Week-6 Challenge: EV Drivetrain

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

2. An Electric Vehicle's Drivetrain 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 is2'. The other parameters of the electric vehicle is given below

Given data -

Armature or Battery voltage `V_b` = 72 V

Armature current `I_a` = 400 A

The motor voltage can be calculated as `V_m= V_b*d` where `V_b` is battery voltage and d is the duty cycle

`V_m = 72V* 0.7 = 50.4V`

Armature resistance `R_a = V_a/I_a`

`R_a = (72V)/(400A) = 0.18 ohms`

The torque-speed relationship is established by this equation

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

where V is the battery or supply voltage,

`Kphi`is the motor constant,

`R_a` is armature resistance,

`omega` is the angular speed of the motor in rad/s,

T is the motor torque.

Substituting all the given data we end up with an equation for torque,

`T = (50.4 * 0.7)/ 0.5 - (0.7^2*omega)/0.5 = 70.56 - 0.98omega` -------- Equation 1

From the data given another equation for the torque is provided

`Ttr = 24.7 + 0.0051omega^2` ----------- Equation 2

By combining equations 1 & 2, we end up with a single equation

`70.56 - 0.98omega = 24.7 + 0.051omega^2`

`0.0051omega^2 +0.98omega - 45.86 = 0` ----------- Equation 3

The final equation represents a quadratic equation `ax^2 + bx +c=0` where the root `x` is given by `(-b+- sqrt(b^2 - 4*a*c))/ (2*a)`

x represents the angular speed `omega` and by substituting the constants we get the values for `omega` as 38.82 rad/s and -230.98 rad/s.

The steady state angular speed of the motor therefore is 38.82 rad/s.

3. Refer to the blog on below topic:

Induction Versus DC brushless motors by Wally Rippel, Tesla

• The author tries to draw out a comparison between induction motor and DC brushless motor used in driving electric vehicles. The author's intention is to underline the principles of operation between both motors and lists out the pro's and cons in terms of performance, control strategies, cooling system,generated torque and manufacturing costs.

This table encaptures the author's comparison between both the motors mentioned in the blog article.

• The author concludes that there is no cleary winner to prefer one over the other. He states that DC brushless drives are better fit for hybrid electric vehicles and AC Induction drives are better for high performance pure electric vehicles. He hopes in the future that these two motors can work side by side.