Week-6 Challenge: EV Drivetrain
1. Which types of Power Converter circuits are employed in electric and hybrid electric Vehicles?
Ans- There are four main categories of Power Converter Circuits,
1. AC to DC Converter (Rectifer)
2. DC to AC Converter (Inverter)
3. DC to DC Converter (Chopper)
a. Step-up Converter
b. Step-down Converter
4. AC to AC Converter
1. AC to DC Converter (Rectifer):-
It takes an AC Power Supply and Converters it to a DC one. Power Generated from the plant is transmitted to our homes at a voltage of 230V at a Frequency of 50Hz, Electricity supplied voltage is 100v to 230v AC, other hands our home appliances such as Mobile charger, Laptop charger nominal voltage is 3.3V to 5V DC Voltage.
So, It is necessary that AC to DC voltage plays a major role.
Rectifiers are of two types
1. Full wave rectifier
2. Half wave rectifier
Full wave rectifier: A full wave rectifier is defined as a rectifier that converts the complete cycle of alternating current into DC.
1. It consists of two diodes.
2. The output DC is a continuous signal.
Half wave rectifier:A half wave rectifier is defined as a rectifier that converts the complete cycle of alternating current into DC..
1. It consists of one diode.
2. The output DC is a disctete signal.
2. DC to AC Converter (Inverter)
DC to AC Converter is known as an Inverter, They are used in Power Electronics application where the power input pure 12V, 24V, 48V DC Voltage that requires power conversion for an AC output with nominal frequency.
Application used in EV/HEV System, battery storage system, traction drives, variable speed drive etc, Converting DC to AC is more complicated because the circuit needs oscillator that reverse the current direction at nominal frequency.
In the case of EVs and HEVs, the battery pack will supply its power to the inverter. The inverter contains a transformer which consists of a primary and secondary winding. The current from the battery is first supplied to the primary winding where the electronic switches will repeatedly reverse the direction of the current to create an AC waveform. This current will return back the way it came from. The AC current in the primary winding will induce an AC current in the secondary winding which then supplies this current to an AC load.
3. DC to DC Converter (Chopper);-
a. Step-up Converter (Boost)
b. Step-down Converter (Buck)
c. Step-up/step-down Converter (Buck/Boost Converter)
It is similar to a Transformer in that it can change the Voltage level, Purpose of this converter is to provide necessary level of power to a particular components. It works by taking the DC supply and passing it through a switching element which converts the waveform to a square wave which is actually AC in nature. Then this AC is passed through a filter which will will turn it back in to a DC supply for use in a load. In EVs, HEVs and conventional vehicles, this type of converter is used for various purposes. E.g. In the case of stepping down the voltage, the car radio may require a stepped down voltage of 12V to operate and would not need the full voltage of the car battery. In the case of stepping up the voltage, the motor of a car may require 500V to operate, but the vehicle may need more voltage than the battery pack can provide.
4. AC to AC Converter:-
This type of converter takes an input AC waveform with a particular voltage and frequency and outputs a different AC waveform with another voltage and frequency. The output voltage and/or freqency can be changed or maintained depending on the application. This converter has many applications in our daily life e.g. lamp dimming, motor speed control.
Types of AC to AC Converter,
a. Cycloconverter (change in Frequency and Magnitude)
The input frequency is changed by changing the angle applied to a Switch, By switching the positive and negative thyistors, we can get an output waveform that has a stepped up/down frequency compared to the input.
b. AC to AC Convert with DC Link,
The input AC waveform is first rectified by the rectifier to remove the negative portions of the AC wave and output only the positive portions. This creates a DC waveform which is then stored in the DC link and is then converted back to an AC waveform using the inverter.
c. Matrix Converter,(IGBT Switch)
AC to AC Conversion without using DC linkage, Matrix converters use bidirectional switches (IGBTs) to conduct current and block voltages of both polarities.
d. Hybrid Matrix Converter
It combines the AC/DC/AC conversion. It is unidirectional switches and 2 converter stages, without the DC link to reduce the size and dimension of converter.
2. What is EV steady state speed if duty cycle is 70%?
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
Torque speed - formula,
w = Vt/Kphi - Ra/(Kphi)^2.T
T = (Vtxk)/Ra - (Kphi^2xw)/Ra
Where given T = 24.7+(0.0051)w^2
Sub T in above equ.
24.7+(0.0051)w^2 = (Vtxk)/Ra - (Kphi^2xw)/Ra
24.7+(0.0051)w^2 = 72x0.7/0.5 - 0.7^2*w/0.5
100.8-0.98w = 24.7+(0.0051)w^2
w^2+192.15w-8992.5 = 0
In order to find the roots use the following formula for a quadratic equ.
ax^2 + bx + c = 0,
x = -b +0r- sqrt b^2-4ac/2a
a=1, b=192.15w, c= -8992.5
x = -192.15w (+or-) sqrt(192.15)^2 - 4(1)(-8992.5)/2(1)
X = 2.0085, X = -194.16,
w = 38.75 rad/sec
3. Develop a mathematical model of a DC Motor for the below equation using Simulink.
ω= V/Kϕ -Ra/Kϕ^2 .T
( Screenshot attached - DC_motor_equ & DC_motor_equ_plot )
4. Conclusion,
DC Brushless,
Require a absolute position sensor less rotor heat is generated, rotor cooling is easier and peak point efficiency higher operate at unity power factor average effiecincy is low Permanent magnets are expensive.
They are easy to control
Induction Motor,
Require an absolute postion sensor. Powerfactor is about 85% peak point efficiency higher compare to DC brushless, Average efficiency is high no magnet there is advantage in cost the control norms are more complex and difficult to control.