Week-6 Challenge: EV Drivetrain
Objective:
1. Understand various types of power converter circuits are employed in an electric and hybrid electric vehicles.
2.To determine the steady speed of EV with duty cycle 70 %.
Introduction:
1.Different types of power converter circuits are employed in an electric and hybrid electric vehicles.
Ans:
The General configuration of EV is as shown in the figure:
The main components of an Electric Vehicle are a DC-AC inverter, DC-DC converter, battery, and electric motor.
Usually, AC motors are used in HEVs or EVs for traction and they are fed by an inverter and this inverter is fed by the DC-DC converter
DC-DC converter:
The most commonly DC-DC converters used in an HEV or an EV are:
Unidirectional Converters:
They cater to various onboard loads such as sensors, controls, entertainment, utility, and safety equipment.
Bidirectional Converters:
They are used in places where battery charging and regenerative braking are required. The power flow in a bi-directional converter is usually from a low voltage end such as a battery or a supercapacitor to a high voltage side and is referred to as boost operation. During regenerative braking, the power flows back to the low voltage bus to recharge the batteries know as buck mode operation.
Buck Converter: The buck converter is step down converter and produces a lower average output voltage than the dc input voltage.
Boost converter: The output voltage is always greater than the input voltage.
Buck-Boost converter: The output voltage can be either higher or lower than the input voltage
DC-AC converters:
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. It plays a significant role in capturing energy from regenerative braking and feeding it back to the battery.
2. An Electric Vehicle's powertrain with 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 is 'd2'.
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, KΦ= 0.7 Volt second
Chopper Switching frequency= 400 Hz
The vehicle speed-torque characteristics are given by the below equation
What is EV steady-state speed if the duty cycle is 70%?
As the torque formula is given, we need to find the speed in rad/sec. Use the other expression available which is given below:
T = torque
V = armature voltage
R = resistance
w = speed rad/sec
Compare the two equations:
24.7 + 0.0051*w^2 = ((v*k)/R) - ((k^2)*w/R)
24.7 + 0.0051*w^2 = (72*0.7*0.7)/0.5 - (0.7^2/0.5)*w = 24.7 + 0.0051*w^2
70.56 - 0.98*w = 24.7 + 0.0051*W^2
w^2 + 192.15*w - 8992.15 = 0
In order to find the roots use following formula
As the speed value can not be negative
w = 38.95 rad/se
3 . Refer to the blog on below topic:
Induction Versus DC brushless motors by Wally Rippel, Tesla
Explain in brief about author’s perspective.
ANS:
The author writes this blog to entail the differences between the two most commonly used motor drives in the automobile industry: the DC brushless motor drive and the induction motor drive. Rippel goes into detail about the differences in 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. In the last lines of the opening paragraph, Rippel writes that "there is no one best engine type" and that it is all a matter of personal preference when it comes to choosing an engine. It is in this same regard that he ends the blog stating that there is still 'no winner' when it comes to motor drive selection either. In spite of both motor drive types having similarities, they also have their benefits and drawbacks.
For DC Brushless drives, we are told that their construction implements 2 or more permanent magnets at the rotor to generate a DC magnetic field. It also consists of a stator core (made up of thin, stacked laminations) It also uses an inverter for the current control, meaning that it cannot run on a DC power supply as the current direction has to be changed repeatedly to maintain a constant torque.
For induction drives, there rotor has no magnets, but instead stacked steel laminations with buried peripheral conductors which form a 'shorted structure'. It is the current in the stator windings that generates a rotating magnetic field that interacts with the rotor.
Induction motors can be used directly with power from the mains and do not require an inverter to start the drive and generate the starting torque. It has the ability to start under the mains load applied. However, DC brushless drives cannot do the same. They cannot generate a starting torque when connected to a mains power supply. They require an inverter to do this.
Induction motor drives cannot run on a DC power supply and are solely dependent on an AC power supply. Also, when they are connected to a 3 phase mains supply, the rotor rotates at a constant speed and also they have a limited starting torque as well as peak torque. In order to rectify this, an inverter can be used to operate the induction drive from a DC power supply or a battery. Also, if the inverter frequency is varied, then the constant speed is no longer an issue - it can be varied.
DC Brushless drives use permanent magnets, and cannot change the strength of their magnetic fields. It would be desirable if they could, because when the maximum torque is needed by the rotor, the magnetic field strength should also be at a maximum level. This allows for the inverter and motor currents to be at a minimum, thereby reducing the power loss by heat dissipation. Similarly, when the torque required is minimum, the magnetic field strength should also be minimized to reduce hysteresis and eddy current losses. The permanent magnets used are also quite expensive costing $50 per kilogram and their magnetic fields are difficult to control if ferromagnetic material gets close enough to the field.
On the other hand, induction drives do not have permanent magnets and so they can vary their magnetic field strength. If used with a smart inverter, they can be operated with minimum hysteresis and eddy current losses and maximum efficiency as the magnetic field strength can be changed according to the requirement by the rotor. These losses do not increase if the size of the induction drive also increases but as a DC brushless drive gets bigger, its losses increase and its efficiency decreases. So induction drives have a cost advantage compared to DC brushless drives
Although, DC Brushless drives generate less heat at the rotors. So rotors more easily and have a higher peak point efficiency than induction drives. In addition, the DC brushless motor can operate at unity power factor i.e. 1, whereas induction drives can operate up to a power factor of approximately 85%.
In conclusion, Rippel has elaborated on the different aspects to compare the induction and DC brushless motor drives and has detailed the advantages and disadvantages of both types. As mentioned earlier, at the end of the day it depends on the preference of the user. If a user is seeking high performance with comparatively lesser costs, then the induction drive would be better suited for those requirements. However, if it is high peak point efficiency, high power factor, and easier control that a user seeks, then the DC brushless drive would be the smarter choice. Both types have their place in the automobile industry and will retain their relevance in their respective applications. Rippel believes that due to the increasing performance of vehicles, one day both may be used side by side under the hood of a vehicle in order to get the best of both drives. This would result in a new kind of hybrid that combines the characteristics of an ordinary vehicle e.g. sedan with that of a sports car.
Conclusion:
When it comes to hybrid or plug in hybrid vehicle market dc brushless motor will dominate. However, Induction motor drive will always dominate for high performance and pure electrics. So, author have not mentioned who is clear winner among two rather he emphasis on advantage and disadvantages of one drive over another.