Week-6 Challenge: EV Drivetrain
1. Which types of power converter circuits are employed in electric and hybrid electric vehicle?
The overall management of electric vehicle load depends on the EV technology.
The basic EV Technology architecture is shown in the figure 1. There are two classifications in EVs as; plug-in hybrid EVs (PHEVs) and Battery EVs (BEVs) respectively.
The different configurations of EV power supply show that at least one DC/DC converter is necessary to interface the FC, the Battery or the Super capacitors module to the DC-link.
The main components of an Electric Vehicle are DC-AC inverter, DC-DC converter, battery, and electric motor.
There are two major power electronic units DC-DC converter and DC-AC inverter
Usually, AC motors are used in HEVs or EVs for traction and they are fed by inverter and this inverter is fed by the DC-DC converter
Every EV has a battery that gives a certain voltage but in order to cope up with various device requirements and functionalities, a proper power circuit is required along the signal path and improves the performance. This device is known as a DC/DC converter.
In general, the DC-DC converters are used to control the unregulated DC input voltage to the desired voltage level with improved efficiency (> 90%). The higher switching frequency operation in DC-DC converters varies between 10 kHz to 1 MHz in order to reduce the sizes of passive and active components. The switching devices such as MOSFETs, IGBTs, and SiC devices are suitable for low, medium, and high power applications. Based on switching the DC-DC converters can be classified as hard-switching and soft switching. In recent times, the soft-switching converters are more dominant and preferred operation in EVs to overcome limitations such as operating frequency, power density, and volume in the hard-switched converters. Mainly, DC-DC converters are classified into two types, non-isolated and isolated converters. The non-isolated DC-DC converters have different power topology circuits such as buck, boost, buck-boost, and Cuck. Whereas, an isolated power topology circuits are also classified into bidirectional, flyback, forward, push-pull, half-bridge, and full-bridge DC-DC converters with a high-frequency transformer.
Generally, the batteries of EVs typically output several hundred volts of DC. Inside the vehicle, DC motor can use up to three times the voltage provided by the battery and the electric components such as radio, dashboard readouts, air conditioning, and in-built computers and displays vary in their voltage requirements running on much lower voltage. In electric vehicles that use a DC motor, actually running the motor can use up to three times the voltage provided by the battery. With the help of the right converter, we can bridge this gap without having to resort to a larger, prohibitively-heavy battery.
The amount of power flow between the input and the output can be controlled by adjusting the duty cycle (ratio of on/off time of the switch )
Usually, this is done to control the output voltage, the input current, the output current, or to maintain a constant power
DC-AC converters (Inverter)
An inverter is a device that converts DC power from the battery to AC power used in an electric vehicle motor. The inverter can change the speed at which the motor rotates by adjusting the frequency of the alternating current.
The use of Inverters can increase or decrease the power or torque of the motor by adjusting the amplitude of the signal.
It plays a significant role in capturing energy from regenerative braking and feeding it back to the battery.
The key component is that it has a direct impact on the on-road performance, driving range, and reliability of the vehicle also as a consequence of their weight and size.
The output voltage waveform of an ideal inverters should be sinusoidal. However, the waveform of practical inverters is non-sinusoidal and contains certain harmonics.
Question:2
I solved it in the course notebook. The images of the calculation are shared.
3. Refer to the blog on below topic:
Induction Versus DC brushless motors by Wally Rippel, Tesla
Explain in brief about author’s perspective.
This blog explains a detailed note on differences between induction motor and brushless DC motor and describes each motor in much detail. At first, he initially narrates the transformation and changes from fuel-powered vehicles to EVs.
Conventional fuel-powered vehicles, engines are mostly flat-heads, Hemis, straight, opposed, and V configurations. And on and on. . There is no one best engine type because it might have ended all the choices and thereafter only the one best engine type would be in production. There are different types of engines to suit personal requirements for the user like price and performance. This is also applicable to electric vehicle drives.
Nowadays the Lead has been replaced by lithium and lithium-ion batteries and its types while DC motors replaced by either DC brushless or induction according to price and performance. The contactors are replaced with modulating inverters.
With brushless DC machines, the rotor includes two or more permanent magnets that generate a DC magnetic field (as seen from the vantage point of the rotor). In turn, this magnetic field enters the stator core (a core made up of thin, stacked laminations) and interacts with currents flowing within the windings to produce a torque interaction between the rotor and stator. As the rotor rotates, it is necessary that the magnitude and polarity of the stator currents be continuously varied in such a way that the torque remains constant and the conversion of electrical to mechanical energy is optimally efficient. The integral part of brushless DC motor is an inverter which provides the current control of the power electronic components.
In induction motor drives, we consider a 3 phase induction motor where the stators are identical to the DC brushless motor and both have three sets of distributed windings that are inserted within the stator core slots available. But the main difference between the two machines is with the rotor part.
Unlike the DC brushless rotor, the induction rotor has no magnets. It is just stacked steel laminations with peripheral conductors that form a shorted structure. Current flowing in the stator windings produce a rotating magnetic field that enters the rotor. In turn, the frequency of this magnetic field is equal to the difference between the applied electrical frequency and the rotational frequency of the rotor. Accordingly, an induced voltage exists across the shorted structure that is proportional to the difference in speeds between rotor and frequency. In response to this voltage, currents are produced within the rotor conductors that are approximately proportionate to the voltage, which gives the speed difference. Finally, these currents interact with the original magnetic field to produce forces of which is the desired rotor torque.
When a 3-phase induction motor is connected to utility type 3-phase power, torque is produced at the outset; the motor has the ability to start under load. No inverter is needed.
The fact that induction motors are directly compatible with conventional utility power gives its edge over brushless and other types. But a brushless DC motor produces no starting torque when directly connected to fixed frequency utility power. And whose phase sequence is maintained in step with the angular position of the rotor.
The 3-phase induction motors have great utility and advantages with some severe limitations. They cannot operate from DC so it requires an AC source. The shaft speed is proportional to line frequency. Hence, when it is used with utility power, they are constant speed machines. But when operated from utility power, they have limited starting torque and limited running peak torque capabilities, when compared to brushless DC machines.
An inverter circuit is required to power an induction machine from a battery or other DC source and variable speed can be achieved by adjusting the inverter frequency with ease of operation. But still, the performance of torque is low compared with Brushless DC machines. If the induction motor is added with some feedback loops such that the inverter produces the desired motor frequency and finally it competes with DC motor and DC brushless for vehicle applications.
Both DC brushless and induction drives use motors having similar stators. Both drives use 3-phase modulating inverters. The only differences are the rotors and the inverter controls and the control algorithm with code.
One of the main differences is that much less rotor heat is generated with the DC brushless drive. Rotor cooling is easier and peak point efficiency is generally higher for this drive. And the DC brushless drive can also operate at unity power factor, whereas the best power factor for the induction drive is about 85 percent. This means that the peak point energy efficiency for a DC brushless drive will be higher than for an induction drive.
Induction machines are more difficult to control and complex. But it achieves stability over the entire torque-speed range and but temperature issue is more difficult with induction than with DC brushless. This means added development costs results in little or no recurring costs.
Finally, the author concluded that DC brushless drives will dominate in the EV markets for HEV, PHEV vehicles whereas the induction drives will dominate the high-performance pure electrics BEVs. Both of them dominates in the future electric mobility market.