Week-6 Challenge: EV Drivetrain

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

Power converter circuits

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 3 types of converters which are widely used in an Electric Vehicle are –

AC – DC Converter, DC – DC Converter and DC – AC Converter

AC – DC Converter

AC to DC converters are most commonly used in Plug-in Hybrid Electric Vehicles (PHEV) & Plug-in Electric Vehicles (PEV), Which is why this convertor also call as on board charger. 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

DC-DC converters can be used to interface the elements in the electric power train by boosting or chopping the voltage levels. By introducing DC/DC converters one can choose the voltage variation of the devices and the power of each device can be controlled. The power of the DC/DC converter depends on the characteristics of the vehicle such as top speed, acceleration time from 0 to 100 Km/h, weight, maximum torque, and power profile (peak power, continuous power).

DC – AC Converter

The DC/AC converter is required to convert the dc power to a suitable ac power for the electrical machine. The power output from battery pack (DC current) is converted using inverter to be used in electric motor (AC current).

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'.

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

Tv = 24.7 + (0.0051) w^2

What is EV steady state speed if duty cycle is 70%?

Ans-

Vehicle Output Voltage (V) is given by: -

V = Battery Voltage (V) * Duty Cycle = 72 * 0.70

Thus, Vehicle voltage = 50.4V

Now, Equation for Vehicle torque is given by: -

Tv = ((Vv − ω. K∅ ) ⋅ K∅ ) / Ra

Where;

Tv = Vehicle Torque (N-m)

Vv = Vehicle voltage (Volts)

ω = steady state speed of vehicle (rps)

K∅ = Motor constant

Ra = Armature Resistance

Substituting all these values in equation, we get:

Tv = ((50.04 − 0.7 ⋅ ω) ⋅ 0.7) / 0.5

Tv = 70.56 − 0.98 ⋅ ω

Tv = 24.7 + 0.0051 ⋅ ω 2

Solving these two equations,

70.56 − 0.98 ⋅ ω = 24.7 + 0.0051 ⋅ ω 2

We get equation:

0.0051 ⋅ ω 2 + 0.98 ⋅ ω − 46.94 = 0

Since this is a quadratic equation, we get two values of ω

ω =38.85 rad/s and ω = -230.95 rad/s

We take the positive value

Steady state speed of vehicle = = 38.85 rad/s

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

ω= V/Kϕ -Ra/Kϕ^2 .T

Mathematical model of a DC Motor

Where;

V = 72 (Volts)

K∅ = 0.7

Ra = 0.5

Torque Block is Ramp Block that range from 0 to 25 as an simulation time is 25 sec

Above file Attach as dcmotoreq.slx

Output

Relation between Speed and Torque

4.Refer to the blog on below topic:

Induction Versus DC brushless motors by Wally Rippel, Tesla

Explain in brief about author’s perspective.

Author stats with describing the dominance of Internal Combustion Engines (ICE) in the world of automobile industry and also discusses different types of engines. He explains that all engines were similar and it was up to the user to select according to their requirement. He also speaks about induction motor and brushless DC motors and presents his view that these two technologies will be widely used for the coming years.

Then he explains the working of both 3-phase induction motor and a brushless DC motor.

Induction motor have some advantages like: it has no magnets. The motor has the ability to start under load. No inverter is needed.

Some disadvantages of Induction motors are: They cannot operate from DC; AC is a must. Shaft speed is proportionate to line frequency.

Hence, when used with utility power, they are constant speed machines. When operated from utility power, they have limited starting torque and somewhat limited running peak torque capabilities, when compared to DC type machines.

Brushless DC motor have some advantages like: much less rotor heat is generated with the DC brushless drive. The peak point energy efficiency for a DC brushless drive will typically be a few percentage points higher than for an induction drive.

Some disadvantages of Brushless DC motor are: as machine size grows, the magnetic losses increase proportionately and part load efficiency drops.

Permanent magnets are expensive and difficult to handle due to very large forces that come into play when anything ferromagnetic gets close to them.

Finally the author couldn’t choose which drive is better than the other but makes it clear that DC brushless drives will likely continue to dominate in the hybrid and coming plug-in hybrid markets, and that induction drives will likely maintain dominance for the high-performance pure electrics.