Week-6 Challenge: EV Drivetrain
1.
A converter is an electrical circuit which 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.
The DC to DC converters are used in a wide range of applications including computer power supplies, board level power conversion and regulation, dc motor control circuits and much more.
Types of Power converters
- AC to DC Converter (Controlled Rectifier)
- DC to DC Converter (DC Chopper)
- AC to AC Converter (AC voltage regulator)
- DC to AC Converter (Inverter)
Fig 1 Power converter specifications
AC to DC Converters.
AC to DC Converters. An AC to DC converter circuit can convert AC
voltage into a DC voltage. The DC output voltage can be controlled by
varying the firing angle of the thyristors. The AC input voltage could be
a single phase or three phase.
AC to AC Converters.
AC to AC Converters. This converters can convert from a fixed ac
input voltage into variable AC output voltage. The output voltage is
controlled by varying firing angle of TRIAC. These type converters are
known as AC voltage regulator.
DC to DC Converters
DC to DC Converters. These converters can converter a fixed DC input
voltage into variable DC voltage or vice versa. The DC output voltage is
controlled by varying of duty cycle.
Power Electronics Application Power Electronics Application
Figure of Power Electronic devices application
Power converters applications in EV & HEV vehicles.
DC/DC converters for electric vehicles
The different configurations of EV power supply show that at least one DC/DC converter is necessary to interface the FC, the Battery or the Supercapacitors module to the DC-link.
In electric engineering, a DC to DC converter is a category of power converters and it is an electric circuit which converts a source of direct current (DC) from one voltage level to another, by storing the input energy temporarily and then releasing that energy to the output at a different voltage. The storage may be in either magnetic field storage components (inductors, transformers) or electric field storage components (capacitors).
AC/DC converter
AC/DC converter which supplies electric power from a commercial power system to an onboard high-voltage battery.
The AC/DC converter consists of a power factor correction (PFC) which converts input AC voltage into DC voltage, an H-bridge circuit which converts the DC voltage into high-frequency AC voltage, a high-frequency transformer which regulates transformation and insulation, and a circuit which rectifies and smooths high-frequency AC voltage.
DC-AC power converters
Broadly speaking, an inverter is an electrical device that converts electricity derived from a DC (Direct Current) source to AC (Alternating Current) of the type that can be used to drive a device or appliance. In a solar power system, for example, the power stored by batteries charged by solar panels is converted to standard AC power by the inverter which provides the power to plug-in outlets and other standard 120-volt devices.
An inverter serves the same kind of function in a hybrid or EV car, and the theory of operation is relatively simple. DC power, from a hybrid battery, for example, is fed to the primary winding in a transformer within the inverter housing. Through an electronic switch (generally a set of semiconductor transistors), the direction of the flow of current is continuously and regularly flip-flopped (the electrical charge travels into the primary winding, then abruptly reverses and flows back out). The in/outflow of electricity produces AC current in the transformer's secondary winding circuit. Ultimately, this induced alternating current electricity provides power for an AC load—for example, an electric vehicle's (EV) electric traction motor.
Fig. EV drivetrain
AC-AC POWER CONVERTERS
ac to ac wireless power transfer (WPT) system is introduced for electric vehicle (EV) charging applications to reduce cost and design complexity. The presented wireless power transfer concept achieves unity power factor (UPF) on the grid-side by using a hybrid frequency ac / ac converter without an additional converter stage and closed loop control compared to the conventional systems.
2.
GIVEN -
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 is 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
--------Eqn (1)
To Calculate - Steady state Speed
Solution -
We Know That -
Angular Speed equation of DC motor is given by -
ω">ω= V/Kϕ">ϕ -Ra/Kϕ">ϕ^2 .T
also,
From the above equation
T = - ( KΦ)^2*ω/Ra + V ( KΦ) / Ra -------Eqn (2)
Given V = 72 v = 72 * 0.7 = 50.4 V
Putting all the given values in equation (2)
T = - ((0.7)^2*ω/0.5) + ((50.4)(0.7)/(0.5))
We get -
T = - 0.98 ω + 70.56 ----Eqn (3)
Equating Equations (1) and (3)
24.70 + (0.0051)*(ω)^2 = - 0.98 ω + 70.56
(0.0051)*(ω)^2 + 0.98 ω - 45.86 = 0
ω = - 230.39 rad/sec and ω = 38.23 rad / sec
3.
To develop a mathematical model of a DC Motor for the equation using Simulink.
Given Equation -
ω">ω= V/Kϕ">ϕ -Ra/(Kϕ">ϕ)^2
To calculate Angular Speed Value (rad/s)
Assuming Parameters -
V = 50.4 V
KΦ = 0.7 volt second
Ra = 0.5 ohm
T = 33.09 Nm
Simulink Model -
Subsystem -
Conclusion - The value of Angular Speed comes out to be 38.23 rad/sec.
4.
Author's Perspective Induction Versus DC Brushless Motors.
- Author shows interest in the new technologies like EV and HEV,
- He mentioned the difference between 3 phase Induction motor and DC brushless motor.
- With brushless machines, the rotor includes two or more permanent magnets that generate a DC magnetic field (as seen from the vantage point of the rotor). In turn, this magnetic field enters the stator core (a core made up of thin, stacked laminations) and interacts with currents flowing within the windings to produce a torque interaction between the rotor and stator. As the rotor rotates, it is necessary that the magnitude and polarity of the stator currents be continuously varied – and in just the right way - such that the torque remains constant and the conversion of electrical to mechanical energy is optimally efficient. The device that provides this current control is called an inverter. Without it, brushless motors are useless motors.
- 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. (Were an inverter needed, Tesla’s invention would have been useless until sometime in the 1960s.) 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.
CONCLUSION -
- Author differentiates the EV and HEV also the types of converters used in the modern Vehicle.
- Author also discusses the best possible option available in the market.