Regenerative Braking, part 1

EV-205a

This course is presented as part of Evannex Universitya free, open learning environment that presents concise, video-based mini-courses for those who have interest in electric vehicles (EVs)

Kinetic energy - 1From Wikipedia:the energy it possesses due to its motionwork required to accelerate an objectthe body maintains it KE unless its speed changesthe same amount of work is done

Any discussion of regenerative braking should begin with a brief explanation of kinetic energy.

>>This, from Wikipedia:

In physics, the kinetic energy of an object >> is the energy that it possesses due to its motion.

It is defined as the >> work needed to accelerate a body of a given mass from rest to its stated velocity. Having gained this energy during its acceleration, >> the body maintains this kinetic energy unless its speed changes. >> The same amount of work is done by the body in decelerating from its current speed to a state of rest.

Kinetic energy - 2Kinetic energyequationKE = (Mass x velocity squared)/2Physics energy cannot be destroyedSo, when speed (v) changesKE must be transformed into something else

>> Represented as a mathematic equation:>> KE = (1/2 mass x velocity squared, measured in Joules

From your high school physics class, you may recall that:>>energy cannot be destroyed

>> So, what happens when the speed of an object of a given mass changes?

>> the objects KE must be transformed into something elseand its this last observation that lies at the core of regenerative braking.

But enough abstractions lets talk about a moving car.

Kinetic energy of a carA moving car: has a very large mass has a velocity KE is large at high speedHow do we stop a conventional car? stop providing energy to the motordepress the brake pedalEnergy is dissipated as heat

>> A moving car is a large object traveling at a given speed: >> it has a very large mass >> and can have significant velocity >> so its KE is impressive.Any unfortunate sole who has backed into his roadside mailbox understands this from personal experience

>> How do we stop a conventional car?As I mentioned, as a car slows, its kinetic energy must be transformed into something else another form of energy.

>> First, you stop providing energy to the motor, thereby allowing air friction (drag) to slow the car>> Next, you depress the brake pedalCalipers press on disks and create friction that slows the rotation of the wheels Energy is dissipated as heat

For ICE vehicles As braking occurs, a significant amount of heat is createdThere is no easy way to recover this heat energy, and it is lostFor an ICE vehicle80 percent of the energy input to propel the car forward is lost to heat and other affects as the car is brought back to zero velocity

>> As braking occurs, a significant amount of heat is created>> There is no easy way to recover this heat energy, and it is lost>> For an ICE vehicle>> 80 percent of the energy input to propel the car forward is lost to heat and other affects as the car is brought back to zero velocity

As an aside, some ICE vehicle manufacturers are also using regenerative braking systems to replace the alternator, which can sap as much as 10 percent of the energy output of a ICE. They store the electricity generated in a super capacitor, rather than a battery. That a good thing to do, but the EV approach is considerably more interesting.

For EVsBraking can recapture as much as 40 to 50% of the vehicles kinetic energy, but NOT if you use the conventional braking paradigmInstead, control electronics recognizes that: power to the electric motor has been discontinuedregenerative braking is then initiated

For electric vehicles,>> Braking can recapture as much as 40 to 50% of the vehicles kinetic energy, but not if your disc brakes are used as the sole mechanism for slowing the car>>Instead, control electronics recognizes that: >>power to the electric motor has been discontinued>>it then initiates regenerative braking

The Electric MotorThe statorThe rotor Current creates a magnetic fieldAC causes field to vary between N and SThe rotor chases the magnetic field and rotates

Source: Tesla Motors

To understand how regenerative braking happens, we have to look at the electric motor. As we discussed in an earlier EVU mini-course,

>> Coils of copper wire run through a stack of thin steel plates and form a stator. >> The rotor is a steel shaft with copper bars running through it. It rotates, and ultimately, turns the wheels of the EV.

But what makes the rotor rotate?

>> The flow of alternating current into the copper windings of the stator creates a magnetic field.>> Alternating current causes the field to vary between N and S, appearing to move in a circular path>> The rotor chases the magnetic field and rotates as a consequence.

One more important pointan electric motor exhibits an interesting duality. It can be a motor or it can be a generator. Lets take a look at that.

Motor to generatorwhen an electric motor receives electric energy as input, it converts the energy derived from an electromagnetic field into mechanical energy transmitted by the motors rotorif input to the motor is electrical energy, output will be mechanical rotationbut if the electric input stops and the mechanical rotation derived for the KE is used as input the motor becomes a generator and produces electrical energy as output

>> When an electric motor receives electrical energy as input, it converts the energy derived from an electromagnetic field into mechanical energy transmitted by the rotor. This provides torque that is rotational energy that causes the drive wheels of a vehicle to rotate.>> stated more succinctly, if input to the motor is electrical energy, output will be mechanical rotation or torque

>> but if the electric input stops, and the mechanical rotation derived from the KE is used as input >> the motor becomes a generator and produces electrical energy as output

In the second part of the EVU mini-course, well examine how these characteristics can be used to implement a regenerative braking system.Well also look at some of the driving characteristics of a regen system.

a free study guide for all EVU mini-courses is available for download from our website For a complete list of mini-courses and the study guide, visit: www.evannex.com

OlharesRouletBeats d'Amor, track 12014-05-18T14:02:41118051.055OlharesRouletBeats d'Amor, track 12014-05-18T14:02:41118051.055