Week-6 Challenge: EV Drivetrain

I AM A STUDENT OF LIVE BATCH-16

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

SOLUTION:

The function of converter

More properly called a voltage converter, this electrical device actually changes the voltage (either AC or DC) of an electrical power source. There are two types of voltage converters: step up converters (which increases voltage) and step down converters (which decreases voltage). The most common use of a converter is to take a relatively low voltage source and step-it-up to high voltage for heavy-duty work in a high power consumption load, but they can also be used in reverse to reduce voltage for a light load source.

main converters used in electric vehicle are as follows:

AC to DC Converters or Rectifiers

An AC to DC converter is also called a rectifier, which converts AC supply from main lines to DC supply for the load. The block diagram of an AC to DC converter is shown in figure below.The essential components in this rectifier include transformer, switching unit, filter and a control block.

Here, the transformer adjusts the primary AC source supply to the input of rectifier stage. Usually it is a step-down transformer that reduces the supply voltage to a circuit operating range.

The rectifier converts the low voltage AC supply into DC supply.

It comprises diode and/or thyristors based on type of rectifier. The output of the rectifier is of pulsed DC and hence it is filtered using filter circuit, which is usually made with a capacitor or a choke.

The control block controls the firing angle of thyristors in case of phase controlled rectifiers. Since the diode is not a controllable device, control block is not needed in case of diode rectifiers.

Rectifiers are majorly classified into two types

• Uncontrolled diode rectifiers-This type of rectifier converts AC voltage from mains into a fixed DC voltage.
• Controlled rectifiers

DC to DC Converters

Many DC operated applications need different levels of DC voltage from a fixed DC source.

Some of these applications include subway cars, DC traction systems, control of large DC motors, battery operated vehicles, trolley buses, etc. They require variable DC to produce variable speed, so a power conversion device is needed.

A DC chopper is a static device that converts a fixed input DC voltage to variable DC output or a fixed DC output of different magnitude (which can be lower or higher) than input value. The block diagram of a DC chopper is shown in figure below.

The chopper circuit is connected between DC input source and DC load. This chopper consists of power electronic switching devices such as thyristors which are connected in such a way that they produce required DC voltage to the load.

The output voltage is controlled by adjusting ON time of the thyristor (or switch) which turn changes the width of DC voltage pulse at the output. This method of switching is called as pulse width modulation (PWM) control.

The output of the chopper can be less or greater than the input and also it can be fixed or variable. These can be unidirectional or bidirectional devices based on the application it is intended for.

DC choppers are mainly used in DC drives, i.e., electric vehicles and hybrid electric vehicles.

DC choppers are classified into three basic types based on input and output voltage levels and are discussed below.

AC to AC Converters

AC/AC converters connect an AC source to AC loads by controlling amount of power supplied to the load. This converter converts the AC voltage at one level to the other by varying its magnitude as well as frequency of the supply voltage.

These are used in different types of applications including uninterrupted power supplies, high power AC to AC transmission, adjustable speed drives, renewable energy conversion systems and aircraft converter systems.

DC to AC Converters or Inverters

These converters are connected between DC source of fixed input, and variable AC load. Most commonly, these DC to AC converters are called as inverters. An inverter is a static device that converts fixed DC supply voltage to variable AC voltage.

Here the fixed DC voltage is obtained from batteries or by DC link in most power electronic converter. The output of the inverter can be variable/ fixed AC voltage with variable/fixed frequency.

This conversion from DC to AC along with variable supply is produced by varying the triggering angle to the thyristors. Most of the thyristors used in inverters are employed with forced commutation technique.

These can be single phase or three phase inverter depending on the supply voltage. These converters are mainly divided into two groups. One is PWM based inverters and other multilevel inverters.

Further, these are classified voltage source inverter and current source inverter. Each type is subdivided into different types such as PWM, SVPWM, etc. Multilevel inverters are more popular in industrial applications.

The inverters overcome the drawbacks of PWM based inverters.

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

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

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

SOLUTION:

Tv=Vv*(Kϕ/ Ra)-(( Kϕ)²/ Ra)* ω ----------eqn 1

Tv=vehicle torque in Nm

Vv=Vehicle voltage in V

ω =steady state speed of vehicle in rps

Kϕ= motor constant

Ra=armature resistance

Now,

vehicle output voltage= battery*duty cycle

= 72*0.70

=50.4V

vehicle output voltage=50.4V

Putting all values in eqn 1

Tv=(50.4*0.7/0.5)-((0.7)²/0.5)* ω

Tv=70.56-0.98 ω² -------- eqn 2

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

--------eqn3

At steady state,

eqn 2 = eqn 3-----------eqn 4

24.7-0.0051 ω²=70.56-0.98 ω²

0.0051 ω²+0.98 ω-45.85=0 ---------eqn 5

Solving eqn5 by quadratic equation method

ω= 38.92 rad/sec

steady-state speed of the vehicle is=ω= 38.92 rad/sec

EV steady state speed if duty cycle is 70 percentage = 38.914983 radian/sec =371.799198 RPM

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

ω= (V/Kϕ) -(Ra/Kϕ^2 ).T

SOLUTION:

RESULT:

4. Refer to the blog on below topic:

Induction Versus DC brushless motors by Wally Rippel, Tesla

Explain in brief about author’s perspective.

SOLUTION:

In Wally Rippel blog he discussed about So two technologies i.e. DC brushless and induction.How do they work? What differentiates them? And what do they have in common.

In starting he said one size does not fit all means in that odious word engines are of the different types some years ago some thought to try to figure out which is best engine and that should be in production but his thoughts got failed as there is no only one best with the different requirements there are different type of engines so this also implies in case of brushless DC and induction motor all are alike and its up to the user to select one according to their requirements due to the imprising technology there has been alternative found and then such as lead acid batteries DC brush motors and contact controllers have been replaced by Lithium batteries brushless DC or induction motors and inverters one tho their has been various replacement brushless DC and induction motors are playing a major role till date to the construction and working of the brushless motors and induction motors are being discussed the status for the three phase induction motor and the DC brushless motor are Virtually identical he said both have three sets of distributed windings that are inserted within the status Core. Both drives use three face modelling inverters the only difference are the routers and the inverter controls the pros and cons of both brusless DC motors and induction motors are being discussed here the advantage of brushless DC Motors are much less rotar it is generated the peak point efficiency will be a few percent hire then the induction motor that disadvantages are the Machines size grows the magnetic losses increases proportionately and part load efficiency drops permanent rotors are also difficult to handle due to very large forces that come into play when anything ferromagnetic gets come close to them . induction motor has some advantages the motor has no magnets the motor can start under no load condition the disadvantages are they cannot open in DC. When operated with utility power they have limited starting torque and Limited dark capabilities when compared to DC type machines by saying Still No Winner in his blog he concludes 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. And The hardware is common for both drives could mean that we will see induction and DC brushless live and work side by side during the coming golden era of hybrid and electric vehicles.