Week-6 Challenge: EV Drivetrain
Que : 1. Which types of power converter circuits are employed in electric and hybrid electric vehicle?
Ans :
Power Converter:
Power electronics is the application to the control and conversion of electric power. A power converter is a process of converting electric energy from one form to another. A power converter is an electro-mechanical device for converting electrical energy. A power converter can convert alternating current (AC) into direct current (DC) and vice versa; change the voltage or frequency of the current or do some combination of these.
There are three types of Power converter circuits used in EV's and HEV's are given below.
LAC to DC (Rectifier)
2. DC to AC (Inverter)
3. DC to DC
1. AC to DC (Rectifier):
A rectifier is an electrical device that converts alternating current (AC), which periodically reverses direction, to direct current (DC), which flows in only one direction. The process is known as rectification since it straightens the direction of the current
Usos:
A boost converter charges the battery in Electric Vehicles (EV). A conventional OBC (onboard charging system) has a bridge rectifier to convert the input AC voltage to DC but, during the rectification process, there is high conduction, and switching losses and heating issues are also attached to it.
• Rectifiers have many uses but are often found serving as components of DC power supplies and high-voltage direct-current power transmission systems.
• Rectification may serve in roles other than to generate direct current for use as a source of power.
As noted, detectors of radio signals serve as rectifiers
• In gas heating systems flame rectification is used to detect the presence of a flame
More complex circuitry that performs the opposite function, that is converting DC to AC, is called an inverter
2. DC to AC (Inverter):
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
The resulting AC frequency obtained depends on the particular device employed Inverters do the opposite of 'converters which were originally large electromechanical devices converting AC to DC
The input voltage, output voltage and frequency, and overall power handling depend on the design of the specific device. The inverter does not produce any power, the power is provided by the DC source.
. DC to DC:
A DC-to-DC converter is an electronic device or electromechanical device that converts a source of direct current (DC) from one voltage level to another. It is a type of electric power conversion. Power levels range from very low (small batteries) to very high (high-voltage power transmission).
Uses:
• DC-to-DC converters change dc power from an onboard 200-800V high voltage battery into lower dc voltages (48V or 12V) to power headlights, interior lights, wiper and window motors, fans, pumps, and many other systems within electric vehicles (EV) and hybrid electric vehicles (HEV).
• DC-to-DC converters are used in portable electronic devices such as cellular phones and laptop computers, which are supplied with power from batteries primarily Such electronic devices often contain several sub circuits, each with its own voltage level requirement different from that supplied by the battery or an external supply (sometimes higher or lower than the supply voltage).
Additionally, the battery voltage declines as its stored energy are drained. Switched DC to DC converters offer a method to increase the voltage from a partially lowered battery voltage thereby saving space instead of using multiple batteries to accomplish the same thing.
DC-to-DC converters which are designed to maximize the energy harvest for photovoltaic systems and wind turbines are called power optimizers.
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 'di and for the braking operation the duty ratio is 'd2
Ans :
The other parameters of the electric vehicle are given below.
Motor and Controller Parameters
Rated Armature voltage 72 v
Rated armature current- 400 A
Ro= 0.50, KO 0.7 volt second
Chopper Switching frequency 400 Hz
The vehicle speed-torque characteristics are given by the below equation
Tv 24.7+ (0.0051)
What is EV steady-state speed if the duty cycte is 70%
Ans.
Given:
Roted Armature voltage (V) 72 V
Rated armature current 400 A
KO- 0.7 voll second
Chopper Switching frequency 400 12
Duty cycte (de)- 70%-0.7
The Average voltage is given by the product of the bottery voltage and the duty cycan
Chopper Switching frequency 400 Hz
Duty cycle (dc)= 70% = 0.7
The Average Voltage is given by the product of the battery voltage and the duty cycle
V₁ - V xodc
7a - 72 * 0.7
r_{a} = 50.4
The Average voltage is 50.4 V.
The Vehicle Characteristics are given in the form of the equation,
Tv - 34.7+ (0.0051) (1)
The derived formula for the motor speed characteristics
Vax hp
T Ra
Ra
By substituting the values.
(50.4 * 0.7)/0.5 - (((0.7) ^ 3 * omega)/0.5)
--(2)
Compare the Equations (1) and (2), we get
T-70.56 0.58 -
70.56-0.96 - 247+ (0.0051)
0.0001 omega ^ 2 + 0.08omega - 45.9t = 0
By using the Quadratic Equation Formula
omega = - b plus/minus (sqrt(b ^ 2 - 4ac))/(2a)
omega=-0.98 pm sqrt 0.98^ 2 -(4*0051 2*0.0051 omega=38.92rad. sec bigvee omega=r 231.07rad -45.86)) . 4
omega = (- 0.98 plus/minus 1.377)/0.0102
We get two solutions for this angular speed,
We can consider only the positive value,
Angular speed = 38.92rad/sec.
Que 3. Develop a mathematical model of a DC Motor for the below equation using Simulink.
ωω= V/Kϕϕ -Ra/Kϕϕ^2 .T
Ans :
Que :
4. Refer to the blog on below topic:
Induction Versus DC brushless motors by Wally Rippel, Tesla
Explain in brief about author’s perspective.
Ans :
According to the blog Induction vs DC Brushless motors by Wally Rippel, Tesla', the writer states the changes in the world years ago and the latest technological trends that are happening. He then explains about the DC Brushless motors and induction motors.
DC Brushless Machines:
According to him, the rotor has two or more permanent magnets that generate a DC magnetic field. This magnetic field will enter the stator core which interacts with the currents flowing in the windings to produce an interaction between rotor and stator. It needs a proper flow of current into the rotor and stator if it's not connected properly the current will not generate and the torque decreases During the Electric and mechanical conversion when the rotor is rotating, the magnitude and polarity of the stator current will continuously vary such that the torque remains constant and optimal efficiency is achieved
Induction Motor:
The 3-phase induction motor was invented by Nikola Tesla sometime before 1889. 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. They have 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. 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. An induced voltage exists across the shorted structure that is proportionate to this speed difference between the rotor and electrical frequency. The currents are produced within the rotor conductors that are approximately proportionate to the voltage, hence there's a speed difference. Finally, these currents interact with the original magnetic field to produce forces
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 is the main reason for their success in contrast, a brushless DC motor produces no starting torque when directly connected to fixed frequency utility power. They really need the aid of an inverter whose phase is maintained in step with the angular position of the rotor.
Differences:
He then explained that there were only a few brushless drives in the past but now most of the Hybrid Electric vehicles use it. There's a difference between brushless and inverter controls is the control. The Dc brushless requires a position sensor but an induction motor requires only a speed sensor.
Less rotor heat is generated in DC brushless drive and Rotor cooling is easier and peak point efficiency is generally higher.
DC brushless drive operates at a power factor of one, whereas the induction drive power factor is 85%
With the increase in the size of the DC Brushless machine, magnetic losses increase and the part-load efficiency decreases, whereas the induction drives, as the machine size increases, losses don't grow. During high-performance tasks, induction drives are suitable.
Finally, we can know that the writer says that the DC brushless will dominate the Hybrid Electric Vehicle Markets but the Induction drive will maintain dominance in the High performance in the pure electric vehicles. As of now, there is no winner out of the two as each has its own pros and cons and excels at its own forte.