative braking is a tried and true method of braking - It has been used succesfully in many applications. Motor switching or reversal is not involved. When the motor generated voltage (so-called back emf) is greater than the supply voltage, the current and energy flow reverse, not the voltage so the motor acts as a generator. No separate generator is involved and no mechanical switching is needed. A hybrid car doesn't have a separate generator- the electric drive motor is expected to do this, whether driven by the gasoline engine or by the wheels on the road, and the control circuitry is designed for this. However the main drawback of regenerative braking is that it cannot bring the vehicle to a stop as its effectiveness decreases with speed. Conventional braking is needed to actually stop or get effective braking at low speed. . For most braking applications, i.e. reducing speed, say from 60 to 30, it is practical and beneficial.
--------- Regenerative braking is a tried and true method of braking - It has been used succesfully in many applications. Motor switching or reversal is not involved. When the motor generated voltage (so-called back emf) is greater than the supply voltage, the current and energy flow reverse, not the voltage so the motor acts as a generator. No separate generator is involved and no mechanical switching is needed. A hybrid car doesn't have a separate generator- the electric drive motor is expected to do this, whether driven by the gasoline engine or by the wheels on the road, and the control circuitry is designed for this. However the main drawback of regenerative braking is that it cannot bring the vehicle to a stop as its effectiveness decreases with speed. Conventional braking is needed to actually stop or get effective braking at low speed. . For most braking applications, i.e. reducing speed, say from 60 to 30, it is practical and beneficial.
-- Don Kelly --
Except for the part of the Motor/Generation ~>
[which needs to be made optimal by design, the nature of an electrical motor in itself isn't cause for much plus on the energy of the batteries to engine cycles]
maybe should have a seperate eff generator on the axles or somewhere, like dynamos that could be engaged with a lever or switch., then a push could get you a long way};-) =AEoy
Many thanks for the various opinions on my original post concerning the method used to store a relatively large electrical charge over a very short time span into a bank of batteries. As an old retired power plant operator who had to care for and maintain various battery packs to provide cranking power for gensets etc and for various UPS applications for instrumentation, computers, one of the cardinal sins of battery care was overcharging and recharging at too high a rate. It was a real no no because it was hard on the cells.
If you visit the WebPages of the car companies who are selling hybrids, there is no shortage of information extolling the advantages of regenerative braking and how the energy is recovered by going back into the battery pack. No one makes a claim of 100% recovery and everyone notes that this works in conjunction with normal brake systems.
In real world driving, there is probably a 4 to 8 second time period in slowing down and stopping for a red light where the energy of a 3000 lb car slowing from say 60k to 30k is converted to watts and stored in that 4 to 8 second time frame. That sounds like a hell of a jolt to me. Try imagining cranking over a cold motor on a cold day for 20 seconds and then putting a charger on to bring the battery back up to full charge in 20 seconds and you will know what I mean. I still have some questions on the method, but I guess the car manufacturers are keeping their cards close to their chests.
Rq
Re: Hybrid cars and regenerative charging of batteries
--------- Regenerative braking is a tried and true method of braking - It has been used succesfully in many applications. Motor switching or reversal is not involved. When the motor generated voltage (so-called back emf) is greater than the supply voltage, the current and energy flow reverse, not the voltage so the motor acts as a generator. No separate generator is involved and no mechanical switching is needed. A hybrid car doesn't have a separate generator- the electric drive motor is expected to do this, whether driven by the gasoline engine or by the wheels on the road, and the control circuitry is designed for this. However the main drawback of regenerative braking is that it cannot bring the vehicle to a stop as its effectiveness decreases with speed. Conventional braking is needed to actually stop or get effective braking at low speed. . For most braking applications, i.e. reducing speed, say from 60 to 30, it is practical and beneficial.
-- Don Kelly --
Except for the part of the Motor/Generation ~>
---------- such as? Please explain your contention. There is no inherent difference between a motor and a generator so a motor can function quite well as a generator.
----------------
[which needs to be made optimal by design, the nature of an electrical motor in itself isn't cause for much plus on the energy of the batteries to engine cycles]
----------------- If so, try it again in English- as is the statement is meaningless.
Motors are what ! Just like Generators ? then lets put a voltage across a generator and see how many rpms we get from it.
Please Mr. Kelly you are standing too close to the Rift
=AE
Daestrom, It seems that there are not a lot of posters here that have a good understanding of motor operation, specifically DC. It is good to see someone here, such as yourself, that does.
I hope you do not mind if I add some general discussion to your comments.
There is a major difference between the Dynamic Braking and regenerative braking. With Dynamic Braking, when the energy is dissipated as heat, The Resistor grids will continue to take load current as long as the motor is turning and it has field. The slower the motor goes, the lower the voltage, and thus the lower the current, and with it , Braking effort. As the motor slows, braking effort can be maintained by increasing the field excitation, or stepping out parts of the resistor, to get more current for the lower voltage. (The machines we do use DC motors with Shunt fields)
If you wish to get Regenerative Braking into a battery, as you mentioned , all that has to happen is the motor field is to be increased so that the motor counter EMF is higher then the battery voltage. Once this happens, Current will drop to zero and then reverse. The reversal of Current will produce a braking torque, which slows the Vehicle. The problem here is that as the vehicle slows, the generated voltage for a given field strength will drop linearly with speed. So as the Vehicle slows The only way to keep regenerating current into the battery (which is basically a fixed voltage source) is to keep increasing the motor field as the speed goes down. There is obviously a speed where no amount of field current is going to give you a voltage higher then your battery voltage, and braking effort will cease.
Needless to say, the design of the Motor will have to be such that at normal road speeds the motor field will be quite a bit lower then it's maximum value, ( in the field weakening region) and as the speed decreases, The field increase to it's maximum value at base speed, After this some other way must be found to stop the vehicle.
Because of this, Regenerating into a battery, using conventional thinking, is not really a viable option. I am not sure how the toyota people do this. maybe they have some sort of Dc to DC converter to step the voltage up or perhaps they break the battery bank into it smaller voltage cells and regenerate into these. Either way they are adding in complexity and inefficiencies.
My Exposure to this application has been the Drives on the 240 Ton Mine Haul Trucks.
Maybe someone else who is closer to the automotove application of DC Drives can shed a little more light on the matter.
Have a good weekend, All Tom
Your conclusion is OK, but I think your car will have a KE of 100 watt-hours, not kilowatt-hours. Same on the energy content of gasoline.
At $.10 a kWh, (what I pay for electricity), your gallon of gas would be worth $3048!
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