ELECTRIC VEHICLE DRIVETRAIN

AIM :- To study about Power Electronic devices used in Electric Vehicle Drivetrain.

OBJECTIVE:- a) To study different types of Power converters circuits employed in electric and hybrid electric vehicle.

                     b) To find EV steady state speed when duty cycle is 70%.

                     c) To develop a mathematical model of a DC motor for the given speed-torque equation using simulink.

                     d) To write a brief review on, "Induction motor versus DC brushless motor".

THEORY:-

The use of electric and hybrid electric vehicles can substantially reduce urban pollution problems. Power electronics permits generation of electric power from environmentally clean photovoltaic, fuel cells and wind energy sources. Widespread application of power electronics, with an eye for energy conservation and generation of power sources from environmentally clean sources, can help in solving problems like acid rains and greenhouse effects.

The origin of power electronics can be traced back to the time when mercury arc devices were employed for the rectification of a.c. to d.c. or the inversion of d.c. to a.c. However, the rapidly increasing usage of power electronics nowadays has resulted from the development of solid state power devices.

                                                                                                                              Fig:1.1

Since, power is associated with the generation, transmission, distribution and utilization of various forms of electrical power in static as well as rotating machinery. While control deals with the response characteristics of the systems incorporating feedback mechanisms for continuous or sampled data. Thus, as a whole power electronics is regarded as the field that is concerned with the use of power electronics for the purpose of controlling and conversion of electric power. To achieve this, the designing of such systems is done by maintaining interaction between source and load by making use of small signal electric control circuits and power semiconductor devices.

Power electronics is known to be a vast as well as complex subject that is advancing with technology and turning out cost-effective with the various inventions. It deals with conversion and controlling of large amount of electric power. This means the converter and controller are its two major components. The fundamental operation of power electronics is the processing and controlling of electric energy by giving sufficient voltage and current so that it will be suitable for various consumer applications.   

                                                                                                                               Fig:1.2

As it is clear from Fig:1.2 that we are having a power electronic converter and controller along with some interfacing units. The power converters change one form of electric power to another form with the use of power semiconductor devices. While power controllers are the ones that are responsible for producing control signals relative to turning ON or OFF of the switching devices present within the circuit. The complete operation of the system will provide the desired signal of a particular frequency as shown in Fig:1.2. Here, the controller acts as a feedback for the system that controls the operation of the controller depending on the feedback signals from the load.

In power electronic based system, the source of electric power is either AC or DC. The DC electric power source can be a DC generator, battery, etc while the AC electric power source can be an alternator or Induction generator. The converter provide AC or DC electric power according to the load with variable voltage and frequency. Further, the parameters on which load is measured like the voltage, Current, etc acts as the input signal for the controller unit. These are known as feedback signals or controller input signal. The control signal generated by the controller is the outcome of the comparison made between the feedback signal and reference input signal. This control signal regulates the turning ON and OFF of the switching devices of the power converter.

                           TYPES OF CONVERTERS USED IN EV AND HEV

Electric Vehicles are structured using different types of Energy storage systems connected with various types of power electronic converters.

                                                                                                                                Fig:1.3

AC/DC POWER CONVERTER :- An AC/DC power converter supplies electric power from a commercial power system to an on-board high-voltage battery. The AC/DC converter is required to work more efficiently to shorten the charging time. The AC/DC converter consists of a power factor correction (PFC) which converts AC voltage into DC voltage, an H-bridge circuit which converts the DC voltage into high frequency, a high-frequency transformer which regulates transformation and insulation, and a circuit which rectifies and smooths high-frequency AC voltage.

The Energy storage systems are charged by taking current and voltage by the grid or charging stations through AC/DC converters. On board chargers, used primarily for AC to DC power conversion which contain several types of power electronic devices like MOSFETs, diodes and magnetic. The advantage of having an on-board charger (compared to off-board) is that the vehicle can be charged from AC power outlets. However, it also requires the vehicle to carry the extra weight of the power electronics and heat sinks. The recent topologies for bi-directional AC/DC converters are efficient isolated and compact and can be used as an on-board battery charger for EV or HEV. At very high power levels, it is likely that the AC/DC power conversion will be carried out off-board to avoid the additional space and weight required in the vehicle. For example, the Tesla Supercharger can provide 120kW of DC power from the charging station and charge the Model S upto 50% in 30minutes. The topologies are divided into two families : single-stage and two-stage.

i) Single Stage:- The single-stage (AC-AC-DC) topologies contains an AC-filter which consist of a single inductor. A full-bridge (FB) switch module with bidirectional switches is used afterwards. The energy storage element is the leakage inductance of the transformer. On the DC-side, after the transformer, there is another FB switch module and a capacitor is used at the output. The capacitor plays the role of the DC-filter and absorbs the high frequency current ripple. The DC-side FB works under Zero Voltage Switching (ZVS) and the AC side FB works under Zero Curent Switching (ZCS) for all of the operating points.

ii) Two Stage :- The two-stage converters use an AC/DC converter to connect AC-side to the DC-link and then an isolated DC-DC converter to connect the DC-link to the battery. The AC/DC can be either full bridge, half bridge or a multilevel converter and the DC-DC converter can be a dual-active bridge (DAB) or a resonant converter. The DC-link can be either a current or voltage DC-link and is usually a voltage DC-link. The control of the power is assigned to a separate controller through the AC/DC converter and the control of the DC voltage or current is through the DC/DC converter using another controller.  

DC/DC POWER CONVERTER :- A d.c. chopper is a static device (switch) used to obtain variable d.c. voltage from a source of constant d.c. voltage. Thus, a DC chopper is a category of power converter and it is an electric circuit which converts a source of DC from one voltage level to another by storing the input energy temporarily and then releasing the energy to the output at a different voltage. The d.c chopper offers greater efficiency, faster response, lower maintenance, small size, smooth control and lower cost. D.C choppers can be classified as:-

Isolated and non Isolated DC/DC converter.

i) Isolated converter: It is the electrical separation between the input and output of a dc-dc converter. An Isolated dc-dc converter uses a transformer to eliminate the dc path between its input and output. The isolated DC-DC converters are suitable in EVs with low and medium power purposes. The various isolated converters for EV applications are Push-Pull converter, Flyback converter,Resonant converter, Zero-voltage switching converter, Multi-Port isolated converter.

ii) Non-Isolated converter: A non-isolated dc-dc converter has a dc path between its input and output. It usually employs ICs specifically intented for that purpose. The non-isolated converter is appropriate for EVs under medium and high power operation.The various non-isolated converters are Cuk converter, Switched-Capacitor Bidirectional converter, Coupled Inductor Bidirectional converter, Quazi-Z-Source Bidirectional converter, Multi-Device Interleaved Bidirectional converter.

A DC/DC converter is used to interface the elements in the electric powertrain by boosting or chopping the voltage levels. The power of DC/DC converters depends on the characteristics of the vehicle such as the top speed, acceleration time, weight, maximum torque and power profile. The DC/DC converters play a key role in converting the unregulated power flow to a regulated one. The DC/DC converter present non-linear behaviour and lightly damped dynamics due to the switching action.

                                                                                                                            Fig:1.4

Since, DC-DC converter is expected to provide constant DC voltage output irrespective to input, output voltage variations and load changes. Therefore control of the output voltage is required to be performed in a closed loop, to accommodate component value change due to temperature, humidity, pressure, etc. The two most common known closed loop controls are voltage mode control and the current mode control.

Voltage Mode Control : In the Voltage mode control scheme,  the converter output voltage is sensed and subtracted from an external reference voltage in an error amplifier. The error amplifier produces a control voltage that is compared to a constant amplitude saw-tooth waveform. The comparator produces a PWM signal that is fed to drivers of the controllable switches. The duty ratio of the PWM signal depends on the value of the control voltage. The frequency of the PWM signal is the same as the frequency of the saw-tooth waveform. An important advantage of the voltage mode control is its simple hardware implementation and flexibility. Voltage mode control provides good load regulation against output voltage variation. However, Line regulation (regulation against variation in the input voltage) is delayed because changes in the input voltage must first seen in the converter output before they can be corrected.

                                                                                                                             Fig:1.5

Current Mode Control : In order to overcome the limitation of line regulation problem, current mode control is introduced which is much faster in response and able to manage output voltage within a limited range incase of line voltage changes. The current mode control scheme has an additional control loop feedback an inductor current signal, converted into its voltage analog which is compared to the control voltage. The saw-tooth waveform of the voltage mode control scheme is changed by a converter current signal in current mode control. In practical implementation, the peak inductor current is used in current mode control. The peak inductor or switch current is proportional to the input voltage for same duty cycle. Hence, the inner loop i.e, the current mode control naturally accomplishes the input voltage feedforward technique. The advantages of current mode control are :

• Better line regulation.
• Eliminate flux imbalance by better current sharing (e.g. Push-pull converter, interleaved topologies).
• Simplified loop stabilization by eliminating output inductor effect.
• Load current regulation (better output current limit / DC protection).

                                                                                                                             Fig:1.6

The different voltage levels are required by the various electronics components in EVs and HEV. The most basic requirement for DC/DC conversion is to power the traditional 12 V loads. When a standard combustion engine vehicle is operating, an alternator connected to the engine provides the power for all electrical loads and also recharges the battery. The internal combustion engine in HEVs can be off for extended periods of time, so an alternator cannot be relied upon to provide power to auxiliary loads. A DC/DC converter charges the 12 V battery from the HV bus, thus eliminating the 14 V alternator.

DC/AC POWER CONVERTER :- The power inverters and converters are used to invert HV battery pack direct current (DC) to alternating current (AC) for motors that propel the vehicle down the road, they also convert AC to DC to charge the HV battery pack. Within an electric drivetrain, the inverter controls the electric motor in a manner somewhat equivalent to how the Engine Control Unit (ECU) of a gas or diesel internal combustion engine vehicle determines the vehicle’s driving behavior; it also captures kinetic energy released through regenerative braking and feeds this back to the battery. As a result, the range of the vehicle is directly related to the efficiency of the main inverter.

                                                                                                Fig:1.7 (3^rd generation prius inverter)

DC power, from a hybrid battery is fed to the primary winding of the transformer within the inverter through an electronic switch (generally a set of semiconductor transistors), the direction of the flow of current is continuously and regularly flip-flopped (the electrical charge travels into the primary winding, then abruptly reverses and flows back out). The in/outflow of electricity produces AC current in the transformer’s secondary winding circuit. Ultimately, this induced alternating current electricity provides power for an AC load for example an electric vehicle’s (EV) electric traction motor.  

                                                                                                                            Fig:1.8

The Insulated Gate Bipolar Transistor (IGBT) are a high-voltage, high-current switch connected directly to the traction motor in the HEV or EV. The more efficient the IGBT, the less power is lost to wasted heat, resulting in better mileage. Unlike IGBT, SiC MOSFETs in particular combine several desirable characteristics, such as high breakdown voltage, low on-resistance and fast switching speed with their inherent advantages of high-temperature capability, high-power density and high efficiency. Moreover, their light weight and small volume favorably affecting the whole powertrain system in an HEV and thus, the performance and cost. As a result, it is expected that the IGBT will be replaced in HEV/EV applications. Tesla, who released a new model S plaid using an SiC inverter.

                                                                                                                            Fig:1.9

The application of these characteristics in power devices means :

• Wide-bandgap : The greater the wide-bandgap, the greater the critical breakdown voltage, the more suitable for high-voltage and high-power applications.
• Saturated electron mobility : The higher the value, the faster the switching speed of the device, which makes the drive power required for high-frequency operation under high voltage smaller with lower energy loss.
• Thermal conductivity : The high thermal conductivity helps in avoiding additional cooling system, which is conducive to the optimization cost and dimensions.
• Small-drain source on resistance per unit area : It can effectively reduce loss.

In addition to Inverters, SiC power devices can also be used in many aspects such as on-board chargers (OBC) and power conversion systems (DC/DC) of electric vehicles.

                                          STEADY STATE SPEED OF EV

 An Electric Vehicle’s powertrain with 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’.

The other parameters of the electric vehicle is given below,

Motor Controller Parameters :

             Rated Armature Voltage = 72V

             Rated Armature Current = 400A

             R_a = 0.5Ω, KΦ = 0.7 Volt second

             Chopper Switching Frequency = 400Hz

The vehicle speed-torque characteristics are given by the below equation :

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

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

Solution. Rated Armature voltage = 72V

                70% duty cycle means signal is ON 70% of the time and OFF the other 30%.

                 Supply Voltage of the battery for 70% duty cycle,

                 `Vo` = V_bat * D

                 `Vo` = 72 * (70/100) = 72 * 0.7 = 50.4V

Now the equation for speed-torque characteristics is,

              `T = (k.phi.Vo - (k.phi)^2omega)/(Ra)`                                                                                                      …1

Where,      `T` = Torque of the motor

                `Ra` = Armature Resistance

                `Vo` = Supply voltage from battery

                `k.phi` = Motor Constant

                `omega` = Speed of the motor

Putting the values in equation …(2),

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

                 `T = 70.56 - 0.98omega`                                                                                                                          ...2

Here, the given vehicle speed-torque characteristics is,

                 `Tv = 24.7+(0.0051)omega^2`                                                                                                               ...3

Equating equation …(2)  and  equation …(3), we get;

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

                 `0.0051omega^2 + 0.98omega - 45.86 = 0`

Calculating the roots of the equation, we get;

                 `omega = (-b +- sqrt(b^2 - 4ac))/(2a)`

`omega1 = (-0.98 + sqrt(0.98^2 - 4*0.0051*45.86))/(2*0.0051)`= 38.915 rad/sec

`omega2 = (-0.98 - sqrt(0.98^2 - 4*0.0051*45.86))/(2*0.0051)` = -231.072 rad/sec

Thus, the Electric Vehicle steady state speed with 70% duty cycle is 38.915 rad/sec (371RPM).

                MATHEMATICAL MODEL OF A DC MOTOR USING SIMULINK

                                                                                        Speed-Torque Equation : `omega = V/(k.phi) - (Ra)/(k.phi)^2.T`

                                                                                                                                Fig:1.10

The variables such as V, Ra and T are created from the command window and are saved in the workspace of the MATLAB. The multiplication and addition block have been used to solve the speed-torque equation of the DC motor. The display block shows the speed of the motor running at the given inputs.

                          INDUCTION VERSUS DC BRUSHLESS MOTORS

There are many engines (flat-heads, Hemis, straight, opposed, and V configuration) that have been designed in the past but no one engine has been choosen as the best. The reason being is that there is no one engine that fulfills all our requirement. Since, the demand changes so does the engine. At present Electric Vehicles have played a vital role in fulfilling the worlds demand, therefore it becames one of the preferred choices. There have been other examples such as the lead acid batteries, DC brush Motors and contactor controllers being replaced by lithium ion batteries, DC brushless motor and the contactors being used for 3-phase modulating inverters respectively. This replacement is due to the demand of better price and performance of the equipments.

The DC brushless machines include a rotor and a stator. The rotor contains two or more permanent magnets that generates a DC magnetic field. This magnetic field enters the stator core and interacts with the current flowing within the windings to produce a torque between the stator and the rotor. This results in the rotation of the rotor with the continuous change in the magnitude and polarity of the stator current. Thus, the Torque remains constant and the conversion from mechanical to electrical is optimally efficient. The inverter was needed to control the current in the DC brushless motor.

However, the 3-phase induction motor invented by Nikola Tesla was somewhat similar to the DC brushless motor in terms of the distributed windings in the stator core. The difference lies in the rotor since, induction rotor has no magnets. Therefore, current flowing in the stator winding  produces a rotating magnetic field in the rotor. The voltage is produced due to the difference between the rotor and the electrical frequency. Thus, resulting in the flow of current in the rotor conductors. Finally, these current interact with the magnetic field producing the torue in the rotor.

There is a need of inverter to control the current incase of DC brushless motor but incase of Induction motor there is no need of inverter. This is mainly due to the direct compatibility of the Induction motor with conventional utility power producing torque at the outset. Whereas incase of DC brushless motor no torque is produced when directly connected to fixed frequency utility power. Although the 3-phase induction motor has great utility but they also have some limitation such as:

• They can be only operated from the AC supply.
• The speed of the shaft is proportional to the line frequency, therefore when used with utility power they are constant speed machines.
• They have limited starting torque and limited running peak torque capabilities as compared to DC machines.

However, these limitations can be removed by simply using an inverter. An inverter can resolve the following problems such as :

• It converts AC to DC supply.
• The variable speed from the Induction motor is adjusted as per the inverter frequency.
• The torque performance is improved by using a feedback loop.

Thus, making the Induction motor capable of competing with the DC brushless motor.

Now let’s compare the two motors with each other i.e,

• Power Factor : The DC brushless drive operate at unity power factor whereas the best power factor for Induction motor is 85 percent.
• Magnetic field : The magnetic fields are easily adjusted incase of Induction motor since magnetic field are proportional to V/R and it does not contain any permanent magnets. This minimizes the eddy, hysteresis and I²R losses.
• Efficiency : DC brushless drive generate less heat thus efficiency is higher for this drive.
• Size : In DC brushless as the machine size increases, the magnetic losses increases proportionally whereas incase of Induction motor the machine size has no effect on the losses.
• Cost : Permanent magnets are expensive and difficult to handle whereas due to field weakening capability of Induction machine, inverter rating and cost is lower.
• Stability : Controllng the Induction motor over the entire torque-speed range and temperature is more difficult than the DC brushless motor.

The DC brushless drive is widely used in hybrid and plug-in hybrid vehicles whereas the Induction motor dominate the pure Electric vehicle market. However, it is still to be seen if these two machines will work side by side or one will dominate the other in the coming near future.

CONCLUSION: We have successfully studied about the different types of power converters and their application in EV and HEV (Fig:1.3). The EV steady state speed using the speed-torque characteristics equation having 70% duty cycle is calculated successfully. Also the simulink model was created for solving the speed-torque equation. The results were simulated successfully and were displayed using the display block which is shown in the Fig: 1.10. The Induction Motor and the DC Brushless Motor comparison were made with respect to each other and a brief review was made focusing on the future demand based on the continous change in the requirement of the EV and HEV.