Week-6 Challenge: EV Drivetrain

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

ANSWER:-

Power electronics deal with the transfer, control and conversion of power. Typically power can be classified into AC and DC. The types of converters are listed below.

• AC to DC : This is also called a rectifier circuit and it converts AC supply to a DC supply.
• DC to AC : This type of power converter is known as an inverter. It changes DC to AC.
• DC to DC : This converts DC from one level to another and is called Chopper. They are classified into Step down and Step up choppers.
• AC to AC : This converts AC to AC and is calley cyclo converter.

The HEV may make use of the following :-

• Rectifier : To convert the AC voltage from a 3 phase AC motor to suitable DC voltage to be stored in battery. This circuit is also present in the charger circuit to convert AC from main line to DC to charge the vehicle battery.
• Inverter : The 3 phase AC motor requires AC to run but the power source is DC , so The inverter converts the DC from battery source to suitable AC voltage for the motor.
• Chopper : Choppers may be used to convert high voltage DC to a lower voltage DC to power other auxiliary devices like sensors, infotainment system, ECU, etc.

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

ANSWER:-

Given:-

Powertrain Battery voltage = 72V,

Armature voltage = 72V,

Armature current = 400A,

Ra = 0.5Ω,

KΦ = 0.7 Volt second,

Chopper Switching frequency= 400 Hz

The vehicle speed-torque characteristics are given by the below equation

vehicle speed torque char. eqn τ=24.7+0.0051ω2τ=24.7+0.0051ω2

Voltage at motor for 70% duty cycle = 0.7*72 = 50.4Volts

Torque speed eqn of DC motor is {(V*kϕ/Ra)−(((kϕ)^2)/Ra)*ω}

substituting values in above eqn we get

T = 70.56-0.98w

solving above eqn and vehicle torque speed eqn we get

70.56-0.98w = 24.7+0.0051w^2

re-arranging we get

0.0051w^2+0.98w-45.86=0

above eqn is similiar to ax^2+bx+c

then w={(−b±√(b2−4ac))/2a}

solving we get w = 38.91 radians/sec

therefore the steady speed of EV with 70%duty cycle is 38.91 rad/s.

3]Refer to the blog on below topic:

Induction Versus DC brushless motors by Wally Rippel, Tesla

Explain in brief about author’s perspective.

ANSWER:-

Wally Rippel says that he's seen a lot of changes in the electric vehicle industry right from when he was young and when they used to use lead acid batteries and dc motor to now where most vehicles use lithium polymer cells and brushless dc motor or induction motor. He states that the two most commonly used motors are 3 phase induction motor and a brushless dc motor also called BLDC motor.

He also states that just like differnt types of engines are suited to different applications then simillarly the motors in an electric vehicle are also of different type. The choice between an induction and BLDC motor depends purely on the application and cost. For instance he describes how both motors work and by far the induction motor is more complex to design and understand as it has more compenents such an inverters that need feedback loops. But the induction motor if ideally designed can produce a large amount of torque for the provided load almost right from the start.

He also states about the construction of the motors, where both have the same stator but they differ in the construction of rotors. He talks about how hybrid electric vehicles will adopt these motors based on their requirements and states that it BLDC motor is more commonly used where as only pure electrics like tesla roadster make the use of induction motor. In the end he claims its not possible to conclude which of the two is the best.