Week-6 Challenge: EV Drivetrain

Answer to Question 1:

A DC-DC converter converts between voltages of different levels, such as high to low or low to high. This is usually seen between traction batteries and low-voltage power electronics.

These are of several types based on application -

1. Buck DC-DC converter - This helps in reducing the voltage from high DC to low DC
2. Boost DC-DC converter - This helps in increasing the voltage from low DC to high DC
3. Buck-Boost DC-DC converter - This does both increase and decrease voltage levels

An inverter converts DC voltage to AC voltage. It mainly converts DC voltage from traction batteries to AC voltage for traction motors.

A rectifier is also used to convert the AC voltage to DC voltage. This is used primarily for charging systems; between generators and traction Batteries in HEV, and charging ports to traction batteries in EVs.

A schematic representation of the two converters in vehicle architecture is shown below:

The vehicle architecture is of a Hybrid Electric Vehicle (HEV) where the red rectangular boxes highlight the DC-DC converters while the inverters are shown with purple highlighted boxes.

Reference:

"Power Converters for Hybrid Electric Vehicles"; M. Nandhini Gayathri; https://www.igi-global.com/gateway/chapter/275540#pnlRecommendationForm

Answer to Question 2:

Given Data:

• Rated Armature Voltage ( Vin) = 72 V
• Rated Armature Current ( Ia) = 400 A
• Armature Resistance ( Ra) = 0.5 ohms
• Motor Constant ( Kphi) = 0.7 V/(rad/sec)
• Switching frequency ( f ) = 400 Hz
• Final drive ratio ( id) = 1 : 1
• Duty cycle ( d) = 70 %

Vehicle Characteristic equation:

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

Calculation:

Average voltage during the ON state of the duty cycle : Vavg = Vin * d = 72 * 0.7 = 50.4 V

Motor torque-speed characteristic equation:

Tm = Vavg * Kphi / Ra - Kphi^2 * w / Ra

Tm = 50.4 * 0.7 / 0.5 - 0.7^2 / 0.5 * w

Tm = 70.56 - 0.98 * w ------------------> ( 2 )

Since the final drive ratio is 1 : 1, the motor torque ( Tm ) = torque available at the axle ( Tv)

Solving equation ( 1 ) and ( 2 ) we get,

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

0.0051 * w^2 + 0.98 * w - 45.86 = 0

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

The roots of equation ( 3 ) are

w = 38.9145 rad/sec or w = -231.0745 rad/sec

The steady-state speed during the ON state is w = 38.9145 rad/sec => w = 371.61 RPM.

Answer to Question 3:

The equation

omega = Voltage supply / K - Armature Resistance / K^2 * Torque

The above system equation is implemented as a sub-system in the simulink model and this is shown below -

Considering armature resistance = 0.5 ohms, supply voltage = 220 V, and K = 0.8 V / (rad/sec), the plot to this equation is as below:

The speed is converted from rad/sec to revolutions per minute. The torque vs speed is linear and inversely proportional.

Answer to Question 4:

	Induction Motor	Brushless DC Motor
Torque-Speed characteristic	The shape of the torque-speed characteristics is sinusoidal.	The shape of the torque-speed characteristics is trapezoidal.
Type of Voltage applied	AC voltage is applied in 3-phases.	Pulsed DC voltage is applied.
Structure	The rotor has steel laminations like a squirrel cage.	The rotor has permanent magnets.
Efficiency	It has a lower efficiency.	It has a higher efficiency.
Slip	There is slip as the rotor rotates at a lower frequency than the rotating magnetic field at the stator.	There is no slip as the rotor and rotating magnetic field are at the same frequencies.
Cost	It is less expensive.	The cost is higher due to permanent magnets