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.
