Week-6 Challenge: EV Drivetrain

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

Solution:- Types of power converters are as follows

• The device which converts AC current to DC current is known as Rectifier. In plug-in type of electric vehicle the battery receives AC current from the source. To convert the AC type into DC type, a rectifier is used, because we can’t charge the battery by direct AC supply.
• The device that converts DC-AC is known as Inverter. The battery of an EV supplies DC power to the motor and if the motor used is an AC motor then to convert the DC to AC an inverter will be used in between.
• The device which converts DC-DC is known as chopper. Essentially, a chopperis an electronic switch that is used to interrupt one signal under the control of another. In power electronics applications, since the switching element is either fully on or fully off, its losses are low and the circuit can provide high efficiency. A chopper is a static device that converts fixed DC input voltage to a variable DC output voltage directly. A chopper is a high speed on/off semiconductor switch which connects source to load and disconnects the load from source at a fast speed. To power some of the electronic accessories which are of some low voltage than the battery voltage a chopper is used.
• The 4^th type of converter is an AC-AC converter. Although until now it has no application in an EV but in other appliances it has its importance. For example this type of converter is used in a regulator of a fan which converts a different Voltage and Frequency to a different Voltage and frequency.

2. An Electric Vehicle's powertrain 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 is 'd2'.

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

Solution:- The Torque-speed characteristics of the motor is given as

`T = (V*k)/R – (k^2 * omega)/R`

Where,

T = Torque

V = Armature voltage

R = Resistance

`omega` = = angular velocity

Now equating the above given equations we get

`24.7 + 0.0051*omega^2 = (V*k)/R – (k^2 * omega)/R`

`24.7 + 0.0051*omega^2 = (72*0.7*0.7)/0.5 – ((0.7)^2 * omega)/0.5`

`24.7 + 0.0051*omega^2 = 70.56 – 0.98*omega`

`0.0051*omega^2 + 0.98*omega – 45.86 = 0`

Dividing by 0.0051 throughout

`omega^2 + 192.156*omega – 8992.1568 = 0`

Solving the above quadratic equation by the below given formula we get

`x_(1,2) = (-b +- sqrt(b^2 – 4*a*c))/(2*a)`

The two values are

`x_1` = 38.9151 OR `x_2`= -231.0711

since the angular velocity cannot be a negative value

therefore, `omega` = 38.9151 rad/sec

3. Refer to the blog on below topic:

Induction Versus DC brushless motors by Wally Rippel, Tesla

Explain in brief about author’s perspective.

The similarities between both the motors is that both of them does not use brushes in their constructions, which means there will be low maintenance cost due to friction

Difference

Induction AC motor Brushless DC motor

The Induction motor experiences a The brushless DC motor prevails here

high energy loss due to energizing because of the permanent magnet used

the electromagnet

The AC motor uses a converter known Whereas this motor uses a sensor known

as VFD which is bulky. It controls the as Hall effect sensor. Due to its small size it

speed by adjusting the frequency. can be easily fitted inside the circuit and it

can align the rotor and stator more precisely

An AC motor losses Torque with an The brushless DC motor provides a flat

Increase in speed torque characteristics over a larger speed

due to the highly precise Hall Effect sensor

and because of a permanent magnet used

as the rotor the energy losses are also low

Conclusion:- The brushless DC motor is used because of its ability to have full control, efficiency, capability at high speed operations. It is more widely used because it is capable of many different applications and also the fact that most application needs speed control.

AC Induction Motor is cost effective at places where there is no need for a controller and can be run directly from the power supply.