Week-6 Challenge: EV Drivetrain

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

Electronic used for conversion and control electric power is know as power electronic and that conversion and control is done by the power converters.

There are typically four types of thr power convertor :

1. AC to DC (rectifiers) – here the common example is the chargers which induced as voltage and supply DC to the battery .
2. DC to AC (inverters) – AC induction motors .
3. DC to DC (chopper)- 1)step up 2)step down
4. AC to AC – common example is fan regulator.

Following are the types of power convertor circuit in electric and hybrid electric and hybrid electric vehicle :-

-plug IN EV’ s- here rectifier are used in AC to DC conversion where grid electric is first introduced with the vehicle . the grid voltage obtained fro m the plug is 110/60Hz or 220v/Hz single phase AC input in household application of plug in EV.

Power convertor – these are classification according to their input and output types namely DC-DC CONVERTOT and AC-AC CYLCLOCONVERTERS. DC -DC CONVERTERS are used to increase or decrease the input voltage stage to desired value at output .therefore , device know as buck converter or stepdown converter decreases the output value according to high input voltage and boost converter or step-up converter generates an increased output voltage.

When the EV drive switched into braking mode , the batteries the are charged by electric motor due to bidirectional structure of converter . therefore ,

BIDIRECTIONAL DC-DC COVERTER IS THE KEY DEVICE IN EV,s.

INVERTERS –

In EV’s inverters are used for DC to AC CONVERSION . they are used to provide required AC voltage or current for AC induction motors from DC BATTERY supply and consequently , the inverter control algorithm perform motor control operational I terms of torque and speeds.

Question 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%?

-FIRSTLY calculate the average battery voltage provided for acceleration and braking operation:

V(avg) = V(bat)*dutyratio

V(avg) = 72*50.4

The vehicle speed -torque characteristic are given by the below equation

TV = 24.7 + (0.0051)ω²

Also, the torque speed characteristic equation is expressed as follows :

T = ( 50.4×(Kφ)^2 )/Ra

-Equating both the equation,we get;_-

(50.4 × 0.7÷0.5) –(ω×(0.7)^2÷0.5) = 24.7 + (0.0051)ω²

70.56-0.98ω = 24.7 + (0.0051)ω²

45.8 = 0.0051 ×ω^2 + 0.98 × ω

Above the torque speed characteristic equation is expressed as follows:

By solving there were two values in negative and positive and speed value can never be in negative .

So, ω=38.95 rad/sec

Question 3:-Develop a mathematical model of a DC Motor for the below equation using Simulink.

ω=( V/Kϕ-Ra/Kϕ^2 .T)

Output graph:-

1. Speed vs time graph

   

2. Torque – speed characteristic graph

Conclusion:-

After persual of above graph , we conclude that as soon as torque increases , speed of DC motor decreases.

Question 4: - Refer to the blog on below topic:

Induction Versus DC brushless motors by Wally Rippel, Tesla

Explain in brief about author’s perspective.

In DC brushless motor, the rotor includes two or more permanent magnets that generate a DC magnetic field. Due to this, magnetic field enters the stator core and interacts with currents flowing within the windings to produce a torque interaction between the rotor and stator. However, the induction rotor has no magnets- (just stacked steel laminations with buried peripheral conductors that form a "shorted structure.)" and current flowing in the stator windings produce a rotating magnetic field that enters the rotor.

• In case of DC brushless motor, when rotor rotates, it is necessary that the magnitude and polarity of the stator currents be continuously varied in such way that the torque remains constant and the conversion of electrical to mechanical energy is optimally efficient. Hence, this current control is provided by inverter.

• A brushless DC motor produces no starting torque when directly connected to fixed frequency utility power. They requires an inverter whose "phase" is maintained in step with the angular position of the rotor. Hence, DC brushless motors are useless without inverters. However, AC induction motors have the capability of producing the torque at the outset when connected to utility type 3-phase power, hence the motor has the ability to start under load. So, No inverter is needed in case of AC induction motorDC brushless motors require an absolute position sensor, whileinduction motor drives require only a speed sensor.

• The speed of DC Brushless motor is controlled by varying current, whereas the speed of Induction motor is controlled by varying frequency.

• In case of DC brushless motors, less rotor heat is generated because rotor cooling is easier and peak point efficiency is generally higher as compared to induction motors.

• The DC brushless motor can also operate at unity power factor, whereas the best power factor for the induction motor is about 85 percent. Hence, we can say that the peak point energy efficiency for a DC brushless motor will typically be a few percentage points higher than for an induction motor.

• The Peak efficiency of Induction motor is less than DC Brushlessmotor but the average efficiency is higher.

• Due to permanent magnets in case of DC brushless motors, the magnetic field strength (B) cannot be ajustable and as a result, sum of the eddy, hysteresis, and P losses cannot be minimized. However in case of induction motors, there are no permanent magnets, hence magnetic field strength (B) can be easily adjustable so that sum of eddy, hysteresis, and P losses can be minimized.The induction motors when operated with a smart inverter has an advantage over a DC brushless motors is that magnetic and conduction losses can be traded so efficiency is optimized.

• In case of DC brushless motors, as machine size grows, the magnetic losses increase proportionately and part load efficiency drops. However in case of induction motors, when machine size increases, losses do not increases necessary.