Week-6 Challenge: EV Drivetrain
1)Which types of power converter circuits are employed in electric and hybrid electric vehicle?
ANSWER
Some of the types of power converter circuits employed in electric and hybrid electric vehicles are as follows,
DC to AC circuit :- This power converter circuit will convert the DC source into AC output. This type of converter is commonly seen in motor circuit. To convert DC to AC we need switches that will continuously turns ON & OFF , which will create a signal similar to AC wave. For this purpose a total of 4 switches are used. The types of switches used here are mainly MOSFET and IGBT.
AC to DC-DC circuit :- This power converter circuit will convert the AC source into DC output. This type of converter is commonly seen in charger circuit, where AC input source has to be converted into DC to charge the battery. To convert AC to DC we use full bridge rectifiers , which will filter out the negative half of the AC wave. There is also a chopper circuit in this arrangement , which basically transfers only the required amount of DC voltage to the battery or from the battery.
2)TO FIND EV STEADY STATE SPEED
AIM
To find the EV steady state speed if the duty cycle is 70%.
SPECIFICATION
The specification of the EV is given below,
Battery voltage - 72V
Rated armature voltage of the motor - 72V
Rated armature current - 400A
Armature resistance,`R_a = 0.5Omega`
Motor constant,`K_phi = 0.7` Volt second
Chopper switching frequency = 400Hz
Vehicle-speed torque characteristics - `T_v=24.7+(0.0051)omega^2` ----Eq(1)
CALCULATION
Duty cycle , `d=0.7`
So we can calculate average voltage as follows,
`V_(o(avg)) = d*V_(bat)`
`=0.7*72`
`=50.4V`
Now to calculate the speed of DC motor in `(rad)/sec`,
`omega_m = V/K_phi-R_a/K_phi^2*T`
`=50.4/0.7-(0.5*T)/0.7^2`
`=72-1.02(T)`
From this we can get an equation for torque in terms of angular speed,
`T = 70.56-omega_m/1.02` ----Eq(2)
Now compare Eq(1) & Eq(2) , we get,
`24.7+(0.0051)omega_m^2 = 70.56- omega_m/1.02`
`(0.0051)omega_m^2 = 45.86-omega_m/1.02`
`omega_m^2 = 8992.15-192.23*omega_m`
`omega_m^2+192.23*omega_m-8992.15 = 0`
This is a quadratic equation whose roots can be founded out using the following equation,
`(-bpmsqrt(b^2-4ac))/(2a)` ----Eq(3)
`sqrt(b^2-4ac) = sqrt(192.23^2-4*(1*(-8992.15))) = 270.03`
Putting this value in Eq(3),
`(-192.23pm270.03)/2 = 38.9 ,-231.13`
Taking the positive root , we get the speed as,
`omega_m = 38.9 (rad)/sec`
CONCLUSION
The steady state speed has been calculated successfully.
3) Refer to the blog on below topic:
Induction Versus DC brushless motors by Wally Rippel, Tesla
Explain in brief about author’s perspective.
ANSWER
The author Wally Rippel is an engineer at Tesla motors. He is a long time proponent of electric vehicles and he also helped in the development of the famous EV1 for General Motors.
In this blog , he basically compares induction motors with brushless DC motors.
In the first section he gives an interesting analogy between engine types and electric vehicle drive types. So in this analogy he mentions how there are different types of engines and still even today no one is able to figure out which was the best type. This happened due to the fact that every vehicle requirements are different for every other vehicles. And this is same in the case of EV’s also.
In the next paragraph he lists some old technologies like lead acid batteries,DC brush motors etc.. that has become obsolete now and has been replace by new technologies like lithium batteries , brushless DC motors, induction motors etc….
Now he explains the working of the two types of the motor.
So a Brushless DC motors has rotors that includes two or more permanent magnets , which produces magnetic fields that enters the stator core and produces torque. An inverter is used to maintain the electrical to mechanical energy optimally efficient by controlling current.
In case of an induction motor the main difference is that , it has stacked steel lamination’s with conductors inside a rotor. The stator will induce voltage across the rotor which will be proportionate to the speed difference between the rotor and electrical frequency. This will produce current inside the rotor and this will interact with the stator magnetic field and produces torque.
Some advantages of Induction motors are , it can start under load when connected to utility type 3-phase power and it is also directly compatible with conventional utility power. Also the magnetic field strength can be controlled by changing the voltage. When the size increases the magnetic losses do not necessarily grow. Average efficiency higher than BLDC type motors. No recurring costs for maintenance.
Some limitations of Induction motors are ,they cannot operate from DC and their shaft speed is proportionate to line frequency so they are constant speed machines without an inverter. Also the torque performance is low compared to DC machines. These are very difficult to control due to the fact that the control laws are complex and difficult to understand.
To overcome these limitations inverters and feedback loops are used.
Some advantages of Brushless DC motor are, less rotor heat is generated and the peak point efficiency is generally higher. They can also operate at unity power factor.
Some of the limitations of Brushless DC motors are , it requires an absolute position sensor for controlling the motor , the strength of the magnetic field produced by the permanent magnets cannot be adjusted. In this type of motors when the size increases the magnetic losses also increases. Also these permanent magnets are expensive. Also the maintenance is costly compared to induction type.
For high performance drives induction machines are preferred. Some notable uses of induction drives is seen in GM EV-1,Tzero, Tesla Roadster.
All the hybrids are powered by DC brushless drives.
Now the author concludes this comparison by saying that the DC brushless will dominate the hybrid vehicles and induction drives will be dominant for the high performance pure electric vehicles and so in the coming era they both will work side by side .