Go Back  

DIY Electric Car Forums > DIY EV Wiki > EV Information


Reply
 
Article Tools Display Modes
  #1  

Default Regenerative Braking

What is regenerative braking?
Regenerative braking is a way of slowing a vehicle down where some or all of the vehicle's kinetic energy is saved rather than being wasted as heat. Electrically this is achieved through the use of a generator (often the traction motor with the terminals reversed) and energy storage devices such as batteries or capacitors. This generator is usually an AC or permanent magnet DC motor.

What sort of range can be gained from this feature?
The typical stated range gain for regenerative braking is about 10%. AC Propulsion states as high as 30%, US Electricar measured as high as 20+%, Toyota RAV4 owners report as high as 25%. This would obviously be more effective in city driving rather than highway where little braking occurs.

Is regenerative braking possible on a series wound DC-motor?
Yes, but it is difficult and can be dangerous to implement. Some controllers, such as the ZAPI H2 have regen abilities built in but some have questioned the controller's reliability. One successful DIY attempt by Otmar Ebenhoech of Cafe Electric is documented here. Early 90's Soleq brand EV's were DC and had regen built in.

Why is it so hard to use regenerative braking on a series wound motor?
Series wound EV motors have field coils instead of magnets, which are not normally energised when the car isn't being powered forward. No current in the fields results in no current generated when the motor is turned by the inertia of the car coasting/slowing down. Specialised controllers can energize the fields, but even that isn't a simple solution because series motors run above 96V are usually "timed", that is the brush, armature, and field magnet positions are calculated and set in an advanced position to optimize the motor producing torque when it is powered by batteries. Attempting to cause that motor to become a generator will mean that the timing optimization is exactly opposite what it should be for producing power instead of consuming it, and the effective result is that excessive arcing and/or plasma is generated on the armature's commutator, usually ruining it. The motor can be timed in a "neutral" manner, but then it makes a less efficient drive motor, and the regen it might put out will be consumed just making up for the efficiency it lost by being mis-timed.

Is regen possible on a Permanent Magnet DC motor?
Regenerative braking is easier with a permanent magnet motor because the magnets do not need to be energised. Regenerative braking is achieved by having the controller reverse the terminals to the motor so that current flows in the opposite direction. Since these motors are also brushed they suffer from the same advanced timing problems when used at voltages greater than 96V. Typically though PM motors are smaller anyway and therefore run with neutral timing and lower voltages. Thus many motorcycles and small EVs run regen using PM motors at less than 96V.

Can an alternator or generator be attached to the drive train to implement Regen?
Theoretically a generator (or alternator) that is connected to the drive train when the brake pedal is pressed would be a simple way to implement regenerative braking. In practice it could be quite difficult to mount a generator of sufficient size (approximately the size of the motor for similar braking power to your acceleration) or enough 12V alternators to charge the individual batteries in the pack in an already crowded engine compartment. Also the additional complexity of a clutch system to remove unnecessary drag when not in use and controlling the current spike into the batteries further complicates things. All of this for a 10% gain in range is often not deemed to be worth the money, time and additional weight it would take to implement it. That said it would be an interesting experiment and challenge, and serve as a good example if someone was willing to try such a set-up.

Hybrid gas/electric automobiles now use a completely different method of braking at slower speeds. While hybrid cars still use conventional brake pads at highway speeds, electric motors help the car brake during stop-and-go driving. As the driver applies the brakes through a conventional pedal, the electric motors reverse direction. The torque created by this reversal counteracts the forward momentum and eventually stops the car.

But regenerative braking does more than simply stop the car. Electric motors and electric generators (such as a car's alternator) are essentially two sides of the same technology. Both use magnetic fields and coiled wires, but in different configurations. Regenerative braking systems take advantage of this duality. Whenever the electric motor of a hybrid car begins to reverse direction, it becomes an electric generator or dynamo. This generated electricity is fed into a chemical storage battery and used later to power the car at city speeds.

What are AC motors like at providing Regen?
Unlike brushed motors, AC motors (ie induction or perm mag) can provide regen very efficiently. AC motors can usually regen at almost the same efficiency as when motoring. Regen for AC motors also comes at no extra cost to the existing controller, although bare in mind that AC motors and controllers cost more than brushed controllers. Before purchasing an AC drive system it's worth investigating its regen capabilities.


Created by mattW, 02-11-2008 at 12:37 AM
Last edited by DavidDymaxion, 08-24-2011 at 12:15 AM
45 Comments , 53455 Views
  #2  
Old 02-18-2008, 05:43 AM
Thalass's Avatar
Thalass Thalass is offline
Senior Member
 
Join Date: Dec 2007
Location: Perth, Western Australia, Australia
Posts: 110
Thalass is on a distinguished road
Default Re: Regenerative Braking

What kind of range boost can I expect using Regen?


If accelerating at a certain rate costs me 200A, how much charging current can I expect from a similar rate of de
celleration?


Just some things I was wondering. :P
Reply With Quote
  #3  
Old 02-18-2008, 01:14 PM
mattW's Avatar
mattW mattW is offline
Super Moderator
 
Join Date: Sep 2007
Location: Sydney, Australia
Posts: 882
Blog Entries: 18
mattW is a glorious beacon of lightmattW is a glorious beacon of lightmattW is a glorious beacon of lightmattW is a glorious beacon of lightmattW is a glorious beacon of light
Default Re: Regenerative Braking

I typically hear that regen gives about a 10% increase in range, obviously that will vary with city/highway driving but 10% is the number that is always thrown around. And I think that the prius regen returned about 30%, so you would get 60A back. These aren't solid numbers from studies or anything, just what most people say when talking regen... hope that helps.
Reply With Quote
  #4  
Old 06-22-2010, 02:20 AM
deanbo deanbo is offline
Member
 
Join Date: Jun 2010
Posts: 62
deanbo is on a distinguished road
Default Re: Regenerative Braking

Can regen create more electricity than the batteries can handle? Or should I say does some of the electricity created by regen get wasted because batteries can only accept so much current all at once?
Reply With Quote
  #5  
Old 06-22-2010, 06:39 AM
major's Avatar
major major is offline
Senior Member
 
Join Date: Apr 2008
Location: Ohio, USA
Posts: 6,094
major has a spectacular aura aboutmajor has a spectacular aura about
Default Re: Regenerative Braking

Quote:
Originally Posted by deanbo View Post
Can regen create more electricity than the batteries can handle? Or should I say does some of the electricity created by regen get wasted because batteries can only accept so much current all at once?
I guess so. Much depends on the system. Like the particular battery. And then the application. Like most often, the scenario used is that the EV is fully charged by the guy who lives at the top of a hill. So he goes down that hill and tries to regen into a fully charged battery. What happens? The system is smart enough not to regen. So there is no "electricity" wasted. Electricity is not generated and therefore no braking torque. What's the guy to do? Duh, use the original braking system. That is what it was designed to do. And if the guy was smart enough to figure out his driving mission ahead of time, maybe he would not fully charge his battery so he could take advantage of the potential energy available at his location.

And yes, some of the electricity used to charge batteries is wasted, whether the charge current comes from a battery charger or from regen. Batteries, like most things, are not perfect. So there are losses associated with charging and with discharging batteries.

Regards,

major
Reply With Quote
  #6  
Old 11-07-2010, 08:50 AM
DJBecker DJBecker is offline
Senior Member
 
Join Date: Nov 2010
Location: Annapolis MD
Posts: 230
DJBecker is an unknown quantity at this point
Default Re: Regenerative Braking

Quote:
Originally Posted by mattW View Post
I typically hear that regen gives about a 10% increase in range, obviously that will vary with city/highway driving but 10% is the number that is always thrown around. And I think that the prius regen returned about 30%, so you would get 60A back. These aren't solid numbers from studies or anything, just what most people say when talking regen... hope that helps.
I suspect the Prius 30% figure is solely for optimistic regenerative cycle efficiency: how much of the power slowing down can be returned in later traction effort if everything is perfect.

A quick back-of-the-envelope calculation shows this to be likely. The motor and controller are each about 85% efficient. A very good battery is about 70% efficient at return charge energy. Multiplying these together, remembering that the power is making two trips through the motor and controller, gives you 0.85*0.85*0.70*0.85*0.85 = 36%. Factor in the drivetrain losses, at 90% efficient (although a dyno operator will give you a 130% factor for wheel-to-shaft horsepower) and you get very slightly under 30%.

Considering that each of these numbers is optimistic (e.g. a cold, loaded differential can absorb 10HP at highway speeds, the regenerated voltage will be far from optimal for charging), I suspect the 30% number is for the traction/power unit only, not as-installed: you won't be able to climb a 300ft hill from the power going down a 1000ft hill.

Consider regenerated power a tiny extra bump in efficiency, with a bigger bump in braking effectiveness. It's not a major win in range, despite what the press stories would have you believe.
Reply With Quote
  #7  
Old 01-05-2011, 10:49 PM
algea07 algea07 is offline
Member
 
Join Date: Sep 2010
Posts: 66
algea07 is on a distinguished road
Default Re: Regenerative Braking

what efficiency does a pm dc motor get in regen? you mentioned that AC motors are quite good but what about pm dc?

might as well ask about SepEx while i'm here.

thanks for your time.
Reply With Quote
  #8  
Old 01-05-2011, 11:20 PM
frodus's Avatar
frodus frodus is online now
Senior Member
 
Join Date: Apr 2008
Location: Portland, OR
Posts: 3,473
frodus will become famous soon enoughfrodus will become famous soon enough
Default Re: Regenerative Braking

it always depends on the particular motor, and only the manufacturer will know that.
__________________
Travis Gintz
Electro Motive Force, LLC.
Reply With Quote
  #9  
Old 01-12-2011, 10:13 AM
mikep_95133 mikep_95133 is offline
Member
 
Join Date: May 2009
Location: San Jose, CA
Posts: 68
mikep_95133 is on a distinguished road
Default Re: Regenerative Braking

I hope it's ok if I updated this part of the wiki on regen. It's very valuable. Soleq's use sep ex to generate regen. I've personally measured 20-40% regen return on my vehicle.

Mike
Reply With Quote
  #10  
Old 01-12-2011, 05:49 PM
Duncan Duncan is offline
Administrator
 
Join Date: Dec 2008
Location: Southland New Zealand
Posts: 2,268
Duncan will become famous soon enough
Default Re: Regenerative Braking

Hi Mikep
Those numbers sound incredibly high - they imply that 30% of your energy is wasted in the brakes
I have only seen numbers of that level in inner city buses with frequent stops
Reply With Quote
Sponsored Links
Reply

Share or Bookmark this

Article Tools
Display Modes

Posting Rules
You may not post new threads
You may not post replies
You may not post attachments
You may not edit your posts

BB code is On
Smilies are On
[IMG] code is On
HTML code is Off

Forum Jump

 

All times are GMT -6. The time now is 10:58 AM.


Powered by vBulletin® Version 3.8.7
Copyright ©2000 - 2014, vBulletin Solutions, Inc.
Copyright 2009 Green Web Publishing LLC
Ad Management by RedTyger