Week-6 Challenge: EV Drivetrain

Objective :

1. Which types of power converter circuits are employed in electric and hybrid electric vehicle? 

2.  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'.

3. Explain in brief about authors perspective. 

Explanations :

1. Power converter circuits :

This technology deals with processing and controlling the flow of electrical energy in order to support coltages and currents in a form that is optimally suited for the instruments.

                 As shown in the abouve block diagram, it consists of an electrical energy source, a power electronic circuit, a control circuit and an electric load. This convertor changes one form of electrical energy to other forms of electrical energy. The power electronic circuit performs both the power part and the control part. The power part transfers the energy from source to load and its consists of power electronic switches, transformers, electric choke, capacitors, fuses and sometimes resistors. The control circuit regulates in the power part of the converter. This block is built with a complex low power electronic that consists of either digital or analog circuit assembly.

Depending upon the functions performed, the following are the 4 different classifications 

a) AC to DC converter which is called "Rectifier" - It converts AC into Unipolar DC current.

b) DC to AC converter which is called "Inverter" - I t converts DC into AC of desired voltage and frequency.

c) DC to DC converter which is called "Chopper" - It converts High voltage DC to low voltage DC to High voltage DC (Step up)

d) AC to AC converter which iscalled "Cycloconverter" -It converts the AC of the desired frequency and desired voltage magnitude from the AC supply.

Except for the AC to AC converter the other 3 are predominantly used in both the HEV and EV.

AC to DC converter (Rectifier) this used for changing the traction battery. Because the source of current is AC and the voltage stored in the traction Battery is DC.

This DC current from the Battery is then transmitted to 2 different converters DC - AC converter (Invester) and DC - DC converter (Chopper).

The inverter is usually found between Battery and the DC induction motor, this motor is directly connected to the wheels and is responsible for the traction. Incase of Regenerative braking, this motor functions as a generator by converting the mechanical energy (from the wheels & axles) to electrical energy which is then stored in the traction battery.

DC - DC converter takes the DC source from the traction battery. The output from this DC - DC converter is used to power all the other components of the EV/HEV like HML, Headlamps, AC, etc.

2. An electric Vehicle's powertrain with 72V battery pack is shown in the diagram below. The duty ration for acceleration operation is 'd1' and for the braking operation the duty ration is 'd2'.

I have solved the problem in my notebook and uploaded the sum.

3. Author's perspective on Induction Vs DC  Brushless motor:

         Author Wally Rippel in his blog explained the working principle of both the Induction and DC Brushless motor. Though Induction motors are old technology and he explained it's significant advantages over the DC brushless motor EV's, at the same time he also explained the DC brushless motor significance in Hybrid and Plug-in HEV and it's dominantly used in the current market. Induction motors though lastly found in General Motor EV1, the author suggested that the Induction motor along with DC Brushless motor will be widely used side by side as per Vehicle requirement.

Working Principle of DC Brushless Motor:

1. The Stator is made of thin and stacked laminations. The stator interacts with the current flowing within its windings to produce a torque interaction between the stator and the rotor.

2. The Rotor here includes 2 or more permanent magnets that generate DC magnetic field. As the rotor rotates it is important that the magnitude and the polarity of the stator current be continuously varied so that torque remains constant and also achieving the efficiency.

3. For the functioning of DC brushless motor it is mandatory to have an inverter which it cannot function.

Working Principle of Induction Motor:

1. The stator is the same as of Induction motor. The only difference is that rotor is different when compared to the DC Brushless. Induction Motor does not have permanent magnets. The Induction motor does not mandatorily require an Inverter to function but with help of a small inverter, the efficiency of the Induction motor will increase.

2. Here the Rotor has stacked steel laminations with buried peripheral conductors that form a shorted structure. Currents flowing in the stator windings produce rotating magnetic fields that enter the rotor.

Pros and Cons of DC Brushless:

Pros

i) Heat generated from the rotor is less, hene rotor cooling is easy and peak point efficiency is higher than the DC Brushless.

Cons

i) Inverter (whose phase is maintained in a step with the angular position of the rotor) is mandatory without which DC Brushless motor is useless.

ii) Magnetic field strength cannot be easily adjusted because of the permanent magnets.

iii) In DC Brushless motor, as its size increases the magnetic losses also increase.

iv) Permanent magnets are expensive.

Pros and Cons of Inductuin Motor:

i) No need for Inverter, the motor has the ability to start under load.

ii) Since they do not have any magnets, controlling the Magnetic field here is easy. Hence Magnetic losses (Eddy+Hysterisis+Current resistance losses) are minimized and the efficiency is maximized.

iii) Induction drives may be the favoured approach where high-performance is desired. Peak efficiency will be a little less than with DC brushless, but average efficiency may actually be better.

iv) Induction Motor are less expensive when compared to DC Brushless.

Cons

i) They cannot operate from DC, AC source is a must.

ii) When operated from utility power, they hace limited starting torque and somewhat limited running capabilities when compared to DC type machines. Even by adding the inverter, the torque performance is low when compared to DC motor.

Conclusion:

        Though DC Brushless continues to dominate the current market with its presence in all the EV and HEV, Induction motors will not become obsolete because as mentioned Induction motors have its unique application and significance when compared to the DC Brushless. So he concluded that both the DC Brushless and Induction motor applications will grow side by side as per the EV characteristics.

Develop a mathematical model of a DC motor for the below equation using Simulink

Equation,

$\omega$=   V/K$\varphi$ -Ra/K$\varphi$^2    .T

V= input voltage=100V

K= Motor Constant

Ra= Armature Resistance =  ohms

T= Torque

BLocks used in Simulink Model

Constant block: Because most of the inputs are constant values, therefore this block has been used.

Divison

Square: To square the K value to 0.7

Ramp: THis block represents torque values.

Scope: Speed vs Torque graph.

Figure-3 Block Diagram

Figure-4 Speed(rpm) vs Torque(N-m)

Conclusion:

• From equation 1, Speed is inversly proportional to torque.