Week-6 Challenge: EV Drivetrain

HEV WEEK 6 CHALLENGE

QUESTION1:

POWERCONVERTERS USED IN ELECTRIC AND HYBRID ELECTRIC VEHICLE:

• Power converters are the devices used to convert and control the electrical power
• The controls achieved by ON &OFF operation of semiconductor switches(MOSFET,IGBT)
• The following are the types of powerconverters used in the EV system

1.AC-DC CONVERTER:

• This is the most common type of converter used in many application
• This converter used to convert the AC input voltage in to DC output voltage
• This converter also called as a rectifier circuit
• In EV,this converter mainly used to charge the battery from the input AC voltage grid or the charging station.
• This can be further classified as Half-wave ,Full-wave and bridge rectifier by the nature of output wave
• But mostly we use bridge rectifier to get continuous DC output voltage
• The below image represents a simple bridge rectifier circuit
• 
• The AC supply connected to a step-down transformer to reduce the input voltage to required rate
• This step down voltage is connected to a bridge rectifier containing four diodes (D1,D2,D3,D4)acting as switches
• The ouput of the rectifier circuit connected to the DC load or the Battery

2.DC-AC CONVERTER:

• This type of converter used to convert DC input voltage into AC ouput voltage
• In EV,it used to power the AC induction motor by the DC input battery power
• This converter also called as inverter
• The below image represents a simple DC-AC converter
• 
• The Input battery voltage is connected to four MOSFET (S1,S2,S3,S4)arranged in this manner
• Switching on S1 & S3,results in the postive flow of power from the battery to load
• Switching on S2,S4 results in the negative flow of power from battery to load
• Continuous cycle of this operation results in the AC output voltage which connected to AC motor
• Inductor and capacitor connected in the circuit act as filter circuit to achieve more sinusoidal nature in the output

3.DC-DC CONVERTER:

• This converter convert the dc input voltage to higher or lower dc ouput voltage.
• In EV,the main battery pack will have high power rating which designed to feed the motor
• But to run the auxiliary loads we have additional battery with lower power rating
• To charge this auxiliary battery ,we have to stpedown the main battery voltage to the required voltage rating
• This converter can be further classified as Buck and Boost converter
• Buck converter gives the lower dc output voltage and Boost converter gives the higher dc output
• In this application ,we are using a Buck converter to charge the auxiliary battery
• The below image represents a buck converter circuit
• 
• In this circuit ,the battery connected in series with MOSFET parallel to a diode
• The reduction in the power done by switching operatons
• The below image represents a boost converter circuit
• 
• In this circuit ,Input battery voltage connected parallel to Mosfet and in series with inductor and diode
• The power stored in the inductor will add up to the input voltage to provide high dc voltage level to load

QUESTION 2:

FINDING THE STEADY STATE SPEED OF THE GIVEN EV POWERTRAIN:

• 

PARAMETERS:

• Battery voltage(Vs) - 72 V
• Armature resistance (Ra) - 0.5 ohms
• Armature current (Ia) - 400 A
• Motor constant (Km) - 0.7 V/s
• Switching frequency - 400 Hz
• Duty cycle - 70%

EQUATION:

• The torque-speed characteristic equation can be written as

T = (V*Km/Ra)-(Km^2*w/Ra)

• Where T-torque of the motor
• V- input voltage of motor
• Ra-armatur resistance
• Km-motor constant
• w-speed of the motor

SOLUTION:

• The given torque speed characteristics equation is

• Tv = 24.7+(0.0051)*w^2 ----------(1)

• The duty cycle is 70%
• So the input voltage V for the motor can be calculated as

motor input voltage(V) = battery voltage (Vs)*duty cycle

V = 72*0.7

V = 50.4 V

• Substituting all known values in the torque-speed equation

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

T = 70.56-0.98*w ----------(2)

• In steady state condition motor torque is equal to load torque

Tv=T

24.7+(0.0051)*w^2 = 70.56-0.98*w

• Simplifying the above equation we get

70.56-24.7-0.98*w = 0.0051*w^2

w^2+192.16*w-8992.16 = 0 --------(3)

• Equation 3 represents a first order differential equation
• Steady state speed can be determine by finding the root of this equation
• Roots of the equation determined in matlab workspace as
• 
• As the vehicle moving in a steady state speed ,positive value of roots considered
• The steady state speed of the vehicle is

w = 38.9145 rad/s

• Converting it to rpm

w = 38.9145 *(60/2*pi)

w = 371.8 rpm

CONCLUSION:

• The steady state speed of the given vehicle powertrain for 70% duty cycle is 371.8 rpm

QUESTION 3:

MATHEMATICAL MODEL OF DC MOTOR:

SIMULINK MODEL:

• 

REPORT:

• The mathmatical model of DC motor designed using the below equation

w = (V/K)-(Ra/K^2) *T

• Where T-torque of the motor
• V- input voltage of motor
• Ra-armatur resistance
• K-motor constant
• w-speed of the motor
• A constant block with value 0.7 used to represent the motor constant K
• The motor voltage defined by another constant block with value 50.46
• These value divided by a divide block to get the term (V/K)
• Torque is represented as constant value 33 using a step block
• The resistancee value of 0.5 is defined in the constant block
• The resistance block divided by the square value of motor constant
• Multiplying torque and this divided value of resistance by square of motor constant using a product block to get the term

(Ra/K^2) *T

• The output of this product block subtracted by the terrm V/K to get the final equation
• The torque and speed is connected to scope blocks
• Run the model
• Outputs plotted in the scope block

RESULT:

• The speed output for the given torque value is plotted as shown below
• 
• In the graph ,yellow line indicates the value of torque
• And blue line indicates the corresponding speed of the motor
• For a torque value of 33 Nm ,speed obtained is 38.41 rad/s

CONCLUSION:

• The mathematical model of DC motor for the given equation is designed and the outputs verified

QUESTION 4:

Induction Versus DC brushless motors by Wally Rippel, Tesla

REPORT:

• In this blog ,author discussed about the difference between induction motors and brushless dc motor
• And their role in the future of electric vehicle
• Difference between both machines is briefly explained below
• 

CONCLUSION:

• The perfect motor for an electric vehicle doesnt exists ,it depends on the application and personal requirement
• Still the induction and DC brushless motors will dominate the EV industry in the future