Week-6 Challenge: EV Drivetrain
AIM:- To study EV Drivetrain
OBJECTIVES:-
1) Studying power converter circuits are employed in electric and hybrid electric vehicle.
2) Calculating EV steady state speed if duty cycle is 70%
3) Explaining in brief about author’s perspective about particular video link mentioned
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Q.1 Which types of power converter circuits are employed in an electric and hybrid electric vehicle?
Ans:-
Power converters:-
A converter is an electrical circuit that accepts a DC input and generates a DC output of a different voltage, usually achieved by high frequency switching action employing inductive and capacitive filter elements.
A power converter is an electrical circuit that changes the electric energy from one form into the desired form optimized for the specific load. A converter may do one or more functions and give an output that differs from the input. It is used to increase or decrease the magnitude of the input voltage, invert polarity, or produce several output voltages of either the same polarity with the input, different polarity, or mixed polarities such as in the computer power supply unit.
Fig 1.1Power converter specification
The DC to DC converters is used in a wide range of applications including computer power supplies, board-level power conversion and regulation, dc motor control circuits, and much more.
The converter acts as the link or the transforming stage between the power source and the power supply output. There are several kings of converters based on the source input voltage and the output voltage and these fall into four categories namely the AC to DC converter known as the rectifier, the AC to AC cyclo converter or frequency changer, the DC to DC voltage or current converter, and the DC to AC inverter.
The converter uses non-linear components such as the semiconductor switches, and linear reactive components such as the inductors, transformers, and capacitors for intermediate energy storage as well as current and voltage filtering. The size, weight, and cost of the converter are largely determined by these components.
There three basic converter circuits that are widely used in DC to DC converters are the buck, boost, and the buck and boost. These configurations are the most used topologies due to their simplicity and use of fewer components. Each has its advantages and drawbacks which determine the suitability for any specific application.
Types of power converter circuits are employed in the electric and hybrid electric vehicle:-
Fig 1.2 Converters for electric vehicle
Depending on the type of function performed, power electronic converters are categorized into the following types.
- AC to DC = Rectifier: It converts AC to unipolar (DC) current
- DC to AC = Inverter: It converts DC to AC of desired frequency and voltage
- DC to DC = Chopper: It converts constant to variable DC or variable DC to constant DC
- AC to AC = Cycloconverter, Matrix converter: It converts AC of the desired frequency and/or desired voltage magnitude from a line AC supply.
AC to DC Converters or Rectifiers
An AC to DC converter is also called a rectifier, which converts AC supply from main lines to DC supply for the load. The block diagram of an AC to DC converter is shown in the figure below.
The essential components in this rectifier include a transformer, switching unit, filter, and a control block.
Fig 1.3 AC TO DC Converters
Here, the transformer adjusts the primary AC source supply to the input of the rectifier stage. Usually, it is a step-down transformer that reduces the supply voltage to a circuit operating range.
The rectifier converts the low voltage AC supply into a DC supply.
It comprises a diode and/or thyristors based on the type of rectifier. The output of the rectifier is of pulsed DC and hence it is filtered using a filter circuit, which is usually made with a capacitor or a choke.
The control block controls the firing angle of thyristors in the case of phase-controlled rectifiers. Since the diode is not a controllable device, a control block is not needed in the case of diode rectifiers.
Rectifiers are majorly classified into two types
- Uncontrolled diode rectifiers
- Controlled rectifiers
DC to DC Converters
Many DC operated applications need different levels of DC voltage from a fixed DC source.
Some of these applications include subway cars, DC traction systems, control of large DC motors, battery-operated vehicles, trolleybuses, etc. They require variable DC to produce variable speed, so a power conversion device is needed.
Fig 1.4 DC to DC converters
A DC chopper is a static device that converts a fixed input DC voltage to variable DC output or a fixed DC output of a different magnitude (which can be lower or higher) than the input value. The block diagram of a DC chopper is shown in the figure below.
The chopper circuit is connected between the DC input source and DC load. This chopper consists of power electronic switching devices such as thyristors which are connected in such a way that they produce the required DC voltage to the load.
The output voltage is controlled by adjusting the ON time of the thyristor (or switch) which in turn changes the width of the DC voltage pulse at the output. This method of switching is called pulse width modulation (PWM) control.
The output of the chopper can be less or greater than the input and also it can be fixed or variable. These can be unidirectional or bidirectional devices based on the application it is intended for.
DC choppers are mainly used in DC drives, i.e., electric vehicles and hybrid electric vehicles.
DC choppers are classified into three basic types based on input and output voltage levels and are discussed below.
AC to AC Converters
AC/AC converters connect an AC source to AC loads by controlling the amount of power supplied to the load. This converter converts the AC voltage at one level to the other by varying its magnitude as well as the frequency of the supply voltage.
These are used in different types of applications including uninterrupted power supplies, high power AC to AC transmission, adjustable speed drives, renewable energy conversion systems, and aircraft converter systems.
DC to AC Converters or Inverters
These converters are connected between the DC source of fixed input and variable AC load. Most commonly, these DC to AC converters is called inverters. An inverter is a static device that converts fixed DC supply voltage to variable AC voltage.
Fig 1.5 DC to AC converters or inverters
Here the fixed DC voltage is obtained from batteries or by DC link in most power electronic converter. The output of the inverter can be variable/ fixed AC voltage with variable/fixed frequency.
This conversion from DC to AC along with variable supply is produced by varying the triggering angle to the thyristors. Most of the thyristors used in inverters are employed with the forced commutation technique.
These can be single-phase or three-phase inverter depending on the supply voltage. These converters are mainly divided into two groups. One is PWM based inverters and other multilevel inverters.
Further, these are classified as voltage source inverter and current source inverter. Each type is subdivided into different types such as PWM, SVPWM, etc. Multilevel inverters are more popular in industrial applications.
The inverters overcome the drawbacks of PWM based inverters.
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Q.2 An Electric Vehicle's powertrain with a 72V battery pack is 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%?
Ans:-
Given,
Rated Armature voltage= 72 V
Rated armature current= 400 A
Ra= 0.5Ω, KΦ= 0.7 Volt second
Chopper Switching frequency= 400 Hz
as we know torque formula
T - Torque
V - Armature voltage
R - Resistance
w - Speed in rad/sec
comparing 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 ****************forming into ax^2+bx+x format by finding out the coefficient
we can find w values
to find roots use the following formula
for a quadratic of the form, 
the solution is
will get values in positive and negative format but our concern is a positive value because it is related to speed
w = 38.95 rad/sec
EV steady-state speed if duty cycle is 70% is 38.95 rad/sec
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Q.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:-in this blog he is comparing the DC brushless and induction motors talking about the advantages and limitation of both
The author is talking about how there were lead-acid batteries, DC brush motors, and contactor controllers. Today, none of these remain Lead has been replaced by lithium and DC by either DC brushless or induction. Contactors
first stated that brushless motors are useless motors without the device that provides this current control is called an inverter.
the stators for the 3-phase induction motor and the DC brushless motor are virtually identical. Both have three sets of “distributed windings” that are inserted within the stator core. The essential difference between the two machines is with the rotor. Unlike the DC brushless rotor, the induction rotor has no magnets – just stacked steel laminations with buried peripheral conductors
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.
In contrast, a brushless DC motor produces no starting torque when directly connected to fixed frequency utility power.
While 3-phase induction motors have great utility, they also have some severe limitations. They cannot operate from DC; AC is a must. when operated from utility power, they have limited starting torque and somewhat limited running peak torque capabilities, when compared to DC type machines.
Adding some feedback loops such that the inverter produces the exact frequency that the motor “desires,” and the induction motor is now capable of competing with DC and DC brushless for vehicle applications.
then the author raised the question of brushless or induction motor?
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.
One of the main differences is that much less rotor heat is generated with the DC brushless drive.
the peak point energy efficiency for a DC brushless drive will typically be a few percentage points higher than for an induction drive.
In an ideal brushless drive, the strength of the magnetic field produced by the permanent magnets would be adjustable. When maximum torque is required, especially at low speeds, the magnetic field strength (B) should be maximum
In contrast, induction machines have no magnets, and B fields are “adjustable,” since B is proportionate to V/f (voltage to frequency). This means that at light loads the inverter can reduce voltage such that magnetic losses are reduced and efficiency is maximized.
Thus, induction drives may be the favored approach where high-performance is desired; peak efficiency will be a little less than with DC brushless, but average efficiency may actually be better.
Induction machines are more difficult to control. The control laws are more complex and difficult to understand. Achieving stability over the entire torque-speed range and temperature is more difficult with induction than with DC brushless. This means added development costs, but likely little or no recurring costs.
Then the author is stating his conclusion about the future of both motors
In this he is talking about how these motors are best in their particular are, DC brushless drives will likely continue to dominate in the hybrid and coming plug-in hybrid markets, and that induction drives will likely maintain dominance for the high-performance pure electrics. then he raised the question.
what will happen as hybrids become more electrically intensive and as their performance levels increase and on that question, he finally concludes that so much of the hardware is common for both drives could mean that we will see induction and DC brushless life and work side by side during the coming golden era of hybrid and electric vehicles.
RESULT :-
1) EV steady state speed if duty cycle is 70% is 38.95 rad/sec
REFERENCES:-
1) Fig 1.1 Power converter specification:-https://www.sunpower-uk.com/files/2014/07/What-is-a-power-converter.jpg
2) Fig 1.2 Converters for electric vehicle:-https://api.intechopen.com/media/chapter/19583/media/image2.png
3) Fig 1.3 AC TO DC Converters:-https://www.electronicshub.org/wp-content/uploads/2015/12/AC-to-DC-Converters.jpg
4) Fig 1.4 DC to DC converter:-https://www.electronicshub.org/wp-content/uploads/2015/12/DC-chopper.jpg
5) Fig 1.5 DC to AC converters or inverters:-https://www.electronicshub.org/wp-content/uploads/2015/12/Inverters.jpg
