15-01-2013, 12:00 PM
Regenerative Braking System for an Electric Bicycle
1Regenerative Braking.pdf (Size: 1.96 MB / Downloads: 92)
The Big Idea
The concept for this project is to demonstrate a regenerative braking system on an electric bicycle. The author will build the regenerative braking system (hereafter referred to as RBS) from scratch but will use existing electric bicycle parts. The ultimate goal is to use the energy regained from the RBS as an acceleration “boost” on the bicycle.
The idea was first conceived as an extension of Matt Corley’s electric bike conversion project [1], but it was later found that the project would be incompatible with Corley’s bike for reasons discussed below. Further inspiration was drawn from a similar project done by Brett White, a hobbyist online. [2]
Through this project the author hopes to gain a better understanding of the technologies employed in modern green-energy solutions such as regenerative braking in the Toyota Prius as well as explore the challenges facing green technology in general.
Introduction
Green energy is perhaps the most relevant area of science and engineering moving forward; as the world runs out of oil, electric transportation solutions as well as energy recovery solutions will become increasingly important.
I decided to formulate a project around these ideas so as to gain a greater understanding in these areas. A regenerative braking system makes use of the fact that electric motors and electric generators are essentially the same things, just run in reverse.
Bicycle
This component of the build is the overall frame onto which the system will be mounted. Since the motor is built for bicycles this will obviously be a bicycle. Initially the author was unable to find a suitable bicycle, so in the meantime he built a tabletop rig to test the wheel (Figure 2). The tabletop rig encountered some problems, namely that it was unable to hold the wheel perfectly still during operation thus causing some minor fraying on the motor lead’s casing. Further use of the tabletop mount is inadvisable as it may cause more permanent damage to the hub motor.
Battery
Currently the battery in use was borrowed from Matt Corley’s ebike. It is a 24 V, 8 Ah NiCad battery. [1] Since the motor and controller are actually rated up to 36 V it would be nice in the future to get a new 36 V battery, which would allow both projects to be complete simultaneously instead of swapping the battery between the two bikes. As shown in Figure 1 by the arrow, the battery supplies current to the controller. Note that this is a one-way relationship.
Controller
As its name suggests, the controller controls the amount of power supplied to the motor. The controller is rated at 36 V and 30 A and was included in the ebike conversion kit. Since the motor is a brushed DC motor, the internal workings of the controller are fairly simple. A delay chip supplies power to the motor at timed intervals, and different coils in the motor are powered each time as the motor turns to align with the permanent magnets in the housing. Note that this controller has a one-way relationship with the battery and motor, meaning that it does not do regenerative braking.
Throttle
The throttle was included in the controller kit. It is a half-twist throttle. Throttles can work in several ways, but the most common and simple way is as follows. Inside the handle is a Hall-effect sensor which can supply some voltage in response to change in magnetic field. As the handle is twisted it changes the Hall-effect sensor’s proximity to a magnet, thereby changing the voltage produced by the sensor. This is interpreted by the controller and the power supplied to the motor is varied. [5]
Motor
The motor purchased for this project is a Crystalyte hub motor. Hub motors are a special type of motor fixed onto the hub of a wheel. Internally hub motors work the same way as regular DC motors. This particular model uses brushed current switching, but brushless models are common as well. The motor is actually the key component in both the forward drive and the regenerative braking functions of the bike. During “powered/pedal assist” mode the motor is used as a motor and can move the bicycle forward. In “regenerative braking” mode the motor acts as a generator and supplies power back to the controller system. The motor’s resistance to turning when wired to generate current is what slows the bike down. Hub motors eliminate the chain drive dilemma. Furthermore this model is not geared, making it an ideal motor with which to do regenerative braking. In fact in the lab it was possible to generate currents of up to 5 A by simply turning the wheel by hand. Other tests conducted on the motor include a maximum RPM test (Figure 4) and a torque test using a 24 V battery. The torque test did not produce any results as the wheel exerted more force than our testing apparatus could handle, but the RPM graph is shown below.
Regenerative Circuit
This component is the part that the author must build over the course of the project. The regenerative braking circuit is in some ways a simple switch that sits between the controller and the motor. When regenerative braking is engaged (using a switch on the handle bar), the regenerative braking circuit cuts the connection from the controller to the motor and instead puts a load on the motor because of some very low resistance components like shunts. A shunt is basically a very low resistance resistor placed in a circuit. One can measure the voltage drop across it, and from that information calculate current, power, etc. (Note also that the lower the resistance in a circuit the greater the load is on the power source, so a component like a shunt puts a heavy load on the regenerative circuit, which is what is needed for braking.) As the wheel is kept spinning by its forward momentum, the torque from the load will slow the wheel down, thus creating the braking effect. The regenerative circuit will also have to protect the controller from any back EMF. Finally the regenerative braking circuit will include a way to store the recovered energy back into a capacitor or into the batteries. The preliminary sketches (Figure 5) are based on Brett White’s design.