Week-6 Challenge: EV Drivetrain

Aim :- Week-6 Challenge- EV Drivetrain

Objective :-

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

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%?

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

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

4.   Refer to the blog on below topic: 

       Induction Versus DC brushless motors by Wally Rippel, Tesla

       Explain in brief about author’s perspective. 

Solutions :-

1. Which types of Power Converter circuits are employed in an electric and hybrid electric vehicle? 

Power Converters :-

• In all fields of electrical engineering, power conversion is the process of converting electric energy from one form to another.
• A Power Converter is an electrical or 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 combination of these.
• The power converter can be as simple as a transformer or it can be a far more complex system, such as resonant converter. The term can also refer to a class of electrical machinery that is used to convert one frequency of alternating current into another. Power conversion systems often incorporate redundancy and voltage regulation.
• Power converter are classified based on the type of power conversion they do. One way of classifying power conversion systems is according to whether the input and output are alternating current or direct current. Finally, the task of all power converters is to process and control the flow of electrical energy by supplying voltages and currents in a form that is optimally suited for user loads.

• Hence, depends on the type of power conversion systems, power converters are classified into four categories :-
• i) AC-AC Converters :- This converter converts an AC waveform to another AC waveform, where the output voltage and frequency can be set arbitrarily (eg. Cycloconverter)
• ii) DC-DC Converters :-  This converter is an electronic circuit or electromechanical device that converts a source of direct current (DC) from one voltage level to another. (Eg. Chopper)
• iii) DC-AC Converters :- A power inverter, inverter or invertor is a power electronic device or circuitry that changes direct current (DC) to alternating current (AC).
• iv) AC-DC Converters :-  The electrical circuits that transform alternating current (AC) input into direct current (DC) output are known as AC-DC converters. (For eg. Rectifiers)

Out of these four converters, only three converters used in Hybrid Electric Vehicle and Electric Vehicle and the same are as under;

(1) DC-DC Converters (Choppers):-

• A DC-to-DC converter is an electronic circuit or electromechanical device that converts a source of direct current (DC) from one voltage level to another. It is a type of electric power converter. Power levels range from very low (small batteries) to very high (high-voltage power transmission).
• Switched DC to DC converters offer a method to increase voltage from a partially lowered battery voltage thereby saving space instead of using multiple batteries to accomplish the same thing.
• In these DC-to-DC converters, energy is periodically stored within and released from a magnetic field in an inductor or a transformer, typically within a frequency range of 300 kHz to 10 MHz. By adjusting the duty cycle of the charging voltage (that is, the ratio of the on/off times), the amount of power transferred to a load can be more easily controlled, though this control can also be applied to the input current, the output current, or to maintain constant power.
• Transformer-based converters may provide isolation between input and output. In general, the term DC-to-DC converter refers to one of these switching converters. These circuits are the heart of a switched-mode power supply. Many topologies exist. This table shows the most common ones.
• 
• DC to DC converter are used to convert the DC input from the charging into step down DC output. This happens during charging. But during vehicle propulsion. DC input is converted into step up DC output for the motor. When the voltage is increased, it is called step up or boost mode. When voltage is reduced, it is called step down or buck mode. This converter are bidirectional for charging and discharging. Charging is also achieved through regenerative braking.
• The main dc-dc converter changes dc power from an on-board 200-800V high voltage battery into lower dc voltages (48V or 12V) to power headlights, interior lights, wiper and window motors, fans, pumps and many other systems within electric vehicles (EV) and hybrid electric vehicles (HEV). This high voltage to low voltage (HV-LV) DC-DC converter is often referred to as an auxiliary DC-DC, or Auxiliary Power Module (APM). Isolation is critical for separating the control systems from high-voltage domains.
• In case of interfacing the Fuel Cell, the DC/DC converter is used to boost the Fuel Cell voltage and to regulate the DC-link voltage. However, a reversible DC/DC converter is needed to interface the SCs module.
• A full-bridge DC/DC converter is the most frequently implemented circuit configuration for fuel-cell power conditioning when electrical isolation is required. The full bridge DC/DC converter is suitable for high-power transmission because switch voltage and current are not high. It has small input and output current and voltage ripples. The full-bridge topology is a favorite for zero voltage switching (ZVS) pulse width modulation (PWM) techniques.
• Due to the automotive constraints, the power converter structure has to be reliable, lightweight, small volume, with high efficiency, low electromagnetic interference, and low current/voltage ripple.

(2) AC-DC Converter (Rectifiers):-

• AC-DC converters are electrical circuits that transform alternating current (AC) input into direct current (DC) output. They are used in power electronic applications where the power input a 50 Hz or 60 Hz sine-wave AC voltage that requires power conversion for a DC output.

• AC to DC converters use rectifiers to turn AC input into DC output, regulators to adjust the voltage level, and reservoir capacitors to smooth the pulsating DC.

• AC-DC converters can have more than one output and may feature overcurrent, overvoltage, or short circuit protection.
• 
• Generally, an AC/DC converter has higher output power than that of a DC/DC converter for driving auxiliaries, and accordingly AC/DC converter’s coil parts occupy a larger space. As the AC/DC converter is also equipped with a reactor for the PFC, the coil parts alone weigh as much as 20-30% of the total weight of the AC/DC converter.
• The AC/DC converter, which supplies power from a commercial system to an on-board high voltage battery, has a power factor correction (PFC) to meet the harmonic-current regulations.
• AC to DC converters are used for charging purpose. Single phase or Three phase AC supply is fed to the Charging port. But this cannot be supplied to the battery. Before it is fed to the battery, it is converted DC using rectifier. This has a unidirectional power flow.
• AC to DC converter is also used at the time of regenerative braking, when motor works as a generator. This facilitates bidirectional power flow.
• The simplest AC/DC converters comprise 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 due to the advantages of smaller transformer requirements and lower noise referred back to the mains power supply.
• 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) is analogous to a relay in that a small amount of voltage can control the flow of a larger voltage and current.

(3) DC-AC Converter (Inverters) :-

• DC/AC converter, also described as “Inverter”, is a circuit that converts a DC source into a sinusoidal AC voltage to supply AC loads, control AC motors, or event connect DC devices that are connected to the grid.
• Similar to a DC/DC converter, the input to an inverter can be a direct source such as battery, solar cell, or fuel cell or can be from an intermediate DC link that can be supplied from an AC source.
• Inverters can be usually classified according to their AC output as single-phase or three-phase and also as half- or full-bridge converters.
• De Lorenzo has designed two configurations to implement this category. One configuration to cover the inverters with PWM control and another configuration to explain the properties of the frequency converter circuit. Regarding the frequency converter and because it is difficult to change the frequency of an AC sine wave in the AC mode, the first job of a frequency converter is to convert the wave to DC because it is relatively easy to manipulate DC in order to make it look like AC. The three main components of all frequency converters are Rectifier, DC Bus and Inverter. They are dedicated to high schools and first years of university.
• 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.
• The inverter does not produce any power; the power is provided by the DC source.
• In an electric drivetrain, the traction inverter converts DC current from the electric vehicle’s battery to AC current to be used by the motor to drive the vehicle’s propulsion system. By improving the traction inverter’s efficiency will enable longer range, fewer charging cycles, and extended battery life with the same battery cost, or the use of smaller, lower-cost batteries to achieve the same range, both of which will help improve the viability of alternative vehicle technologies.
• For EVs, the semiconductors used in traction inverters have a significant impact on efficiency, power density, and cooling requirements.
• The three-phase AC motors used in today’s EVs run at voltages up to 1,000V and switching frequencies up to 20 kHz. This is very close to the operational limits of the silicon-based metal-oxide semiconductor field-effect transistors (MOSFETs) and insulated-gate bipolar transistors (IGBTs) currently used in traction inverters.
• Without a significant technical breakthrough, silicon-based MOSFETs and IGBTs will have difficulty meeting the higher operational requirements of next-generation EVs.
• The AC motors used in Electric vehicle are “three phase” motors, which means that they operate on three separate AC electrical currents which are offset from each other in time. This means that phase 2 reaches its positive peak a little after phase 1 does; and phase 3 reaches its peak a little after phase 2. It’s like a Stadium Wave of voltage. This has the effect in the motor of creating a set of magnetic fields which effectively rotate, which is what causes the rotor (the rotating part of the motor) to turn. Hence, the inverter in our EVs doesn’t produce just one AC output – it produces three. You can see in this photo that there’s a two-way connection for the DC input (+ and -), and three connectors for the AC cables that feed the motor:

                                                                                                                   Fig. Inverter

• The inverter in an EV is also called a VFD – Variable Frequency Drive. The sine wave AC power can be generated in a wide range of different frequencies. In other words, the rate at which the voltage cycles from positive to negative and back again can be changed dramatically. This is what’s needed to control the rotation speed of the motor.
• By changing the AC frequency, that magnetic stadium Wave speeds up or slows down, and the motor changes speed and vehicle speed also changes.
• One of the great things about an electric motor is that it can generate useful propulsion torque over a very wide range of rotation speeds; this is quite different from a gasoline or diesel engine, which is one reason why they need complex multi-speed transmissions.
• Inverter converts DC power into cleverly constructed, smoothly sinusoidal, three-phase, variable-frequency AC power.
• “Regeneration” – is the behavior of EVs where when you take your foot off the accelerator pedal, the motor becomes a generator which sends charge to the batteries. This helps to improve the range on the road – and to extend the lifetime of your brake components. So, when the motor is acting as a generator, it produces three phases of sinusoidal AC power. The inverter has to take these and convert them to a single DC output which has a higher voltage than the batteries, in order to charge them.

2.  An Electric Vehicle's powertrain with a 72V battery pack is shown in the diagram below. The duty ratio for acceleration operation is 'd1' and for the braking operation the duty ratio is 'd2'.

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 the duty cycle is 70%?

------->

The motor voltage can be calculated as `Vm = Vb * d`

where,

`Vb`= Battery Voltage

`d`= Duty Cycle =`70% = 0.7`

Hence,

`Vm = 72 * (0.7)`

`Vm=50.4 V`

The given vehicle speed-torque characteristics is;

`Tv = 24.7 +((0.0051)omega^2)` ............................................(1)

Also, we know that speed-torque characteristics of DC motor is,

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

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

`omega = 72 - 1.024081T`..........................................................(2)

After rearranging the terms, we got,

`(1.0204081)*T = 72 - omega`

Dividing LHS & RHS by 1.0204081, we get,

`T = (72/(1.0204081)- omega/1.0204081)`

 `T = 70.56 - (omega/1.0204081)`..................................................(3)

Substitute the value of equation(1) in equation(3), we get,

 `24.7 + ((0.0051)*omega^2) = 70.56 - (omega/1.0204081)`

 Multiply throughout by 1.0204081, we get,

`25.20408 +((0.005204)*omega^2) = 72 - omega`

 `((0.005204)*omega^2)+(omega) = 72-(25.20408)`

`((0.005204)*omega^2)+(omega) = 46.79592`

`((0.005204)*omega^2)+(omega) -46.79592 = 0`................................(4)`

We know that above equation(4) is in the form of `ax^2 + bx + c = 0`

Comparing it with equation(4), we get,

a = 0.005204

b = 1

c = -46.79592

Also, we know that `x = (-b +- (sqrt(b^2 - 4ac)))/(2a)`

`x = (-1 +-(sqrt(1-(4*0.005204*(-46.79592)))))/(2*0.004204)`

`x = (-1 +-(sqrt(1+0.97410)))/(0.010408)`

`x =(-1 +-(sqrt(1.97410)))/0.010408`

`x =( (-1 + 1.40502)/0.010408) or ((-1-1.40502)/0.010408)`

`x = 0.40502/0.010408 or -2.40502/0.010408`

x =38.9142  or -231.0741

Assuming the forward direction is the positive and that the vehicle is in steady speed moving in the forward direction, we will accept the first solution and reject the second solution.

Therefore,

`omega = 38.9142 (rad)/(s)`

Hence, the steady-state angular speed of the motor there is `38.9142 (rad)/(s)`.

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

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

 Output Graph :-

1) Speed vs Time graph

2)  Torque-Speed Characteristics Graph

Conclusion :- After perusal of the above graph, we conclude that as soon as torque increases, speed of DC motor decreases .

 4. Refer to the blog on below topic: 

     Induction Versus DC brushless motors by Wally Rippel, Tesla

     Explain in brief about author’s perspective. 

------>

After going through the blog of Induction motors vs DC brushless motors by Wall Rippel, Tesla, we have understood that,

• In DC brushless motor, the rotor includes two or more permanent magnets that generate a DC magnetic field. Due to this, magnetic field enters the stator core and interacts with currents flowing within the windings to produce a torque interaction between the rotor and stator. However, the induction rotor has no magnets – (just stacked steel laminations with buried peripheral conductors that form a “shorted structure.)” and current flowing in the stator windings produce a rotating magnetic field that enters the rotor. 
• In case of DC brushless motor, when rotor rotates, it is necessary that the magnitude and polarity of the stator currents be continuously varied in such way that the torque remains constant and the conversion of electrical to mechanical energy is optimally efficient. Hence, this current control is provided by inverter. 
• A brushless DC motor produces no starting torque when directly connected to fixed frequency utility power. They requires an inverter whose “phase” is maintained in step with the angular position of the rotor. Hence, DC brushless motors are useless without inverters. However, AC induction motors have the capability of producing the torque at the outset when connected to utility type 3-phase power, hence the motor has the ability to start under load. So, No inverter is needed in case of AC induction motor.
• 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. 
• Since 3-phase induction motors have great utility, but they cannot operate from DC source, they always require an AC supply. 
• In case of Induction motors, shaft speed is proportionate to line frequency. Hence, when Induction motors used with utility power, they are constant speed machines. Finally, when operated from utility power, they have limited starting torque and limited running peak torque capabilities, when compared to DC brushless motors.
• DC brushless motors require an absolute position sensor, while induction motor drives require only a speed sensor.
• The speed of DC Brushless motor is controlled by varying current, whereas the speed of Induction motor is controlled by varying frequency.
• In case of DC brushless motors, less rotor heat is generated because rotor cooling is easier and peak point efficiency is generally higher as compared to induction motors.
• The DC brushless motor can also operate at unity power factor, whereas the best power factor for the induction motor is about 85 percent. Hence, we can say that the peak point energy efficiency for a DC brushless motor will typically be a few percentage points higher than for an induction motor.
• The Peak efficiency of Induction motor is less than DC Brushless motor but the average efficiency is higher.
• Due to permanent magnets in case of DC brushless motors, the magnetic field strength (B) cannot be ajustable and as a result, sum of the eddy, hysteresis, and I² losses cannot be minimized. However in case of induction motors, there are no permanent magnets, hence magnetic field strength (B) can be easily adjustable so that sum of eddy, hysteresis, and I² losses can be minimized.
• The induction motors when operated with a smart inverter has an advantage over a DC brushless motors is that magnetic and conduction losses can be traded so efficiency is optimized.
• In case of DC brushless motors, as machine size grows, the magnetic losses increase proportionately and part load efficiency drops. However in case of induction motors, when machine size increases, losses do not increases necessarly.
• The Peak efficiency of Induction motor is less than DC Brushless motor but the average efficiency is higher.
• Since permanent magnets are very expensive, hence DC brushless motors are costly as compared with Induction motors.
• Achieving stability over the entire torque-speed range and over temperature is more difficult with induction motor than with DC brushless motor as induction motor is difficult to control.
• DC brushless motors are mostly suitable for Hybrid Electric vehicle (HEV) & Plugin hybrid vehicle (PGHEV) and Induction motor is suitable for Electric vehicle(EV).