Week-6 Challenge: EV Drivetrain

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

ANS:

The task of a power converter is to process and control the flow of electrical energy by supplying current and voltages that are optimally suited for the user load. The block diagram of a power electronic converter is given below:

This circuit consists of 4 parts:

• electrical energy source
• power electronic circuit
• control circuit
• electric load

This convertor changes one form of electrical energy to the other.

The power electronic circuit consists of both the power part and control part. The power part transfers the energy from source to load and it consists of power electronic switches (SCR or TRIAC), transformers, electric choke, capacitors, fuses, and sometimes resistors.

The control circuit regulates the elements in the power part of the convertor. This block is built with a complex low power electronic circuit that consists of either analog or digital circuit assembly.

Depending on the type of function performed, power electronic converters are categorized into the following types.

• AC-DC(rectifiers)
• DC-AC(inverters)
• AC-AC(cyclo converter)
• DC-DC(chopper)

RECTIFIERS:

A rectifier is an electrical device comprised of one or more diodes that allow the flow of current only in one direction. It basically converts alternating current into direct current. Rectifiers can be mold in several shapes as per necessity like semiconductor diodes, SCRs (silicon controlled rectifiers), vacuum tube diodes, mercury-arc valves, etc.

INVERTERS:

A power inverter, or inverter, is a power electronic device or circuitry that changes direct current (DC) to alternating current (AC). The input voltage, output voltage and frequency, and overall power handling depend on the design of the specific device or circuitry. The inverter does not produce any power; the power is provided by the DC source. A power inverter can be entirely electronic or maybe a combination of mechanical effects (such as a rotary apparatus) and electronic circuitry.

CHOPPER:

A chopper is basically a dc to dc converter whose main function/usage is to create adjustable dc voltage from fixed dc voltage sources through the use of semiconductors.

CYCLO CONVERTERS:

A cycloconverter (also known as a cycloinverter or CCV) converts a constant voltage, constant frequency AC waveform to another AC waveform of a different frequency. A cycloconverter achieves this by synthesizing the output waveform from segments of the AC supply (without an intermediate DC link).

TYPES OF  POWER CONVERTERS EMPLOYED IN EV AND HEV

1. AC - DC convertor 

1.    The AC-DC converters or rectifiers are used to charge the battery from external sources.
2.    They are also called as rectifiers.
3.    The external power stations use an AC supply and only DC can be stored in the battery.
4.    This is mainly used in plug-in hybrids where the battery can be charged externally.

2. DC-AC converter

1.     the inverter supplies current to the battery pack for recharging during regenerative braking.
2.     Provides electricity to the AC induction motor for propulsion.
3.     The inverter unit choreographs how these divergent voltages and current types work together.

because of the use of transformers and semiconductors (and the accompanying resistance encountered), enormous amounts of heat are emitted by these devices. Adequate cooling and ventilation are paramount to keeping the components operational. For this reason, inverter/converter installations in hybrid vehicles have their own dedicated cooling systems, complete with pumps and radiators, that are entirely independent of the engine's cooling system.

3. DC-DC converters

•  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)
• The amount of power flow between the input and the output can be controlled by adjusting the duty cycle (ratio of on/off time of the switch). Usually, this is done to control the output voltage, the input current, the output current, or to maintain constant power. 
• The most commonly used DC-DC converters are: 

         1. Unidirectional Converters:

               They cater to various onboard loads such as sensors, controls, entertainment, utility, and safety equipment.

         2. Bidirectional Converters:    

              They are used in places of regenerative braking and battery charging. The power flow is from a low voltage end like a battery or a supercapacitor to a high voltage side which is referred to as boost operation.

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

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

 ANS:

As the torque formula is given, we need to find the speed in rad/sec. Use the other expression available which is given below:

`T=V(Kphi)/(Ra)-((Kphi)^2)/(Ra).omega`

T = torque

V = armature voltage

R = resistance 

w = speed rad/sec

Compare the two equations:

`24.7+(0.0051)omega^2=V(Kphi)/(Ra)-((Kphi)^2)/(Ra).omega`

`24.7+(0.0051)omega^2=(72*0.7*0.7)/0.5-(0.7^2)/0.5*omega`

`24.7+(0.0051)omega^2=70.56-0.98omega`

`omega^2+192.15*omega-8992.15=0`

In order to find the roots use following formula

For a quadratic equation of the form `ax^2+bx+c=0`the solutions are

`X1,2=(-b+- √(b^2-4*a*c))/(2*a)`                                       

As the speed value can not be negative

w = 38.95 rad/sec

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

 `omega=V/(Kphi)-(Ra)/(Kphi)^2*T`

ANS:

Mathematical model of DC motor is shown below.

                                      Fig. Speed vs Torque characteristics of DC motor

4.   Refer to the blog on below topic: 

            Induction Versus DC brushless motors by Wally Rippel, Tesla

            Explain in brief about author’s perspective. 

ANS:The blog by Wally Rippel which tells the evolution of EV from years as an introduction. He briefed that the lead acid batteries are been replaced by lithium ion which is more efficient in all perspective, brushed DC motor being replaced by brushless DC (BLDC) and induction motor (IM).

He described the working principle, construction of both BLDC and IM and also compared both of them in terms of perfromance,efficiency,size etc which is explained below.

The construction of BLDC motor is such that the stator is made of thin and stacked laminations. The stator interacts with the current flowing within its winding to produce a torque interaction between stator and rotor. The rotor is made of two or more permanent magnets and it rotates only when the simultaneous action of current flow in the stator windings (ON and OFF for some time) happens. The BLDC type motor must have an inverter circuit othewise this motor is a total waste.

The construction of stator of an IM is same as that of the BLDC but we provide an AC supply from grid here. The rotor is of wounded or squirel cage type and no permanent magnet type are used here. Here for its working there is no need of inverter circuit but for better efficiency they are used. The working of IM is such that the stator magnetic field creataor is of rotating type in which the rotor is placed, when the current passes through the rotor the resultant magnetic fielf vector is generated and rotor tries to catch up with the resultant, as a result the rotor rotates.

He also compared the BLDC motor with the IM 

1. In BLDC the heat generated in rotor is less when compared to the IM rotor.

2. The BLDC motor needs an inverter for its working but the IM doesn't need one  but for better performance and efficiency IM also uses inverter and feedback circuits.

3. By increasing the size of BLDC motor for more power output, it may produce more losses when compared to the BLDC motor in small size but increasing the size of IM will not produce huge change in its losses.

4. Permanent magnets are expensive i.e BLDC motor are more expensive than IM.

5. since IM do not have any permanent magnet controlling the magnetic field is easy and easy to reduce hysterisis and eddy current losses, but in BLDC motor we cannot adjust the rotor magnetic filed strenght because of the permanent magnets.

6. Peak efficiency will be more in BLDC motor than IM but the average efficiency are almost same.

7. The IM provide a little low torque than the BLDC type.

By accomodating all the factors walley Ripple came to the conclusion that BLDC will rule the HEV and IM will be usefull in high power application electric vehicle. He also concluded that there are no ideal motor and each types of motor have there own advantages and disadvantages and selection is only based on the purpose.