Week-6 Challenge: EV Drivetrain

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

Electric Vehicle Hybrid Electric Vehicle and Fuel Cell Electric Vehicle:

An Electric Vehicle is an vehicle which is propelled by an Electric Traction Motor which is powered by an Li-ion Battery Pack. An Electric Vehicle may have two power sources such as battery and Super Capacitors. An Hybrid Electric Vehicle have Internal Combustion Engine and Battery Pack and depending on HEV Architecture such as Series, Parallel and Series-Parallel the vehicle may be propelled by an Electric Traction Motor alone or by combination of ICE and Electric Motor. In FCEVs the vehicle has 2 sources Fuel Cell and Battery Pack.

In all above vehicles the battery pack receives power from an external charger circuit which is called as an Electric Vehicle Supply Equipment, if the power received is AC then the Vehicle On board Charger converts it to dc and  it supplies power to the battery. So Power Converters plays an important Role in operation of electric vehicle. The power and voltage required for all components in EV, HEV and FCEV is different so power converters converts the voltage from one level to another level and from one form to another form. Its main purpose is to process and control the flow of electric energy by supplying Voltage and Current in a form that is suited for the Components and devices.

The Types of Power Converters used in EV, HEV and FCEV are:

1. AC-DC Converter (Rectifier)

2. DC-DC Converter (Step up and Step Down Converter)

3.DC-AC Converter (Inverter)

 

 

 

 

1.AC-DC Converter (Rectifier):

AC-DC Converter is commonly known as Rectifier and has wide range of applications in Power Electronics. The Rectifier circuit converts AC supply into DC Supply and this process of conversion is called Rectification. A step down transformer is used in Rectifier to lower the AC Supply Voltage and then through Rectifier circuit it is converted to suitable operating range DC Voltage which is supplied to the Devices.

In Elelctric Vehicle it is used in On Board Charger Circuit. The On Board Charger is capable of taking both AC and DC Supply while some BEV has different ports for AC Supply Charging and DC Charging. If the EVSE supplies AC power to the charger then On Board Charger take input as AC supply and convert it to DC supply and will charge the battery pack. The Rectifier Circuit consists of Diodes, Capacitors , Resistors and Regulators. There are different types of Rectifiers such as Half Wave Rectifier, Center Tapped Rectifier and Full Wave Bridge Rectifier. Depending  on the type of applications the rectifier is used.

a) Half Wave Rectifier:

A Half Wave Rectifier consists of only one diode or SCR,  Transformer a Resistive Load and a Filter Capacitor. Diode operates only in forward biased mode. When a  AC Supply waveform is passed through a half-wave rectifier, diode gets forward biased and Half-wave rectifiers only allow one half-cycle (i.e  positive or negative half-cycle) of the AC voltage through and will block the other half-cycle on the DC side.

         

                                                       HALF WAVE RECTIFIER WITH RESISTIVE LOAD

When AC Supply is passed Diode allows only positive Cycle and blocks negative half cycle and at the output a pulsating DC Waveform is obtained. If the Diode direction is revrsed then it will allow Nrgative Half cycle of AC waveform and block Positive hakf cycle. Diodes are used in applications where fixed dc voltage is needed and thyristors are used where variable dc voltage is required.

 To smoothen the dc Voltage waveform or to reduce the ripples in dc waveform a Capacitor or Inductor is used. Filters are components used to convert (smoothen) pulsating DC waveforms into constant DC waveforms. They achieve this by suppressing the DC ripples in the waveform.

               

                                                        HALF WAVE RECTIFIER WITH FILTER CAPACITOR

When an AC Supply Waveform is applied to Half Wave Rectifier with Capacitor Filter the Capacitor removes the unwanted AC component ie Ripple remaining when converting the AC voltage waveform into a DC waveform.If a high value Capacitor is used then the DC Output will be more like Constant DC but still some small amount of Ripples will be  left on the output side which pulsates the DC waveform.

 

b. Center Tapped Rectifier:

                                                                         Center Tapped Rectifier

It is a rectifier which is designed by using two diodes as well as a center tapped transformer for converting the whole AC signal to DC and  is called as  center tapped FWR.  This is called as “full wave center tapped” because there are two full cycles in one complete cycle of AC voltage. This means that it produces twice as much DC voltage as a half-wave rectifier would produce from an equivalent AC source.

The AC voltage Source is given to the primary side of Transformer . A Center tapped is conected to the center of secondary winding which divides the input voltage into two parts. The higher portion is connected to Diode D1 and lower portion is connected to Diode D2. The center point is considerd as ground point.

When Ac supply is applied  during positive half cycle A will be +ve and B will be -ve and Diode D1 will be forward biased and D2 will be reversed biased. When D1 is forward biased current flow will be through D1 Load Resistor Rl. So output will have pulsating DC waveform.

During negative half cycle A will be -ve and B will be +ve and D2 will be forward biased and D1 will be reversed biased. The current direction will be through D2, Load Resistor Rl.

c. Full Wave Bridge Rectifier:

 A full wave Bridge Rectifier consists of 4 diodes which are connected inseries pair and during the half cycle only 2 diodes conduct.

                                       

                                                                               Full Wave Bridge Rectifier

When AC supply is applied to the circuit during positive half cycle Diode D1 and D2 are forward biased and D3 and D4 are reversed biased. The current flow will be through D1 through load resistor through D2 and back to the Source.

During negative half cycle Diode D3 and D4 are forward biased and D1 and D2 are reversed biased. The current flow will be through D3 through load resistor through D4 and back to the Source.

Since the direction of current flowing through load  is unidirectional so the voltage developed across the load is also unidirectional. The output will be Pulsating DC waveform.

To reduce the ripples in dc waveform a capacitor filter is connected to smoothen out the output.

 

When AC supply is applied the capacitor charges during increase in voltage and stores the electrons and it releases during the decrease in voltage ie discharges  and reduces the AC Voltage ripples.

 

d. Three Phase FW Rectifier:

 

A three-phase full-wave diode rectifier is obtained by using two half-wave rectifier circuits. The advantage of this circuit is that it produces a lower ripple output than a half-wave 3-phase rectifier. This is because it has a frequency of six times the input AC waveform. The Diodes conducts in pairs, each pair conducts for 60 degree and each diode conducts for 120 degrees. 

 

2. DC-DC Converter (Step up and Step Down Converter)

DC-DC converter is a converter which converts dc voltage from one voltage level to another voltage level. A converter which steps up the dc voltage level is called Step up Dc converter (Boost Converter) and one which steps down the voltage is  called Step down converter (Buck Converter). In BEV battery pack supplies high voltage dc supply and Auxilary loads such as Headlights, Rear Lights, wipers, Indicators, Aux Systems etc works on low voltage DC Supply so a step down converter or Buck Converter is used to step down the voltage. The AC Traction motor requires High Voltage so before the Motor receives power from Traction Inverter the violtage from Battery pack is boosted to voltage level which is required by Traction motor by using Step up converter or Boost Converter. This Converter  should be Bi directional ie it should allow the power flow from battery pack to motor and from Motor/Generator to battery pack during Regenerative Braking.

a. Buck Converter(Step Down Converter): 

A Buck Converter or Step down converter is a converter which steps down the dc voltage level while steping up the cuurent.

A Buck Converter consists of a Switch (such as Transistors, IGBT or MOSFETS) a Diode, Inductor and a Capacitor.The input voltage source is connected to a controllable solid state device which operates as a switch. The second switch used is a diode The switch and the diode are connected to a low-pass LC filter which is appropriately designed to reduce the current and voltage ripples. The load is a purely resistive load.

The controlled switch is turned On and Off by using Pulse Width Modulation Method. Time based Modulation is mostly used for DC-DC Converters. It is simple to construct and use. The frequency remains constant in this type of PWM modulation.

The Buck Converter operates in 2 modes

Mode1: Switch is On and Diode is Off 

Mode2: Switch is Off and Diodde is ON

 

                                  Mode 1                                                               Mode 2

The switch is on for a time T_ON and is off for a time T_OFF. Time Period is defineds  as

T = Ton + Toff

and the switching frequency,

The duty cycle,

Since the switch is closed for a time T_ON = DT , Δt = DT.

During Mode 2 the energy stored in the inductor is released and is ultimately dissipated in the load resistance, and this helps to maintain the flow of current through the load.

.

Buck Converters are used for low power applications.

b. Boost Converter (Step Up Converter):

A boost converter is used to step up the dc voltage to a specified output. A boost converter consists of a Switching device, Diode, Capacitor, Inductor.

                                                                        BOOST CONVERTER

A boost converter is used between Battery Pack and Traction Inverter. Boost Converter also has 2 modes of operation like Buck Converter.

Mode1: Switch is On and Diode is Off 

Mode2: Switch is Off and Diodde is ON

                                           Mode 1                                                                            Mode 2

During Mode 1 Switch is turned ON and Diode is off and all the current will flow through the switch and back to the voltage source and charging the inductor.

Time Period is T =Ton +ToffDC Converter

Switching frequency f = 1/T

Since the switch is closed for a time T_ON = D_Tso Δt = DT.

During Mode 2 when switch is turned off and diode is turned on so the polarity of the inductor is reversed. The energy stored in the inductor is now  released and is ultimately dissipated in the load resistance and this helps to maintain the flow of current in the same direction through the load and also step-up the output voltage as the inductor is now also acting as a source in conjunction with the input source.

 

c. Buck Boost Converter:

A Buck Boost Converter is type of DC-DC Converter whose output voltage is  either lesser or greater than the input voltage. Buck Boost Converter consists of Inductor, Switching Device, Capacitor and diode. It allows bi dirctional power flow. If the switch is ON then the inductor feed the energy from the input and it stores the energy. If the switch is opened it discharges the energy.

 

3. INVERTER (DC-AC)

An Inverter is used to convert the dc power from battery pack to Ac power which is upplied to Traction Motor to propel the vehicle. Traction Inverter acts as an Speed controller for controlling the speed of the motor by varying the frequency of AC Supply. It plays a significant role in capturing energy from regenerative braking and feeding it back to the battery. The Inverter generates a pulsated Sinusoidal Square wave, so to get a Pure Sine wave Pulse Width Modulation Technique is used to get pure sine wave and reduce the AC harmonics so losses can be minimised.

A 3 phase inverter is used in Electric Vehicles as the traction motor used is 3Phase Ac Synchronous Motor. 3 phase Inveretr consists  of 6 Switching Devices which have high switching frequency which generates a lot of heat  so a thermal management system is also employed in order to keep the Inverter temperature under control. The ratio of the switch used in the inverter is 50% and switching can be done after every 60-degree angle. Switches like S1, S2, S3, S4, S5, S6 will complement each other.

The Inverter conducts in 120degree and 180degree Conduction Mode. In 180degree conduction mode, each device is carried at 180°. Where they are activated at intervals of 60 degree. In 120degree of Conduction mode, each electronic device will be in a 120 ° conductor position. 

 

 

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%?

 

Solution:

Armature Voltage = 72V

Duty Cycle = 70% = 0.7

So Actual Armature Voltage = 72 * 0.7 = 50.4V

Switching frequency = 400Hz

Ra = 0.5Ω, KΦ (Motor Constant )= 0.7 Volt second

Torque Equation of Motor

T = (V*Kϕ/Ra) - (K$\varphi$^2/Ra)*w

Substituting all the known values 

T = (50.4 * 0.7/0.5) - ((0.7)^2/0.5)

T = 70.56 - 0.98w  .......................................(1)

The given Equation is 

............................(2)

Comparing the Torque equation of Motor and Vehicle

70.56 - 0.98w = 24.47 + 0.0051*w^2

0.0051*w^2 + 0.98w - 46.09 = 0

Solving the above quadratic equation

w = 39.08 rad/sec and -231.24 rad/sec

So EV Steady State Speed if duty Cycle is 70% is 39.08rad/sec or 373.18 RPM.

 

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

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

The mathematical Model of DC Motor is shown below 0.

                                                               DC MOTOR MATHEMATICAL MODEL

Here Supply Voltage is considered as 50.4V , Ra = 0.5 ohm and Kϕ = 0.7 from above example

 

Since the Speed of Dc motor is inversely proportional to the torque the graph is also linearly decreasing ie Speed of Motor is decreasing as the load torque on the vehicle is increasing.

 

 

4. Refer to the blog on below topic: 

            Induction Versus DC brushless motors by Wally Rippel, Tesla

            Explain in brief about author’s perspective. 

• The Author Wally Rippel is a long-time proponent of electric vehicles. Prior to joining Tesla Motors, he was an engineer at AeroVironment, where he helped develop the EV1 for General Motors and was featured in the documentary movie, Who Killed the Electric Car? Wally has also worked for the Jet Propulsion Laboratory on electric vehicle battery research, among other projects. In 1968, as a Caltech undergraduate student, he built an electric car (a converted 1958 Volkswagen microbus) and won the Great Transcontinental Electric Car Race against MIT. 
• He Compares the Configurations of Gas Powered Vehicles and thought that the best engine type out of all this will be in production but there is no one best engine type, rather there are different types of engines to suit personal requirements, such as price and performance. This is also true for electric vehicle drives.
• In 1960 for vehicles Lead Acid Batteries, Brushed DC Motor and Contactor Controllers were used but today the Lead Acid batteries are replaced by Lithium Ion, DC Motor by Brushless DC Motor or Induction Motor and Contactor Controllers by Modulating Inverters.
• Then he Compares the Construction and operation of Brushless DC Motor and Induction Motor. Brushless Motor and Induction Motor Stator Constructions is similar but the difference is in Rotor Construction. Brushless DC Motor has permanent Magnets which produces a  DC magnetic field. 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.
• When the rotor rotates the magnitude and volatge polarity needs to be varied continuously so that torque reamins constant and this is acheived by use of Inverter.
• Then he explains the Working of Induction Motor and says it has few advantages over Brushless motor like it does not require any inverter and when connected to 3 phase power source torque is produced at the outset and he motor has the ability to start under load.
• Then he explains the limitations of 3 Phase Induction Motor over Brushless DC Motor that the IM cannot runon DC Supply it requires AC supply only and when operated from utility power, they have limited starting torque and somewhat limited running peak torque capabilities, when compared to DC type machines.
• So by adding Inverter without feedback it can power the 3 phase IM  and control the Speed by varying the Frequency of Inverter. But Torque performance is low, so by adding feedback loops to inverter it can produce exact frequency that the motor requires.
• The DC Brushless Motors and Induction motors have same stator construction and 3 phase modulating inverters but the difference is rotor and inverter controls.
• DC Brushless Motors have few better advantages like during its operation less rotor heat is generated and rotor coolong is easier and it has peak point efficiency which is higher than Induction Motor. The DC brushless drive can also operate at unity power factor, whereas the best power factor for the induction drive is about 85 percent. The Brsuhless Drive have efficiency which is larger than Induction motor by few percentage.
• The DC Drive have few drawbacks like since it uses Permanent magnets the magnetic field Density (B) cannot be varied easily. 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  losses and thereby optimizes efficiency. At lower Torque values the B field should be reduced such that eddy cuurent and hysterisis losses should be reduced.
• Induction Machines has advantage over DC machines that it does not use a permanent magnet so magnetic field strength (B) is adjustable since  since B is proportionate to V/f (voltage to frequency). 
•  By using Smart Inverters magnetic and conduction losses can be traded such that efficiency is optimized and thereby performance is increased.
• Another drawback of DC machine is that as its size grows the magnetic losses increase proportionately and part load efficiency drops but this not the case with Induction machine.
• Induction machines are favorable to use where High Performance is required although peak efficiency will be less than that of Brushless DC.
• Permanent Magnets are costly and when used in rotors, the rotors are not capable of handling higher power when ferromagnetic substance comes in contact with it. So Induction Motors have cost advantage and field weaking capability reduces the inverter ratings and costs are lower as compared to Brushless DC.
• But the speed control of Induction Motors is quite difficult and control laws are complex as comapared to Brushless DC.
• So the author is not able to justify that which Motor is superior as both motor has some advantages and disadvantages over each other.
• But the author concludes that both DC Brushless drive and Induction Motor will work side by side during the coming golden era of hybrid and electric vehicles.
• DC Brushless Drives will most probably dominate the Hybrid and Plug-in hybrid Elecyric vehicles market and Induction motors will maintain dominance for the high-performance pure electrics.