How does regenerative braking work?
Hybrid and electric vehicles use this bit of tech a lot to improve efficiency, but how does it work?
Regenerative braking is a technology employed on hybrid and full-electric cars, and is a large contributor to their high efficiency. When a conventional car applies the brakes, the car is slowed down, and its kinetic energy is converted into heat energy at the pads and rotors. This wastes the energy we put into accelerating the car. Regenerative braking is the process of gathering electrical energy (or powering up a kinetic energy storage device) by slowing down the car, thus converting the kinetic energy of the car back into stored energy that can be used later.
Basic diagram of a DC motor, from www.electrical4u.com
So how does regenerative braking work in a technical sense? Well, essentially we turn the electric motor into a generator. If you every played with a dynamo, or just spun a DC motor fast with an LED across the teminals, you would quickly see that a motor turned by an external force acts as a generator. This is because when you spin the motor you move its coils through the magnetic field from its permanent magnets. When you move a conductor (in our case, a metal such as copper) through a magnetic field, it induces a current, hence in our case, spinning the motor creates an electrical output that we can use to charge our batteries. The presence of this current also means the magnets now apply a force on the conductor that resists its direction of travel. In the case of our motor/generator this is felt as resistance at the shaft, essentially the same as braking.
Now obviously, most car motors are a lot more complex than a simple DC motor, but the principle is still fundamentally the same. If you want a more specific explanation of Tesla tech, check out the bonus bit below the video!
Braking produces huge amounts of energy, regen braking can help, but it can't absorb all of it at this point in time.
So why don't we do 100% of our braking regeneratively? Well for a start, that generated current has to go somewhere, and typically batteries can discharge much faster than they charge. This means that we can't actually charge the battery properly in most heavy braking situations. Which means you need additional secondary storage, or have to waste the electricity by dumping it into a resistor or some other high load. It should also be noted that for race car applications we can also use a flywheel or other type of kinetic energy storage device to regeneratively brake, as opposed to an electric motor.
For more details on the concept of regenerative braking, as well as some of the advantages and disadvantages, check out the video below!
BONUS ADVANCED EXPLANATION: TESLA
It gets a little more complex with with the induction motors used in Teslas, as there are no permanent magnets there. Induction motors have a conductive core (rotor) that current is induced into, and sets of coils on the outside (stator) that induce a rotating magnetic field. When the current passing through these coils is controlled with a variable frequency inverter we can precisely control motor speed and torque, as well as provide regenerative braking.
An induction motor stator diagram, showing the multiple coils used to generate a rotating magnetic field. From www.allaboutcircuits.com
In this case, you use the inverter control to decrease the frequency applied by the stator windings to below that of the rotor, this is effectively a reversed slip of the rotor against the field. As long as the inverter continually manages the frequency lower than the rotational rate this will cause the total current in the stator to be generated rather than applied (i.e. reversed current). You will always need some input current to excite the rotor, as there is no electricity present in the rotor without a magnetic field present. This current can be supplied from the batteries or directly from the generated current, via the inverter, and is smaller than the amount of current/power generated. Keep in mind that because we have 3 phases, it is possible to be applying a current on one phase and be receiving a larger current in reverse on another phase (generated power).
So, regarding the induction braking system, fundamentally we provide power to the motor to keep it excited (reactive power), however this causes the motor to generate more power then we are applying (active power). We can subtract the reactive power from the active power and this will give us the usable power that we are generating. The reactive power can be taken from the active power and supplied back to the generator/motor. This process generates a torque on the rotor, which slows the car down, turning the kinetic energy into electrical energy.
A tesla motor, with the rotor visible through a cutout.
Dr Kyle Forster is a qualified Aerodynamicist, race car engineer, and the man behind JKF Aero, a firm that offers a variety of aerodynamic consultancy services for racing purposes. If you have any questions for Kyle or have any suggestions for future videos, drop them in the comments below!