Week-6 Challenge: EV Drivetrain

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

Hybrid electric vehicle (HEV) :-

• In HEV it contains more than one source.
• But at least one of the source can supply electrical power for propulsion.
• Based on the combination of sources there are different types of HEV,but most commonly used is internal combustion engine (ICE) along with electrical motor.

Electric vehicle (EV) :-

• In this vehicle is run through pure electrically.
• Its only contain electric motor for propulsion that is single source.

Power converter circuits used in EV and HEV are follows :-

a. Rectifier(charger) :-

• It convert AC supply to DC supply required for the battery to charge.
• Battery output voltage can vary from 400v to 800v .
• Battery may be charged by the on board charger (AC/DC) using a regular outlet or larger fast charger, external to the vehicle.

b. Inverter :-

• Inverter circuit is used to convert the DC supply to AC supply which is required by AC motor to run.
• The inverter is a bi-directional converter that accepts the high voltage from the battery and converts it into AC supply voltage suitable for the traction.
• During the deceleration that is regenerative breaking period the motor absorbs the torque and provide the AC voltage to the inverter.
• The inverter controls the amount of the energy generated by the motor and consequently the intensity of the braking. The harvested energy is
  applied back to the battery as a charge.

c. Chopper :-

• Chopper is the circuit which convert DC supply at one voltage level to DC supply at other voltage level
• It is also called as DC to DC converter.
• The chopper circuit are of three type :-
• Boost chopper(step up )
• Buck chopper (step down)
• Buck-Boost chopper.
• Mainly in vehicle the step down chopper is used as it is used by vehicle auxillaries which can operate on low voltage DC supply.
• The auxillaries are headlight,power (streering,brake and window),air conditioning,music system ,etc.

2. An Electric Vehicle's powertrain with 72V battery pack in shown below:-

• Rated armature voltage = 72v
• Rated armature current (Ia) = 400A
• ra = 0.5 ohms
• kϕ = 0.7 volt-second

The vehicle speed torque characteristic of is :-

T = 24.7+(0.0051)*ω^2.....equation 1

The DC motor speed -torque characteristics is given by:-

(kϕ)*V/ra - (〖(kϕ)〗^2*ω)/ra....equation 2

As steady state speed is calculated at duty cycle of 70%

Therefore average voltage applied to dc motor supply terminal is V=0.7*72 = 50.4volt

Putting all the values in equation 2 we get:-

(0.7)*50.4/0.5-〖(0.7)〗^2*ω/0.5

T=70.56-0.98*ω.....equation 3

plotting equation 1 and3 we get :-

• In this plot torque in Nm and speed in rad/sec.
• The plot is in red colour is of DC motor characteristics and plot is in blue is of vehicle characteristics.
• The intersection of both the plot is the steady state speed point and that has to be calculated.
• The matlab script for plotting above plot are as follows :-

  clc
  speed =linspace(0,50,10);
  torque = 24.7+((0.0051)*speed.*speed);  %torque-speed characteristic of vehicle
  speed1=linspace(0,50,10);
  torque1=70.56-(0.98*speed1);   %torque -speed characteristic of DC motor
  plot(speed,torque);
  hold on
  plot(speed1,torque1);
  xlabel('speed(rad/sec)');
  ylabel('Torque(Nm)');
  

• In this code using linspace function speed in rad/sec is defined with 10 evenly spaced point.
• Vehicle and DC motor torque-speed characteistic is deifined.
• With the help of plot function graph is plotted.
• To merge both the graph ued hold on function.

To calculate the steady state speed equate both eqaution1 and 3 :-

24.7+(0.0051)*ω^2 = 70.56-0.98*ω

ω^2 + 192.15*ω - 8992.16=0

By solving the above quadratic equation we get two value of speed :-

ω=-231.98 , 38.915 rad/sec

Taking positive value ,therefore steady state speed for EV = 38.915 rad/sec = 743.22 RPM.

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

ω">ω= V/Kϕ">ϕ -Ra/Kϕ">ϕ^2 *T

Before modelling some parameter has to be assumed which are as follows :-

• V=72 volt
• ra=0.5 ohms
• kϕ = 0.7 volt-second
• Kt=2
• B (damping constant)=3
• J (inertia constant) =5
• L(Inductance of motor)=0.02H

di/dt=(V-(Ia*ra)-k*dϴ/dt)*1/L

〖(dϴ)/dt〗^2 =(Kt*Ia-B*dϴ/dt)/J

ω">ω= V/Kϕ">ϕ -Ra/Kϕ">ϕ^2 *T

All the above equation are implemented in simulink model :-

• The above model contains sone blocks as follows :-
• Add
• divide
• square
• gain
• intergrator
• goto and from
• scope
• constant
• outport

Torque and speed plot of the equation ω">ω= V/Kϕ">ϕ -Ra/Kϕ">ϕ^2 *T are shown below :-

• In the above plot torque in Nm and speed in rad/sec are plotted.
• The plot in blue colour is of torque and plot is in yellow is of yellow.
• It is shown from the graph as speed increases the torque reduces as decaying exponential .

4. Refer to the blog on below topic:

Induction Versus DC brushless motors by Wally Rippel, Tesla

Explain in brief about author’s perspective.

Brushless DC motor (BLDC) :-

• Construction wise BLDC motor is different from Brushed DC motor.
• In BLDC motor permanent magnet is placed on rotor and coils are placed on stator.
• The movement of rotor is with the help of attraction and repulsion principle .
• On the stator three coils are placed and they are switched on and off according to rotor rotational position which is determine by position sensor with the help of Hall effect.
• For switching the stator supply current and electronic control is required that is inverter.
• BLDC motor are also known as self synchronous AC motor , variable frequency synchronous motor, permanent magnet synchronous motor(PMSM) and electronically commutated motor(ECM).

Three phase Induction motor (IM):-

• Construction wise IM has stator and rotor ,stator of IM are supplied from three phase ac source and rotor is squirrel cage or slip ring.
• As three phase supply is given to stator it producess alternating magnetic field and it cutts rotor conductor to induced emf in rotor .
• Due to which rotor generates its own magnetic field and the interaction of two magnetic field results in torque in IM.

Comparision between two motors :-

1. Both motors that is BLDC and Induction motor are widely used in electric vehicle .
2. Less heat is generated ,easy cooling and peak point efficiency is much higher in case of BLDC than induction motor.
3. Starting torque of BLDC is relatively more as compared to IM.
4. BLDC power factor is good than IM.
5. Eddy and hysterisis loss can be minimised by reducing magnetic flux density "B" .But the control of B cannot been easy in case of BLDC .
6. Control of B can be easily done in case of induction motor as B is proportional to V/f.
7. As machine size of BLDC increases magnetic losses are increase proportionally but this is not the case in induction machine.
8. Without the inverter BLDC is useless and in IM also inverter is used for the conversioN of DC to AC .

Finally as per the author perpective :-

• Both the motor has it own advantage and disadvantage and they have there dominance in there respective position where it have been used.
• As BLDC has good peak point efficiency so it has its dominance in hybrid and plug in hybrid vehicle market.
• While if we see overall performance of induction motor is good so it has dominance in pure electric vehicle market.
• So, as there hardware is nearly similar they can work side by side in EV and HEV.