Week-6 Challenge: EV Drivetrain
QUESTION 1. Which types of power converter circuits are employed in electric and hybrid electric vehicle?
ANSWER-1 Power Electronic Converters: -In Electric and Hybrid Electric Vehicles (EV/HEV), the power electronic converters are considered as the key elements that interface their power sources to the drivetrain of the EVs.
In order to design highly efficient converters for the EV's power system, advanced DC/DC and DC/AC converters are required to adapt the output voltage and current levels with high power quality.
The 3 types of converters which are widely used in an Electric Vehicle are:
AC - DC Converter (On Board Charger): -AC to DC converters are most commonly used in Plug-in Hybrid Electric Vehicles (PHEV) & Plug-in Electric Vehicles (PEV). These vehicles get their propulsion power from an external electricity source (charging stations) which is alternating current. The battery needs to be charged using a direct current source. For this purpose, AC to DC converter is employed in a PHEV & PEV.
DC - DC Converter: -DC-DC converters can be used to interface the elements in the electric power train by boosting or chopping the voltage levels. By introducing DC/DC converters one can choose the voltage variation of the devices and the power of each device can be controlled. The power of the DC/DC converter depends on the characteristics of the vehicle such as top speed, acceleration time from 0 to 100 Km/h, weight, maximum torque, and power profile (peak power, continuous power).
DC - AC Converter (Inverter): -The DC/AC converter is required to convert the dc power to a suitable ac power for the electrical machine. The power output from battery pack (DC current) is converted using inverter to be used in electric motor (AC current).
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ANSWER-2
AIM:Find the EV steady state speed if duty cycle is 70%?
GIVEN: An Electric Vehicle's powertrain with 72V battery pack
The duty ratio for acceleration operation is 'd1' and for the braking operation the duty ratio is 'd2'.
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
SOLUTION:Vehicle Output Voltage (Vv) is given by: -
Vv = Battery Voltage (V) * Duty Cycle
Vv = 72 * 0.70
Thus, Vehicle voltage = 50.4V
Now, Equation for Vehicle torque is given by: -
Tv = [(Vv — w.KΦ)KΦ] / Ra
Where; Tv= Vehicle Torque (N-m)
Vv= Vehicle voltage (Volts)
w= steady state speed of vehicle (rps)
KΦ= Motor constant .
Ra= Armature Resistance
Substituting all these values in equation, we get:
Tv = [(50.04 — 0.7w) 0.7] / 0.5
Tv = 70.56-0.98w --------- equation-2
Tv = 24.7 + (0.0051) w^2 -------- given equation-1
Solving these two equations,
70.56 — 0.98w = 24.7 + 0.0051w^2
we get; w= 38.85rad/s and -230.95 rad/s --------- w= -230rad/s neglected due to negetive value
EV steady state speed = 38.85 rad/s
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AIM: Develop a mathematical model of a DC Motor for the below equation using Simulink.
ωω= V/Kϕ -Ra/Kϕ^2 .T
SOLUTION:
Let us draw a model for the above equation.
Let V = 50.4 Volts
Kϕ= 0.7
Ra= 0.552 and
also let consider torque as a linearly increasing signal.
MODEL IN SIMULINK:
:-
RESULT:
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ANSWER-4
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 in to 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 in 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 does 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 minimum 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 their 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 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 a 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.