How does regenerative braking systems work?
When a vehicle is cruising down the road, it has
kinetic energy, which is simply defined
as the energy something possesses because it is in motion.
At the most basic level, regenerative braking means
re-capturing the kinetic energy of the vehicle's motion and turning it
into another type of energy. Commonly, this is done by converting
kinetic energy into electricity to recharge the vehicles battery.
When you apply the vehicles brakes, instead of traditional
brake pads clamping down and letting the kinetic energy dissipate as
heat, regenerative braking systems use their
electric motors to slow the car and
generate electrical energy (hybrids still have conventional friction
brake systems that are used at higher deceleration rates). Hybrid cars,
where regenerative brakes are the most common, and electric cars can
reverse the flow of power through their electric motors backwards to
slow the car down. In one of those convenient engineering coincidences,
electric generators are the same as electric motors.
When you apply electrical current to a motor it turns,
converting electricity into mechanical torque, hence when you apply a
mechanical torque to the motor it induces electric current so it can be
used as a generator. Thus, using kinetic energy to turn the motor
generates energy. Put this collected energy into the battery and, the
next time you step on the accelerator, some of the energy you just saved
is used to get you moving again. Of course, friction and other energy
losses mean that you don't get to use all of the energy you captured (no
potential for a perpetual motion machine here, sorry), but this is one
reason the Prius and the Insight, for example, get such high mpg ratings.
Regenerative
Braking Limitations:
Traditional friction-based braking is used with mechanical regenerative
braking for the following reasons:
-
The
regenerative braking effect rapidly reduces at lower speeds,
therefore the friction brake is still required in order to bring the
vehicle to a complete halt, although malfunction of a dynamo can
still provide resistance for a while.
-
The
friction brake is a necessary back-up in the event of failure of the
regenerative brake.
-
Most
road vehicles with regenerative braking only have power on some
wheels (as in a 2WD car) and regenerative braking power only applies
to such wheels, so in order to provide controlled braking under
difficult conditions (such as in wet roads) friction based braking
is necessary on the other wheels.
-
The
amount of electrical energy capable of dissipation is limited by
either the capacity of the supply system to absorb this energy or on
the state of charge of the battery or capacitors. No regenerative
braking effect can occur if another electrical component on the same
supply system is not currently drawing power and if the battery or
capacitors are already charged. For this reason, it is normal to
also incorporate dynamic braking to absorb the excess energy.
-
Under
emergency braking it is desirable that the braking force exerted be
the maximum allowed by the friction between the wheels and the
surface without slipping, over the entire speed range from the
vehicle's maximum speed down to zero. The maximum force available
for acceleration is typically much less than this except in the case
of extreme high-performance vehicles. Therefore, the power required
to be dissipated by the braking system under emergency braking
conditions may be many times the maximum power which is delivered
under acceleration. Traction motors sized to handle the drive power
may not be able to cope with the extra load and the battery may not
be able to accept charge at a sufficiently high rate. Friction
braking is required to absorb the surplus energy in order to allow
an acceptable emergency braking performance.
For these
reasons there is typically the need to control the regenerative braking
and match the friction and regenerative braking to produce the desired
total braking output. The GM EV-1 was the first commercial car to do
this. Engineers Abraham Farag and Loren Majersik were issued 2 patents
for this 'Brake by Wire' technology. |
