Week-6 Challenge: EV Drivetrain

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

- Power Converters - :

- Power electronics is the application to the control and conversion of electric power. A power converter is a process of converting electric

energy from one form to another. A power converter is an electro-mechanical device for converting electrical energy. A power converter can

convert alternating current (AC) into direct current (DC) and vice versa; change the voltage or frequency of the current or do some

combinationof these.

- There are three types of Power converter circuits used in EV's and HEV's are given below - :

- AC to DC (Rectifier)

- DC to AC (Inverter)

- DC to DC (Chopper)

- AC to DC (Rectifier) - :

- AC to DC converters are one of the most important in power electronics because there are lot of real applications are based on this

conversions. The AC to DC conversion process is known as rectification. The rectifier converts AC supply into the DC supply at load end

connection. Normally transformers are used to adjust the AC source to get step down transformer to reduce the voltage level to have better

operation range for DC supply. As we all know that power generated from a powerplant is transmitted to our homes at a voltage of 230V at a

frequency of 50Hz which we can use to power our electrical appliances. So, in the case of electric and hybrid electric vehicle, as the battery is

a DC voltage source, when we have to charge the battery, we plug the charger of the vehicle on the supply or grid which is AC supply. So,

rectifier is the main circuit used in the charger circuit in which the 230 V AC supply is converted into 12V or 48V DC supply. An AC to DC

converter will prevent the current changing direction, so that it only flows unidirectionally. In the circuit, there is a step down transformer, a

diode, a capacitor and a low dropout voltage regulator.

- Rectifiers can be classified as - :

- Half-wave and Full-wave Rectifier.

- Controlled and uncontrolled Rectifier.

- Center-tapped and bridge Rectifier.

- DC to AC (Inverter) - :

- 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.

- The resulting AC frequency obtained depends on the particular device employed. Inverters do the opposite of "converters" which were

originally large electromechanical devices converting AC to DC.

- DC to DC (Chopper) - :

- DC to DC converters in an electric vehicle may be classified into unidirectional and bidirectional converters. Unidirectional DC to DC

converters cater to various onboard loads such as sensors, controls, safety equipments, etc,. They are also used in DC motor drives and

electric traction. Bidirectional DC to DC converters find applications in places where battery charging, regenerative braking and backup power

are required. The power flow in a bidirectional converter is usually from a low voltage end such as a battery or a supercapacitor to a high

voltage side and is refered to as boost operation. During regenerative braking, the power flows back to the low voltage bus to recharge the

battery (buck mode). As a backup power system, the bidirectional DC to DC converter facilitates the safe operation of the vehicle when ICEs

or electric drives failto drive the motor. DC to DC converters are also known as choppers. These type of converters are operated using high

frequency switching operation.

- The Chopper or DC to DC converter topologies are classified as - :

- Buck Converter

- Boost Converter

- Buck-Boost Converter

- AC to DC Converter (Rectifier)- Used in EV charger.

- DC to DC Converter (Chopper)- Used to supply DC power to Auxiliary like radio and car headlights.

- DC to AC Converter (Inverter)- Used to supply Power to Motor.

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'.

- Given - :

- 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

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

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

ω= V/Kϕ - Ra/Kϕ^2 .T

- Block diagram of above mathematical expression in MATLAB - :

- The Simulink Model is modelled to execute the Given Equation which is the Mathematical Model of DC Motor

- V, Kϕ, Ra values are assigned through Constant Blocks.

- Torque value is given as a Ramp Signal.

- The Resultant Speed value obtained after execution of the equation is in Radian per Second.

- Output result of the scope - :

- The Output Curve is a line with negative slope which means as the Torque increase Speed decreases.

4. Refer to the blog on below topic , Explain in brief about author’s perspective - :

- According to the blog ‘Induction vs DC Brushless motors by Wally Rippel, Tesla’, the writer states the changes in the world years ago and the

latest technological trends that are happening. He then explains about the DC Brushless motors and Induction motors.

- DC Brushless Machines - :

- According to him, the rotor has two or more permanent magnets that generate a DC magnetic field. This magnetic field will enter the stator

core which interacts with the currents flowing in the windings to produce an interaction between rotor and stator. It needs a proper flow of

current into the rotor and stator if it’s not connected properly the current will not generate and the torque decreases. During the Electric and

mechanical conversion when the rotor is rotating, the magnitude and polarity of the stator current will continuously vary such that the torque

remains constant and optimal efficiency is achieved.

- Induction Motor - :

- The 3-phase induction motor was invented by Nikola Tesla sometime before 1889. The stators for the 3-phase induction motor and the DC

brushless motor are virtually identical. Both have three sets of distributed windings that are inserted within the stator core. The essential

difference between the two machines is with the rotor.Unlike the DC brushless rotor, the induction rotor has no magnets. They have just

stacked steel laminations with buried peripheral conductors that form a shorted structure. Currents flowing in the stator windings produce a

rotating magnetic field that enters the rotor. The frequency of this magnetic field as seen by the rotor is equal to the difference between the

applied electrical frequency and the rotational frequency of the rotor itself. An induced voltage exists across the shorted structure that is

proportionate to this speed difference between the rotor and electrical frequency. The currents are produced within the rotor conductors that

are approximately proportionate to the voltage, hence there’s a speed difference. Finally, these currents interact with the original magnetic

field to produce forces.

- When a 3-phase induction motor is connected to utility type 3-phase power, torque is produced at the outset. The motor has the ability to

start under load. No inverter is needed. The fact that induction motors are directly compatible with conventional utility power is the main

reason for their success. In contrast, a brushless DC motor produces no starting torque when directly connected to fixed frequency utility

power. They really need the aid of an inverter whose phase is maintained in step with the angular position of the rotor.

- The author's comparison between the two motors based on their working, performance characteristics, efficency points, and suitability for

the EV application can be summarized and is mentioned in the below table - :

DC brushless motor	AC Induction motor
Uses a permanent magnet as a rotor, to produce a magnetic field.	Uses alternating current in a stator to produce a rotating magnetic field.
Stator core involves current flowing within the windings to produce a torque interaction between the stator and rotor. Requires inverter for operation.	The stators for the 3-phase induction motor and the DC brushless motor are virtually identical. It has no magnets. Does not require an inverter for operation but integrating it with one gives a smoother operation of this motor for EV.
Requires angle position sensor for operation	requires speed sensor for operation
Does not produce much heat. Cooling is easier	Has heating issues and efficiency depreciation as an effect.
Has higher peak efficiency point	Comparatively lower efficiency point compared to DC Brushless motor.
Permanent magnets are expensive and the motor cost increases as a result. Handling becomes difficult because of the strong magnetic forces between ferromagnetic materials and magnets.	Requires just coils to make the motor and is inherently cheaper to produce and easier to handle as coils need to be excited to produce any magnetic field.
Easier to control for lower to medium performance ranges.	Achieving stability over the entire torque-speed range and temperature is more difficult with induction than with DC brushless.
difficult to alter motor parameters such as field strength because of permanent magnets.	induction machines have no magnets and B fields are “adjustable,” since B is proportionate to V/f (voltage to frequency).
Speed is controlled by the PWM method.	Speed is controlled by altering RMS speed by altering the frequency of feeding in voltage.
As the motor size increases, magnetic losses increase proportionately and load efficiency drops.	induction motors do not follow the same trend and are thus desired for high-performance applications as average efficiency is better than DC machine counterpart.