Week-6 Challenge: EV Drivetrain

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

ANS:

Power converter:

Power conversion is converting electrical energy, from one form to another such as converting between AC and DC, or changing the voltage or frequency; or some combination of these.

Following are some converters used in regular basics :

DC to DC Convertor:

Chopper:

It is convert variable DC to constant DC or constant DC to variable DC. In electrical engineering a DC-DC convertor is a catagory of power convertor. An electrical circuit which converts a source of direct current(DC) from one voltage level to another by storing the input energy temporary and then realising that energy to output at different voltages. The storage may be either magnetic field storage component (inductor,transformer) or electric field storage component (capacitors). We can control or varry a constant DC voltage with the help of chopper.

There are two types of chopper:

1) Step down chopper.

2) Step up chopper.

Step down chopper:

The output voltage is less than the input voltage. The step down chopper is buck converted is used to reduced the input voltage ;evel at the output side. Circuit diagram of step down chopper as shown in below fig.

When the switch is turned ON, Vs is directly appear across load Vs=Vo as shown in below fig.

When switch is turned OFF then Vs is disconnected from load Vo=0 as shown in below fig.

The voltage waveform of step down chopper as shown in below fig.

Ton→ It is the interval in which chopper is in ON state.
Toff → It is the interval in which chopper is in OFF state.
Vs→ Source or input voltage.
Vo→ Output or load voltage.
T → Chopping period = Ton+ Toff

Step up chopper:

The step up chopper or boost convertor is used to increase the input level of output side. Step up chopper as shown below fig.

When the switch is ON it short circuit the load. Hence output voltage during Ton is zero during that period inductor get charged (Vs=VL) as shown in below fig.

When the switch is turned OFFB inductor discharged through the load, so we wil get summation of both source voltage Vs and inductor voltage VL as output voltage as shown in below fig.

The voltage waveform of step up chopper as shown in below fig.

The following devices can convert DC to DC:

• liner regulate
• voltage regulator
• motor-generator
• Rotary converter
• switched-mode power supply.

AC to DC Convertor:

A rectifire is an electrical device that convert AC to DC which flow only in one direction.

There are mainly two types of rectifier:

1) Uncontrolled rectifier.

2) Controlled rectifier

Usually the type of rectifires are classified based on their output.

• Single phase rectifier
• Three phase rectifier
• Controlled rectifier
• Uncontrolled rectifier
• Full wave rectifier
• Half wave rectifier
• Brifdge rectifier
• Center tapped rectifier

The commonly used rectifier is a Bridge rectifier:

As we know our charged is stored in DC form in the battery. For this battery charging we need DC supply, but the generated power supply and external power supply is AC and battery need DC supply. So for this conversion we need rectifier which convert AC to DC.

It comprises a diode/ thyristor based on the type of rectifier. The output of rectifier is a pulse DC and hence it is filter using a filter circuit. Which is usually made a choke/ capacitor. Charger are the best example of AC tio DC convertor.

The following devices can convert AC to DC:

• Rectifier
• Mains power supply unit (PSU)
• motor-generator
• rotary converter
• switched-mode power supply.

AC to AC Convertor:

AC to AC convertors connect an AC source to AC load by controlling the amount of power supply to the load. This convertor convert the AC voltage at one level to another level by varing magnitude as well as frequency of the supply voltage.

The following devices can convert AC to AC:

• transformer autotransformer
• voltage converter
• voltage regulator
• cycloconverter
• variable frequency transformer
• motor-generator
• rotary converter
• switched-mode power supply.

These are used in different types of applications including uniterrupted power supply(UPS), high power AC to AC transmission, renewable energy conversion system.

DC to AC convertor:

An invertor converts the DC electricity from source such as batteries or fuel cell to AC electricity at any required voltage used in an electric vehicle motor. This can change the speed at which motor rotates by adjusting the frequency of alternating current.

The use of this converters increase or decrease the power or torque of the motor by adjusting amplitude if the signal play significant role capturing energy from regenerative braking and feeding it back to the battery.

Sinewave invertor is good quality AC power. They used Pulse Width Modulation(PWM) to produce true sine wave. As we know our induction motor required AC power supply but we have DC power supply in battery. So we have to convert this DC supply to AC supply to motor. Which is done by invertor is convert DC to AC supply as per requirement how much need supply to respected speed.

The following devices can convert DC to AC:

• power inverter
• motor-generator
• rotary converter
• switched-mode power supply.

From all the above converters we used Some majorly used converters are following :

1) Rectifier (AC to DC)- EV charger.

2) Inverter (DC to AC)- For induction motor.

3) Chopper (DC to DC)- For axillary equipment.

Model of converter used in Eletrcle vehicle:

Q.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%?

ANS:

We have,

Va=72 V,

Ia=400 A,

Ra=0.5 ohm,

KΦ= 0.7 Volt-sec,

f=400 Hz

The spped torque characteristics of 100% duty cycle is given as,

Tv=24.7+0.051w^2....................(1)

The relation between input/output voltage & duty cycle is,

Vout=D*Vinput

so,

Vout=70%duty cycle*72

= 0.7*72

=50.4 V

For DC motor torque produced is given by,

T=((v*k)/R-((k^2/R)*w))

=((50.4*0.7)/0.5-((0.7^2/0.5)*w

T=70.56-0.98w............................(2)

In steady state condition motor torque is equal to load torque,

so, by equating 1&2 queation we get,

0.0051w^2+0.98w-45.86...........................(3)

Solving the quadratic equation,

w=38.9 rad/sec.

Result: The EV steady state speed is 38.9 rad/sec, when the duty cycle is 70%.

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

ωω= V/Kϕϕ -Ra/Kϕϕ^2 .T

ANS: Here I assume constant values,

v=72,

k=0.7,

Ra=0.5

Above all are the constant but torque/speed is not a constant its a variable so, i have used ramp block as torque. The model shown below,

Here I defined constant values in the workspace, as shown below,

The output graph as shown below,

Q.4. Refer to the blog on below topic:

Induction Versus DC brushless motors by Wally Rippel, Tesla

Explain in brief about author’s perspective.

ANS:

Wally Ripple is a long time proponent of an electric vehicle. Prior to joining tesla motors he was an engineer at Airo Vironment, where he helped devlop the EV1 for General Motor and was featured in the documentary movie who killed the electric car. Wally has also worked for the jet-propolsion on electric vehicle battery research, among other project. In 1968 as a caltech undergraduate student he build electric car(a coinverted 1958 volkswagen microbus) and won the great transcontinental electric car race against MIT.

In this blog authors perspective is:

• They compared the old and today's technology like a battery, drive machine like motors. he wants to get to know it will get stable this technology any were.
• They want to know that which motor is better for electric vehicle BLDC or an induction motor.
• So they compared are these two technologies? How do they work? What differentiates them? And what do they have in common?
• Firstly he goes through the BLDC motor, explained the working principle
• then he moves on to induction motor drives. A forerunner of the 3 phase induction motor was invented by Nikola tesla. sometime before 1889. Curiously, the stators for the 3-phase induction motor and the DC brushless motor are virtually identical. Both have three sets of “distributed windings” that are inserted within the stator core. The essential difference between the two machines is with the rotor.
• After that he informed about the induction motor working principle, advantages, limitation, etc.
• He gives the about power supply to motors.
• He's main motto is which one is batter for electric vehicle.
• But As per the Application both motors are suitable for cars for more torque BLDC is preferable and induction is for constant speed.
• So when there is a need for hybrid performance, we could see a completely new combination of drives with these two drives.
• So he meant there is no winner in the future of EV and HEV for drives, there can always be a new design and combinations.