Week-6 Challenge: EV Drivetrain
1 ) AIM - Understanding types of power converter circuits employed in electric and hybrid electric vehicles.
Converter is a device that increases or decreases the voltage AC OR DC of a power source depending on applications. Power converters plays an important role in electric vehicles.
While selecting power converter some design considerations are essential for Automotive applications such as,
Light weight
High efficiency
Small volume
Low electromagnetic interference,
Low current ripple drawn from the fuel cell or battery
DC - DC Converters -
The need of the voltage from battery according to the applications varries.
To meet specific amount of voltage or power we introduce DC to DC converters which can choose the voltage variation of the devices and the power of each device can be controlled.
Use of high power DC to DC Converter is necessary for electric vehicle power supply system.
The power of the DC to DC Converter depends on the characteristics of the vehicle such as weight, Top Speed, acceleration time from 0 to 100 Km/h , maximum torque and power profile such as peak power, continuous power.
Generally for passenger cars the power of the converter is more than 20 KW and it can go up to 100 KW.
Several dc-dc converters such as,
Isolated Converters -
- Half Bridge Converter
- Full Bridge Converter
- Push Pull Converter
- Boost Converter
- Buck Converter
Non isolated converters -
- Boost converter
- Buck converter
- Buck boost converter
- Cuck Converter
- Charge Pump converter
Uni-directional
Bidirectional
are used in EV and HEV .
Bidirectional DC DC Converter is placed between the inverter and the energy source of an electric vehicle.
Rectifier or AC - DC Converter -
Rectifier is a device that converts AC power to DC power used in EV motor.
AC to DC converter is is used as charger to convert the input AC voltage to DC , to charge the battery from AC supply.
There is also use of bidirectional rectifiers in electric vehicles and a Hybrid Electric vehicles.
Inverter or DC to AC converter -
An inverter is a device that converts DC power to AC power used in EV motor.
In electric drive train , the inverter controls the Electric motor.
Not only does the inverter drives the electric motor it also captures energy released through regenerative braking and feeds this back to the the battery. The inverter DC to AC converter can change the speed at which the motor rotates by adjusting the frequency of the alternating current.
Using inverter the power or torque of the motor can be increased or decreased by adjusting the amplitude of the signal.
2 ) AIM - finding steady state speed with given parameters.
The given parameters are,
Rated Armature voltage= 72 V
Rated armature current= 400 A
Ra= 0.5Ω, KΦ= 0.7 Volt second
Chopper Switching frequency= 400 Hz
The duty cycle is 70 % = 0.7
The average Voltage Va is,
`Va =` Supplied Voltage × Duty Cycle
`Va=72×0.7 =50.4 V`
The DC motor torque speed equation is,
`Tm = V.(kphi)/(Ra) - (k^2phi^2)/(Ra).omega`
Substituting the given values in above equation we get,
`Tm = (50.4 × 0.7) /0.5 -((0.7) ^2.omega) /0.5`
`therefore Tm= 70.56 - 0.98omega`
At steady state speed condition, the Torque of both vehicle and motor will be same.
Tv = Tm
Equating given equation of Torque Tv and obtained equation Of Torque Tm we get,
`24.71(0.0051).omega^2= 70.56 - 0.98.omega`
`therefore 0.0051omega^2 +0.98.omega - 45.86 = 0`
By Solving The Quadratic Equation,
`omega =38.91 (rad) /sec`
Or
`omega =371.56 RPM`
3 ) AIM - Developing a mathematical model of a DC Motor for the given equation using Simulink.
The blocks used are constant , ramp , square block , gain block.
The equation is,
`omega =V/(k.phi) - (Ra) /(k.phi).T`
The value of `omega` which is in RPS is converted to RPM by multiplying with 60/2π .
Simulink Block -
RESULT -
From the graph we can see that, at 0 Torque , The Speed is Maximum which is 982.7 RPM .
4 ) Explain in brief about author’s perspective.
In the author's perspective there are different types of engines to suit our personal requirements such as price and performance and this is also true in case of electric vehicle drives.
He compared his hair with lead acid battery , DC brush motor and contactor controllers.
Today none of these remained including his hairs.
Because today lead acid batteries has been replaced by lithium and DC Motors by either DC brushless or induction motors.
So what differentiate DC brushless motors and induction motors ?
He described as ,
Brushless DC Motors have rotor and stator in which rotor includes two or more permanent-magnets.
Stator includes windings, which acts as electromagnet.
The Torque is produced by interaction of magnetic fields produced by both rotor and stator Core.
As the rotor rotates the magnitude and polarity of the stator currents must be continuously varied in a right way.
For that inverter is used as current controller. Without inverter brushless Motors are useless motor.
Three phase induction motor was invented by Nikola Tesla before 1889. The stator for both Motors are identical. Because stator core of of both the Motors have three sets of distributed windings.
The rotor part is different in both Motors. The rotor construction of induction motor includes stacked laminations with buried peripheral conductors which are of shorted structure.
When theatre is excited it creates rotating magnetic field, due to induced EMF in the Rotor, Torque is produced due to interactions of magnetic fields.
The induction motor is able to start at under the loaded conditions and they don't require inverter for their operation, where is DC motor required inverter for their operations.
But induction motor can't run on direct DC source for battery source ,so using inverter we can operate induction motor on battery or DC source.
And we can also control the speed of induction motor by adjusting the inverter frequency.
To match the exact performance of induction motor with DC brushless motor we can add some feedbacks loops search that the inverter produces the exact frequency according to the motor requirements.
Now all the Hybrid electric vehicle uses brushless DC motor very General Motors EV 1,Tesla Roadster uses induction motor.
Both the used Motors have similar stator both drive uses three phase modulating inverters.
The peak point Energy Efficiency for DC brushless motor is higher than for an induction motor.
The heat generated with DC brushless motor is less compared with induction motor.
The speed of the induction motor can be controlled by variable frequency method.
Permanent magnets used in rotor of DC motor are expensive and we also need to keep ferromagnetic materials away from the magnetic Rotor
According to his conclusion DC Motors are used in Hybrid electric vehicles and induction motors are used in electric vehicles and they will still in use because there are almost of same characteristics.