Figure 3.2-2: Block diagram of the basic components used in our configurationFigure 3.2-3: Example of a specific PWM controller board [4]3.3 Software Design3. ApproachTo understand the approach used to design the electric motor configuration for the GM equinox electric vehicle, some basic knowledge is required. General knowledge of the electric hybrid vehicles electrical system is presented first to give the required background knowledge on different hybrid configuration vehicles. This section will also explain why the parallel hybrid configuration was chosen over the series configuration. The placement of the motor will also be discussed in the first section. The following sections will talk about the actual design of the electric motor configuration. Each part of the electric motor configuration will be explained in detail to fully develop the technical view of system. The software design will be discussed last with use cases and an information model to explain its operation.3.1 Theory3.1.1 Hybrid VehiclesFor the past seventeen years, the U.S. Department of Energy (DOE) has sponsored over twenty competitions to develop more efficient vehicles. The contests challenge engineering students to re-designa vehicle to minimize energy consumption, emissions, and greenhouse gases while maintaining or exceeding the vehicle's utility and performance. Hybrid vehicles combine a small combustion engine withan electric motor and battery pack. Over these years many electric hybrids have been developed and modified. However, the theory behind these electric systems has remained the same. There are two types of hybrid configurations, parallel and series configurations. The type of configuration chosen will have a high impact on which type motor configuration chosen for the vehicle. Conventional vehicle braking methods waste the energy generated when accelerating. HEV's have the advantage that they can capture the energy lost when slowing down or stopping the vehicle. This is known as regenerative braking. There are different methods of achieving this. The most common is to recharge the battery packs with the captured energy. The following section will discuss in detail the two types of configurations and why the parallel configuration was chosen as well as a brief discussion of the type motors that can be chosen. 3.1.1.1 Series Configuration When using a series configuration, the internal combustion engine (ICE) converts chemical energy of fuelinto mechanical power. This mechanical power of the engine is then converted to electrical power througha generator. The electrical power of the generator is then used as needed to charge the batteries which drive the electric motor. In this configuration, the drive-train of the electric vehicle is powered solely from the electric motor connected to the battery packs. Some benefits for using a series configuration are reduced engine power cycling because the engine never idles, a transmission may not be needed, and more options are available for mounting the engine and vehicle components [1]. Figure 3.1-1 shows a series configuration. Figure 3.1-1: Model of a series hybrid systemOne drawback to a series configuration is that it must have a larger electric motor than the parallel configuration due to the fact that the electric motor must power the entire drivetrain by itself. Another downside of the series configuration is that it requires larger battery packs than the parallel configuration. The motor is also working much harder trying to keep the batteries charged and propel the vehicle. 3.1.1.2 Parallel ConfigurationAs shown below in figure 3.1-2, in a parallel configuration the ICE powers the drive-train along with the electric motor. The sum of the ICE’s power and electric motor’s power is available to the drive-train. A controller must determine the load share of the ICE and the electric motor depending on the total requiredpower from the drive-train, the operating efficiency required, and the limitations of both machines. For instance, a vehicle may begin accelerating from a stop using only the electric motor, bypassing the ICE’s inefficient low-speed region. Also, when much power is needed, e.g. during acceleration, hill climbing, or passing, both the electric motor and ICE can run the drive-train. This sort of flexibility allows the vehicle designer to optimize the system for performance or efficiency by simply changing the control strategy.Figure 3.1-2: Model of a parallel hybrid systemAnother advantage of the parallel configuration is that when the motor and ICE are coupled to the same driveshaft, no dedicated generator is necessary. The system is also more robust and can use smaller individual components. The disadvantage of a parallel configuration is that it is more complex. Mechanically, two power sources must be connected to the system. Also, the system controller must account not only for how much power to put into the drivetrain but also how much each component will contribute [1].3.1.1.3 Regenerative BrakingWith parallel and series hybrids, regenerative braking captures the energy that is typically wasted as heat during braking and charges the battery packs. This is a huge gain for overall system efficiency because the system is able to recover a portion of the energy used to drive the vehicle and use it again. This improvement is especially noticeable in stop-and-go driving situations when the vehicle often brakes. Despite the name though, the other main condition for regenerative braking is coasting downhill. When maintaining a constant downhill speed, kinetic energy is removed from the system and stored at the same rate that gravitational potential energy replaces it. The ability to capture energy in these scenarios relies on the fact that a motor, when its rotor is driven by an outside force, can also act as a generator [2].3.2 Hardware design3.2.1 Chosen ConfigurationAfter consideration of both configurations, the parallel configuration was chosen for the competition. The reasons for choosing this configuration are described in the following statements. As shown above, the parallel configuration is more powerful than the series configuration. This is important because the hybridelectric vehicle has to meet or exceed the original specification of the GM equinox. Having a more powerful system is essential to achieving the original specification. The parallel configuration does not need a generator to provide
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