I've been reading about Regenerative braking systems and their use in hybrid-engine setups. Most articles say something to the effect of, 'the electric motor can double as a generator to slow the vehicle and return power to the battery'.
However, I've also read that Capacitors are the only thing capable of using the stored energy quickly enough to be of any use during acceleration..
Would someone please explain how regenerative braking actually works, more specifically the part about the electric motor switching tasks and becoming a generator, and how as a generator it can slow the vehicle while storing energy created from the breaking?
** Time to get yourself a small DC motor and do some experiments.
If you connect it to a battery via a SPDT switch, wired to short circuit the motor in the off position - you will see how a motor can be used as a break.
Then wire the switch so the motor connects to a low value resistor in the off position.
The problem with motor voltage during breaking will then probably dawn on you.
You treat the back emf of the motor like a low voltage, but high current source and step up the voltage and feed it back to the battery
- that's regenerative braking, for a DC motor.
An AC traction motor is a little different, but the same principle - trigger a semiconductor to transfer energy back to the battery at the right phase angle when decelerating.
Or use a variable displacement pump/motor/accumulator and use hydraulics to recover energy.
Almost all systems use storage batteries - not capacitors. Capacitors are good for relatively low energy storage and fast energy transfer. Batteries are good for large energy and slow speed. You don't need high speed for vehicle drive systems - unless you're doing something cutting edge with "super caps."
--
----== Posted via Newsfeeds.Com - Unlimited-Unrestricted-Secure Usenet News==----
http://www.newsfeeds.com The #1 Newsgroup Service in the World! 120,000+
Newsgroups
----= East and West-Coast Server Farms - Total Privacy via Encryption =----
While I have zero experience in these systems, I would've thought the obvious approach would be to use capacitance in parallel with the batteries for that reason. I fully admit that there may be problems with this approach that I'm not aware of.
The motor doesn't actually 'switch' functions in the usual sense of the word. Putting it very, very simply, what happens is that the coils of wire on the armature cut the fields around the magents, & if you put power into the coils, the resulting field around the armature 'pushes against' the magnets, & if you 'push' the armature through the fields around the magnets, it induces power into the coils. The important thing to remember is that both effects can happen *at the same time*, eg; when you're powering a motor in a vehicle thats' going up a hill, so gravity is 'pushing' the motor in the opposite direction. This results in back-EMF that fights the power you're putting into the motor. So if your vehicle is rolling forward via inertia, but spinning the motor armature, you can collect that energy & use it to charge your battery, & the act of tapping that power 'fights' the motion, thus acting as a brake. You can demonstrate the concept yourself by taking a powerful motor with permanent magnets (eg; any DC or stepper motor), & spinning the shaft with your fingers while nothing is attached to the leads. You then shorts the leads together & try it again, & you'll find that the shaft is much harder to turn. (This is a really fun & effective way to demonstrate these concepts to students, BTW, & you can also prove that it generates power by using a DC motor with a light bulb & an ammeter instead of a short across the leads.)
--
W "Some people are alive only because it is illegal to kill them."
. | ,. w ,
\\|/ \\|/ Perna condita delenda est
---^----^---------------------------------------------------------------
Yea, obviously I know next to nothing about electromagnetics, but I appreciate those of you who took the time to reply.
Let's see if I've got this correct:The force of the magnet inside the motor is strong enough so that when power is drawn from the motor to charge the battery, the momentum of the vehicle (which is turning the drivetrain) -- that motion is not significant enough to fight against the manget, which in turn "brakes" the vehicle?
Um. Another way of putting it might be to say that you can convert the potential energy of the vehicles motion into electricity by using it to spin the motor shaft, & by taking that energy, you're slowing down your vehicle. And bear in mind that by changing the amount of electricity you're draining, you change the amount of breaking.
Yet another way of looking at it is to think of the forward motion of a heavy vehicle as being like the spinning of a heavy flywheel. That motion is form of energy, as is a manetic field in a coil of wire, or a charge in a capacitor. You can add energy to any of those things, or subtract energy from them, & (disregarding friction & other inefficiencies) the energy is equivalent, regardless of the form it takes, & can be transferred back & forth.
--
W "Some people are alive only because it is illegal to kill them."
. | ,. w ,
\\|/ \\|/ Perna condita delenda est
---^----^---------------------------------------------------------------
This probably isn't the right way to think about what is going on.
In a permanent magnet motor, the drive coils see a changing magnetic field as the motor shaft rotates.
This generates a voltage - the "back EMF" - between the ends of the coil. If you apply a higher voltage to drive current through the coil against this "back EMF" you produce a torque roughly proportional to this current which tends to make the shaft spin even faster, and you are using the device as a motor to accelerate your vehicle.
If you let the "back EMF" drive current through the coil in the opposite direction, into a battery, capacitor or resistor, you are using the device as a generator to decelerate your vehicle. Again, the decelerating torque produced is roughly proportional to current circulating through the motor coil.
Any current circulating through the coils also generates a voltage across the resistance of the coil, which generates heat - if you allow the coils to get too hot they can heat the permanent magnets in the motor above their Curie point, and they will cease to be permanent magnets, so you do have to pay attention to the amount of current that ends up circulating through the coils. The heating effet is proportional to the current squared, so the polarity of the current doesn't matter when you are calculating the heat being dissipated
In fact the voltage across any individual coil is an alternating voltage that changes polarity a number of times as the shaft rotates through 360 degrees. Permanent magnet DC motors include a mechanical switching arrangement - the "commutator" - that rectifies this alternating voltage into a direct voltage roughly proportional to the speed at which the motor shaft rotates, which reverses polarity when the direction of rotation is reversed.
If you want to follow the behaviour of the motor in more detail, you have to start worrying about the inductance of the individaul motor coils, which affects the rate at which the current through the individual coils can change, and - even later - you can start worrying about the extent to which the current through the coils induces a magnetic field which can add to or subtract from the magnetic field being produced by the permanent magnets.
When you get to this level, permanent magnet electric motors start looking a lot like brushless DC motors (which use built-in electronic swiches rather than mechanical commutators) and stepping motors (where you are expected to supply the electronic switches).
On a sunny day (15 Mar 2007 17:17:49 -0700) it happened snipped-for-privacy@gmail.com wrote in :
If you have a DC electromotor, you can use it as generator too. So if you turn it it will generate a voltage. If you connect a load to it, then it will take more power to turn it. If you connect an empty capacitor to it, via a diode, it will start charging that cap. It cannot discharge the cap if it stops because of that diode. Now if you short that diode, then the energy in the capacitor will cause the motor to turn again.
If you want to really graphically demonstrate this effect, get about
3 feet of insulated wire, a magnetic compass, and a bar magnet. Make a loop of the wire (i.e., connect the ends to each other), but make it kinda narrow and rectangular:
(the parens signify the magnetic field around the magnet)
and the compass will deflect. The magnet and wire on the left is the generator, and the compass on the right is the motor.
If you move a wire through a magnetic field, it generates a voltage. If you take the same wire and just let it lie there, but pass some current through it, the wire will move. They're exactly the same effect, just kinda like in opposite directions.
Ouch. I hate it when people use 'break' for 'brake', or 'loose' for 'lose'. You can sure tell that I wrote that post on Friday night, after a few beers. :^/
--
W "Some people are alive only because it is illegal to kill them."
. | ,. w ,
\\|/ \\|/ Perna condita delenda est
---^----^---------------------------------------------------------------
As far as how the regenerative braking goes, I'd opt for batteries that can handle as fast of a charge as they will a discharge, which is, admittedly, a pretty tall order for batteries - but something tells me that to do it with capacitors would take about 15 tons of capacitors and bus bars, in a package about the size of a city bus. =:-O
The problem you have to remember, is the limited cycle life of batteries. This largely rules them out for this type of application. Capacitors are fine to handle the momentary needs of regenerative braking, where the energy recovered, can be used only a few seconds latter on the next acceleration. Sizes are not prohibitive for 'single cycle' operation like this. Where problems appear, is doing things like long hill descents, but most systems are 'smart', and simply use more from the mechanical brakes when this happens.
Apparently not. The Prius after all uses batteries.
Any calculation I've done for for EV's has shown that if the storage device has sufficient energy density for the job, the power density is always(*) more than sufficient. Supercaps are an example where the reverse does not hold.
Supercaps hold an advantage only for high power low energy situations, the higher the portion of they hybrid that is electrified the less applicable supercaps will be and the more applicable batteries will be.
Robert
(*) I certainly haven't done an exhaustive battery comparison so it wouldn't surprise me if there were a standby battery chemistry that had too low a power density to match this criteria.
--
Posted via a free Usenet account from http://www.teranews.com
Yes. This was why in the early days, you only 'leased' the batteries. They were unsure how long they would last. It is still the commonest failure on these vehicles. When they are used for a lot of stop start driving, the battery life plummets... :-(
Commonest failure? Really? Toyota claims better than a 180,000 mile lifetime and I know there are taxis that have more than 300,000 km on them which suggests stop and go isn't a huge issue. I have an EPRI report that also reached much the same lifetime conclusions.
Robert
--
Posted via a free Usenet account from http://www.teranews.com
ElectronDepot website is not affiliated with any of the manufacturers or service providers discussed here.
All logos and trade names are the property of their respective owners.