Week-6 Challenge: EV Drivetrain

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

Ans.1)

In an electric and hybrid electric vehicle, power electronic is responsible for converting and controlling electric power in the system. Some of the major power electronic components and functionalities are used in the electric vehicle system. These include an inverter, converter, and onboard charger. Operations at high voltage or high current can be efficiently executed by utilizing power electronic devices, as they deliver a faster switching rate at higher efficiency. In addition, power electronics control both unidirectional as well as the bidirectional flow of energy, depending on the usage, and the regenerated energy can be sent back to the utility.

The three types of converters used in an electric vehicle are:

AC - DC Converter:-

AC to DC converters is most commonly used in Plug-in Hybrid Electric Vehicles (PHEV) & Plug-in Electric Vehicles (PEV). These vehicles get their propulsion power from an external electricity source (charging stations) which is alternating current. The battery needs to be charged using a direct current source. For this purpose, AC to DC converter is employed in a PHEV & PEV.

DC-DC Converter:-

A DC to DC converter is a category of power converters and it is an electric circuit that 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).

DC/DC converters can be designed to transfer power in only one direction, from the input to the output. However, almost all DC/DC converter topologies can be made bi-directional. A bi-directional converter can move power in either direction, which is useful in applications requiring regenerative braking.

DC - AC Converter:-

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.

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'.

Ans2) Given,

Battery pack voltage (V) = 72V

Rated armature current = 400 A

Armature Resistance (Ra) = 0.50

Motor constant (Kϕ) = 0.7

Chopper Switching frequency= 400 Hz

Duty Cycle = 70%

T_v = 24.7 + (0.0051) ω²..................Eqn(1)

Vehicle Output Voltage (V) is given by,

Vv = Battery Voltage (V) * Duty Cycle

V=72*0.70

Thus, Vehicle voltage = 50.4V

Now, the torque equation for the motor,

T = VKϕ/R_a – ((Kϕ)²/R_a)*ω

T = 50.4*0.7/0.5 – ((0.7)²/0.5)*ω

T = 70.56 - 0.98 ω....................Eqn(2)

solving equation (1) & (2),

comparing both torques of the vehicle and torque of the motor,

24.7 + (0.0051) * ω² = 70.56 - (0.98) * ω

0.0051 ω² + 0.98 ω - 45.86 = 0

by solving this by quadratic equation we get,

we get 2 values of ω as

ω = 38.915 rad/sec

and

ω = -231.072 rad/sec

So, the EV steady state speed if the duty cycle is 70% = 38.915 rad/sec.

3. Develop a mathematical model of a DC Motor for the below equation using Simulink.

ω">ωω= V/Kϕ">ϕϕ -Ra/Kϕ">ϕϕ^2 .T

Ans3).Given,

ω = V/Kϕ - Ra/Kϕ^2 .T

We assumed the parameter,

V= 200,

Kϕ =0.7,

Ra = 0.5,

T = 80,

So before plotting a function, we need to identify the basic blocks which are required for carrying out the plotting of a function in Simulink. So, the fundamental blocks which are required for our equation are as follows.

1) Constant:-

So, for the given example, we have three content as 200,0.7 & 0.5.

2) Square:-

In the given example, we need to use the square blocks for a value of constant 0.7.

3) Multiplication:-

So, after dividing and adding all the blocks & fixing the connection, we need to use the multiplication block for getting the final output as mentioned in the given example.

4) Divide:-

we need to use the divided block for getting the final output as mentioned in the given example.

5) Add block:-

Default Add Block

So, after getting the above default block named 'Add block', we need to change the sign from ++ to +- as

wherever is required according to the question, they want to double click on the above block and change the default sign.

7) Display:-

The Display block shows the value of the input data. You can specify the frequency of the display.

The Simulink model is as shown below:

4. Refer to the blog on the below topic:

Induction Versus DC brushless motors by Wally Rippel, Tesla

Explain in brief about the author’s perspective.

The author starts by clearly stating that all engines are not alike, and there is no one best engine, which is similar to electric drives.

With time technologies have developed it could be said for today's technologies that they might get better, as we can see

Lead batteries replaced by lithium, DC brush motors replaed by DC brushless or induction, Contactor Controllers with modulated inverters

The author was focusing on the similarities and the differences between DC brushless motors and Induction motors

So, brushless motors need an inverter for current control, which is not required in IM

from a stator view, 3 phase IM and DC brushless motos are identical, both drives use 3-phase modulating inverters. they vary by the rotor and inverter control, and digital sensors

For Ideal brushless, the B field should be adjusted to reduce I^2 R losses, but there is no easy way of changing B with permanent magnets. whereas in IM, no magnets are there therefore, the B field is adjustble. this increases the performance

IM is favored for high performance and average efficiency is better if not peak efficiency. It will retain a cost advantage over permanent magnet machines, but achieving stability is difficult due to complex control laws.

In the end, the autor was clear that DC brushless will dominate hybrid, and plugin hybrid, Induction drives will maintain dominance for high-performance pure electric.

But, the main conclusion he came to was:

If the hybrids become more electrically intensive and as their performance level increases, DC and IM will live and work side by side in the near future of hybrid and electric.