Week-6 Challenge: EV Drivetrain

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

Introduction:- We have different types of coverter which are using in the Electric vehicle. As we know we drive EV by motor or by motor & engine. In both the cases we required battery source.

Battery source are give and take only DC supply but battery power will be use for Motor which is AC induction motor. Also for charging the battery we have AC supply but battery needs DC power supply.

So these are the factors which needs to provide by Power Converter.

We have different type of power converter and day by day it is getting modified as per space, cost, efficiency, heating etc constarints. 

AC-DC Converter:- For providing Power to the battery we need AC-DC converter which i mean Rectifier circuit. Also as per specific voltage level of battery we again required DC-DC converter which help us to reach the battery specific voltage to charge it. 

The simplest AC/DC converters comprise of a transformer following the input filtering, which then passes onto a rectifier to produce DC. In this case, rectification occurs after the transformer because transformers do not pass DC. However, many AC/DC converters use more sophisticated, multi-stage conversion topologies as depicted in figure 1 due to advantages of smaller transformer requirements and lower noise referred back to the mains power supply.

either natural (in the case of a simple diode) or forced, as in the case of devices that are more sophisticated.

High efficiency power supplies can use active devices like MOSFETs as switches in such circuits. The reason for using topologies that are more complex is usually for efficiency improvement, to lower noise or to act as a power control. Diodes have an intrinsic voltage drop across them when they conduct. This causes power to be dissipated in them, but other active elements may have much lower drop and therefore lower power loss. SCR and TRIAC circuits are particularly common in low cost power control circuits like the light dimmer example below.

used to directly steer and control current delivered to the load as the input mains alternates. Note that these implementations are not galvanic when they do not have a transformer in the circuit – only useful in circuits that are appropriate like direct mains connected light control. They are also used in high power industrial and military power supplies where simplicity and robustness is essential.

Rectifiers are implemented using semiconductor devices that conditionally conduct current in one direction only, like diodes. More sophisticated semiconductor rectifiers include thyristors. Silicon controlled rectifiers (SCR) and triode for alternating current (TRIAC) are analogous to a relay in that a small amount of voltage can control the flow of a larger voltage and current. The way these work is they only conduct when a controlling ‘gate’ is triggered by an input signal. By switching the device on or off at the right time as the AC waveform flows – current is steered to create a DC separation. There are many circuits for doing this, with signals tapped off the AC waveform used as control signals that set the phase quadrants thyristors are on or off. This is commutation, and can be

DC-AC Converter:- Again charged battery required to run the vehicle but our motor is AC induction motor so we need DC-AC converter for run the motor. Also this AC supply will not be sufficient to run the AC motor so we will use AC-AC converter. 

An Electric Vehicle is a vehicle that uses a combination of different energy sources, Fuel Cells (FCs), Batteries and Supercapacitors (SCs) to power an electric drive system as shown in fig. In EV the main energy source is assisted by one or more energy storage devices. Thereby the system cost, mass, and volume can be decreased, and a significant better performance can be obtained. Two often used energy storage devices are batteries and SC_S. They can be connected to the fuel cell stack in many ways. A simple configuration is to directly connect two devices in parallel, (FC/battery, FC/SC, or battery/SC). However, in this way the power drawn from each device cannot be controlled, but is passively determined by the impedance of the devices. The impedance depends on many parameters, e.g. temperature, state-of-charge, health, and point of operation. Each device might therefore be operated at an inappropriate condition, e.g. health and efficiency. The voltage characteristics also have to match perfectly of the two devices, and only a fraction of the range of operation of the devices can be utilized, e.g. in a fuel cell battery configuration the fuel cell must provide almost the same power all the time due to the fixed voltage of the battery, and in a battery/supercapacitor configuration only a fraction of the energy exchange capability of the supercapacitor can be used. This is again due to the nearly constant voltage of the battery. By introducing DC/DC converters one can chose the voltage variation of the devices and the power of each device can be controlled.

In electric engineering, a DC to DC converter is a category of power converters and it is an electric circuit which 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).

DC/DC converters can be designed to transfer power in only one direction, from the input to the output. However, almost all DC/DC converter topologies can be made bi-directional. A bi-directional converter can move power in either direction, which is useful in applications requiring regenerative braking.

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 a constant power. Transformer-based converters may provide isolation between the input and the output. The main drawbacks of switching converters include complexity, electronic noise and high cost for some topologies. Many different types of DC/DC power converters are proposed in literature. The most common DC/DC converters can be grouped as follows:

Objective 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%?

Result:- Provided Duty cycle:- 70%=0.7

   Vout=Vbatt*duty cycle

=Rated armature current*07

=72*0.7

=50.7Volt

Tm=(V*KΦ)/Ra-((KΦ^2)/Ra)*w

In this equation we have

T= Total torque

V= output voltage= 50.7Volt

Armature resistance(Ra)= 0.5 ohm

Motor constant(KΦ)=0.7v/s

Tm=(50.7*0.7)/0.5-((0.7^2)/0.5)*w

Tm= 70.56-0.98w-----------------(1)

Tv= 24.7+(0.0051)w^2-----------(2)

By calculating both the equation

0.0051w^2+0.98w-45.86=0

By solving this equation we get 

w=38.92rad/sec & 231.02rad/sec

38.92 rad/sec is the EV steady state speed if duty cycle is 70%

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

               $\omega$= V/K$\varphi$ -(Ra/K$\varphi$^2 ).T

Simulink link:- https://drive.google.com/file/d/1tBGS6WsWe3xyZkhjQcONkHmfoW44eEhl/view?usp=sharing

In this model i have considered the volatge 50 V, Motor constant(K)-0.7 & Armature resistance constant(Ra)- 0.5ohm

Objective 4:- Refer to the blog on below topic: 

            Induction Versus DC brushless motors by Wally Rippel, Tesla

            Explain in brief about author’s perspective. 

Result:- So basically first of all Author is talking about two different technologies which are Brushless DC motor & Induction motor. These will use in EV & Hev application. So he has define the charcterstics and advantage disadvantage of the technology.

Brushless DC machine:- As per author With brushless machines, the rotor includes two or more permanent magnets that generate a DC magnetic field (as seen from the vantage point of the rotor). In turn, this magnetic field enters the stator core (a core made up of thin, stacked laminations) and interacts with currents flowing within the windings to produce a torque interaction between the rotor and stator. As the rotor rotates, it is necessary that the magnitude and polarity of the stator currents be continuously varied – and in just the right way - such that the torque remains constant and the conversion of electrical to mechanical energy is optimally efficient. The device that provides this current control is called an inverter. Without it, brushless motors are useless motors.

Induction machine:- Unlike the DC brushless rotor, the induction rotor has no magnets – 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. In turn, 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. Accordingly, an induced voltage exists across the shorted structure that is proportionate to this speed difference between the rotor and electrical frequency. In response to this voltage, currents are produced within the rotor conductors that are approximately proportionate to the voltage, hence the speed difference. Finally, these currents interact with the original magnetic field to produce forces – a component of which is the desired rotor torque.

Major difference between them:-

1)- Let’s move on to induction motor drives. A forerunner of the 3-phase induction motor was invented by Nikola Tesla sometime before 1889. Curiously, 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.

2)- 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.

3)- 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. Finally, when operated from utility power, they have limited starting torque and somewhat limited running peak torque capabilities, when compared to DC type machines.

4)- Without feedback inverter induction motor can't achieve the desired torue by system compare to Brushless DC motor.

5)- DC brushless drives require an absolute position sensor, while induction drives require only a speed sensor; these differences are relatively small.)

6)- One of the main differences is that much less rotor heat is generated with the DC brushless drive. Rotor cooling is easier and peak point efficiency is generally higher for this drive. The DC brushless drive can also operate at unity power factor, whereas the best power factor for the induction drive is about 85 percent. This means that the peak point energy efficiency for a DC brushless drive will typically be a few percentage points higher than for an induction drive.

7)- the strength of the magnetic field produced by the permanent magnets would be adjustable. When maximum torque is required, especially at low speeds, the magnetic field strength (B) should be maximum – so that inverter and motor currents are maintained at their lowest possible values. This minimizes the I² R (current² resistance) losses and thereby optimizes efficiency. Likewise, when torque levels are low, the B field should be reduced such that eddy and hystersis losses due to B are also reduced. Ideally, B should be adjusted such that the sum of the eddy, hysteresis, and I² losses is minimized. Unfortunately, there is no easy way of changing B with permanent magnets.

8)- In contrast, induction machines have no magnets and B fields are “adjustable,” since B is proportionate to V/f (voltage to frequency). This means that at light loads the inverter can reduce voltage such that magnetic losses are reduced and efficiency is maximized. Thus, the induction machine when operated with a smart inverter has an advantage over a DC brushless machine

9)-With DC brushless, as machine size grows, the magnetic losses increase proportionately and part load efficiency drops. With induction, as machine size grows, losses do not necessarily grow. Thus, induction drives may be the favored approach where high-performance is desired; peak efficiency will be a little less than with DC brushless, but average efficiency may actually be better.

10)- As per author cost in more in permanent magnet aslo difficult to handle as compare to induction machine.Due to low field area of infuction machine small inverter and controlling required in induction machine. Since spinning induction machines produce little or no voltage when de-excited, they are easier to protect.

11)- If we talking about controoling then as per author induction machine are difficult to control because of complex logics to maintain stability for the enitre torque speed behaviour. And the system generated more heat as compare to brushless DC machine.

Conclusion:- As per authore both technology is good and they are comaptiable as per their application and different categories. These will become the part of the upcoming era of EV.