Week-6 Challenge: EV Drivetrain

                                                               Week-6 Challenge: EV Drivetrain  

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

Answer: From the architecture of the HEV, 
for:

Power converters are used in electric/hybrid vehicles to manipulate the form and magnitude of voltage and current fed from and to the vehicle.

DC/DC converter - This converts one value of DC voltage into another value of DC voltage ( increase/ decrease). The DC/ DC converters used in EVs may be for the following purposes:

• Providing low voltage (12V or 14V) for low voltage loads,
• Boosting the voltage level of the battery such that it matches the level of the DC bus voltage for driving the AC motor inverter
• if the traction motor is an AC motor.
• Motor propulsion if the traction motor is a DC motor.

AC/AC converter - Regulated AC output from AC input. The above converters either increase the magnitude of the output voltage ( boost converters) or reduce the output voltage ( buck converter). This converter can convert from a fixed ac input voltage into variable AC output voltage. The output voltage is controlled by varying firing angles of TRIAC. These types of converters are known as AC voltage regulators.  

AC/DC converter(Rectifier) - These convert AC voltage into regulated DC voltage. An inverter is a device that converts DC power to the AC power used in an electric vehicle motor. The inverter can change the speed at which the motor rotates by adjusting the frequency of the alternating current. It can also increase or decrease the power or torque of the motor by adjusting the amplitude of the signal. Used to get back regen power from vehicle to the battery, embedded on the floor of an EV.

DC/AC converter (Inverter) - These convert DC voltage into regulated AC voltage. Form and magnitude are regulated. Usually emplyed when an AC motor is used to drive the vehicle. In the case of EVs and HEVs, the battery pack will supply its power to the inverter. The inverter contains a transformer which consists of a primary and secondary winding. The current from the battery is first supplied to the primary winding where the electronic switches will repeatedly reverse the direction of the current to create an AC waveform. This current will return the way it came from. The AC in the primary winding will induce an AC in the secondary winding which then supplies this current to an AC load e.g. the AC traction motor.

A Typical HEV/EV has the following setup for the converters which are most used in those cases are given: 

Problem Statement- 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'.

The other parameters of the  electric vehicle are 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 the duty cycle is 70%?

Answer:  Provided 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

 Here, the problem is given with a duty cycle of 70%.

So, V(motor)=V(battery)*0.7= `V_m= V_b*0.7` = 72 * 0.7 = 50.4 Volts.  

Now, from Torque-Speed Characteristics we have the following equation:

 `T=V*(Kphi)/R_a -(Kphi)^2/R_a *omega` ..................equation (1).

Putting the given values in equation (1) we get: T= `50.4*0.7/0.5- (0.7)^2/0.5*omega` =  `70.56 - 0.98*omega` ......equation (2).

The equation provided is : `T_v= 24.7 + (0.0051)*omega^2` .......equation (3).

Now, comparing equation (2) & (3) we get: `70.56 - 0.98*omega - 24.7 + (0.0051)*omega^2` =0;

--------> `0.0051*omega^2+ 0.98*omega - 45.86` =0  ........equation (4).        

We can solve this quadratic equation using the quadratic formula:

`x=-b+- root2(b^2-4ac)/(2a)`     

Applying Sridhar-Acharya Theorem and eliminating the negative value we get and Simplifying this, we get:

`x=−0.98±1.3770.0102`  

So, `omega= 38.92` rad/sec.   

Assuming the forward direction is positive and that the vehicle is in steady speed moving in the forward direction, we will accept the positive value solution and rejected the negative one.     

Problem Statement- 3:  

Refer to the blog, based on the below topic: 

                                                             Induction Versus DC brushless motors by Wally Rippel, Tesla

Explain in brief about author’s perspective.

Answer:  

The author Wally Rippel, writes this blog to entail the differences between the two most commonly used motor drives in the automobile industry: the DC brushless motor drive and the induction motor drive. Rippel goes into detail about the differences in construction, working, and cost between the two drive types. However, with what he has written, he has not concluded that one type is better than the other. In the last lines of the opening paragraph, Rippel writes that "there is no one best engine type" and that it is all a matter of personal preference when it comes to choosing an engine. It is in this same regard that he ends the blog stating that there is 'Still No Winner' when it comes to motor drive selection either. Despite both motor drive types having similarities, they also have their benefits and drawbacks. 

For Induction drives, their rotor has no magnets, but instead stacked steel laminations with buried peripheral conductors which form a 'shorted structure'. It is the current in the stator windings that generates a rotating magnetic field that interacts with the rotor. 

For DC Brushless drives, we are told that their construction implements 2 or more permanent magnets at the rotor to generate a DC magnetic field. It also consists of a stator core (made up of thin, stacked laminations) It also uses an inverter for the current control, meaning that it cannot run on a DC power supply as the current direction has to be changed repeatedly to maintain a constant torque.

Induction motors can be used directly with power from the mains and do not require an inverter to start the drive and generate the starting torque. It has the ability to start under the mains load applied.  However, DC brushless drives cannot do the same. They cannot generate a starting torque when connected to a mains power supply. They require an inverter to do this. 

Induction motor drives cannot run on a DC power supply and are solely dependent on an AC power supply. Also, when they are connected to a 3 phase mains supply, the rotor rotates at a constant speed and also they have a limited starting torque as well as peak torque. To rectify this, an inverter can be used to operate the induction drive from a DC power supply or a battery. Also, if the inverter frequency is varied, then the constant speed is no longer an issue - it can be varied. 

DC Brushless drives use permanent magnets, and cannot change the strength of their magnetic fields. It would be desirable if they could because when the maximum torque is needed by the rotor, the magnetic field strength should also be at a maximum level. This allows for the inverter and motor currents to be at a minimum, thereby reducing the power loss by heat dissipation. Similarly, when the torque required is minimum, the magnetic field strength should also be minimized to reduce hysteresis and eddy current losses. The permanent magnets used are also quite expensive costing $50 per kilogram and their magnetic fields are difficult to control if ferromagnetic material gets close enough to the field. 

On the other hand, induction drives do not have permanent magnets and so they can vary their magnetic field strength. If used with a smart inverter, they can be operated with minimum hysteresis and eddy current losses and maximum efficiency as the magnetic field strength can be changed according to the requirement by the rotor. These losses do not increase if the size of the induction drive also increases but as a DC brushless drive gets bigger, its losses increase and its efficiency decreases. So induction drives have a cost advantage compared to DC brushless drives

Although, DC Brushless drives generate less heat at the rotors. So rotors more easily and have a higher peak point efficiency than induction drives. Also, the DC brushless motor can operate at unity power factor i.e. 1, whereas induction drives can operate up to a power factor of approximately 85%. 

In conclusion, Rippel has elaborated the different aspects to compare the induction and DC brushless motor drives and has detailed the advantages and disadvantages of both types. As mentioned earlier, at the end of the day it depends on the preference of the user. If a user is seeking a high performance with comparatively lesser costs, then the induction drive would be better suited for those requirements. However, if it is high peak point efficiency, high power factor, and easier control that a user seeks, then the DC brushless drive would be the smarter choice. Both types have their place in the automobile industry and will retain their relevance in their respective applications. Rippel believes that due to the increasing performance of vehicles, one day both may be used side by side under the hood of a vehicle to get the best of both drives. This would result in a new kind of hybrid that combines the characteristics of an ordinary vehicle e.g. sedan with that of a sports car. 

Tesla Roadster (uses I/M for high-performance cars)

Nissan Leaf (uses DC Brushless motor)

The author's comparison between the two motors based on their working, performance characteristics, efficency points, and suitability for the EV application can be summarized by the following table: