Week-6 Challenge: EV Drivetrain
1. Which types of power converter circuits are employed in electric and hybrid electric vehicles?
The four types of power converters employed in electric vehicles are,
- AC-AC converter.
- AC-DC converter.
- DC-AC converter.
- DC-DC converter.
AC-AC converter:
AC to AC converters is used for converting the AC waveforms with one particular frequency and magnitude to AC waveform with another frequency at another magnitude. This conversion is mainly required in case of speed controlling of machines, for low frequency and variable voltage magnitude applications as well. We know that there are different types of loads that work with different types of power supplies like single-phase, three-phase supply, and the supplies can be differentiated based on the voltage and frequency range also. We require a particular voltage and particular frequency for operating some special devices or machines. For speed control of induction motors, AC to AC converters (Cycloconverters) is used majorly. For obtaining a desired AC power supply from the actual power supply, we need some converters called AC to AC converters.
DC-DC converter:
DC/DC converters can be based on a number of different designs, the underlying function remains the same. We might be talking about a step-up converter, which turns a low-voltage input into a high-voltage output, or a step-down converter, which does the opposite.
The batteries of an electric vehicle typically output several hundred volts of DC. The electric components inside the vehicle, however, vary in their voltage requirements, with most running on a much lower voltage. This includes the radio, dashboard readouts, air conditioning, and in-built computers and displays.
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 DC-DC converter, we can bridge this gap without having to resort to a larger, prohibitively-heavy battery.
AC-DC converter:
In electronics, the Rectifier circuit is the most used circuit because almost every electronic appliance operates on DC (Direct Current) but the availability of the DC Sources is limited such as electrical outlets in our homes provide AC (Alternating current). The rectifier is the perfect candidate for this job in industries & Home to convert AC into DC. Even our cell phone chargers use rectifiers to convert the AC from our home outlets to DC. Different types of Rectifiers are used for specific applications. We mainly have two types of voltage types present that are widely used these days. They are alternating and direct voltage types. These voltage types can be converted from one type to another using special circuits designed for that particular conversion. These conversions happen everywhere. Our main supply which we get from power grids are alternating in nature and the appliances we use in our homes generally require a small DC voltage. This process of converting alternating current into the direct current is given the name rectification. Converting AC to DC is preceded by the further process which can involve filtering, DC-DC conversion, and so on. One of the most common parts of an electronic power supply is a bridge rectifier.
In applications of EV wireless charging, the rectifier and output filter capacitor is needed to convert the high-frequency AC to DC, in order to charge the power battery. Rectifier and the circuit after it are usually equivalent to a pure resistance load to design the system or control strategy
DC-AC converter:
An inverter converts the DC electricity from sources such as battery or fuel cells to AC electricity. The electricity can be at any required voltage; in particular, it can operate AC equipment designed for mains operation, or rectified to produce DC at any desired voltage.
An inverter is a device that converts DC power to the 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. It can also increase or decrease the power or torque of the motor by adjusting the amplitude of the signal.
2. An Electric Vehicle's Drivetrain with a 72V battery pack in shown in the diagram below. The duty ratio for acceleration operation is 'd1' and for the braking operation the duty ratio is2'. The other parameters of the electric vehicle are given below,
Motor and Controller Parameters:
Rated Armature voltage= 72 V
Rated armature current= 400 A
Ra= 0.5Ω, K`T=(V*K(Φ))/(Ra) - (k(Φ)^2)/(Ra)*w`= 0.7 V-s
Chopper Switching frequency= 400 H
The vehicle characteristics are given by the equation
TTR=24.7+0.0051w2
What is EV steady-state speed if the duty cycle is 70%?
Solution:
Given:
Rated armature voltage = 72 V
rated armature current = 400 A
Ra = 0.5 ohms
kΦ = 0.7 V-s
chooper switching frequency = 400 Hz
The vehicle characteristics are given by the equation
TTR=24.7+0.0051w2
From DC machine Torque speed relation:
`T= (V.k(Φ))/(Ra) - (k(Φ)^2)/(Ra) *w`
Voltage (motor)=Voltage of armature * duty cycle
= 72*0.7 = 50.4 volts.
T(motor)= (50.4*(0.7))/0.5 - ((0.7)^2/(0.5))*2 -------> (1)
given, TTR = 24.7+0.0051 w2 -------->(2)
Equating (1) and (2),
70.56-0.98w = 0.0051W^2 + 24.67
0.0051w^2+0.98w -45.89=0
By solving the above quadratic equation,
w= -231.08 rad/sec and w=38.92 rad/sec.
since vehicle speed should be in positive we can say that the value of w= 38.92 rad/sec.
3. Refer to the blog on the below topic:
Induction Versus DC brushless motors by Wally Rippel, Tesla
Explain in brief about the author’s perspective.
Solution:
The author writes this blog to give the differences between the two most commonly used motors in the automobile industry: the DC brushless motor and the induction motor. He goes into detail about the differences in the construction, working, and cost between the two drive types. However, with what he has written, he has not arrived at the conclusion that one type is better than the other. He even stated that there is still no winner when it comes to motor drive selection either. both the motor types have similarities, own kind of benefits and drawbacks.
The main difference 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. The DC brushless drive can operate at the unity power factor, whereas the best power factor for the induction drive is about 85 percent. The peak point energy efficiency for a DC brushless drive will typically be a few percentages higher than the induction motor drive.
Unlike the DC brushless rotor, the induction rotor has no magnets – just stacked steel laminations with buried peripheral conductors that form a “shorted structure.” Currents flowing in the stator windings produce a rotating magnetic field that enters the rotor. In turn, the frequency of this magnetic field as “seen” by the rotor is equal to the difference between the applied electrical frequency and the rotational “frequency” of the rotor itself. Accordingly, an induced voltage exists across the shorted structure that is proportionate to this speed difference between the rotor and electrical frequency. In response to this voltage, currents are produced within the rotor conductors that are approximately proportionate to the voltage, hence the speed difference. Finally, these currents interact with the original magnetic field to produce forces – a component of which is the desired rotor torque.
Thus the induction machine, when operated with a smart inverter, has an advantage over a DC brushless machine-Magnetic and conduction losses can be traded such that efficiency is optimized. This advantage is increasingly important as performance is increased.
In DC brushless machine, as the size of the machine grows, the magnetic losses increase proportionally and part load efficiency drops.
Conclusion:
DC brushless drives will likely continue to dominate in the hybrid and common plug-in hybrid markets, and that induction drives will likely maintain dominance for the high-performance pure electrics.