Week-6 Challenge: EV Drivetrain
ANS1.
Objective: To understand which types of power converter circuits are employed in electric and hybrid electric vehicle.
Theory: There are three types of Power Converter circuits employed in Electric and Hybrid Electric vehicles which are ,
DC-AC Converter or Inverter: An inverter is a device that converts DC power to the AC power used in an electric vehicle motor. The inverter can change the speed at which the motor rotates by adjusting the frequency of the alternating current. It can also increase or decrease the power or torque of the motor by adjusting the amplitude of the signal. Broadly speaking, an inverter is an electrical device that converts electricity derived from a DC (Direct Current) source to AC (Alternating Current) of the type that can be used to drive a device or appliance.
Application: In EV or HEV from the above picture it is clear that an inverter normally uses to convert from the battery power which is DC to AC and fed to the motor which is connected to wheel axel or transmission.
AC-DC Converter: It is the most commonly used converter where ACV is converted into DCV. These converter is also known as rectifier where bridge is used in the ckt. Single phase AC or three Phase AC connected to transformer(normally step down) and then feeded to either diode or thyristor in half wave or full wave fashion and the converted DC is feeded to load terminal.
Application:
- In EV or HEV from the above picture we can see a charger is connected to the car. This situation is true for a plug in type EV/HEV or in some scenario the charger is built inside car. This Charger is commonly Known as AC to DC converter .
- Also used for regenerative braking.
DC-DC Converter/Chopper: The soul purpose of this type of converter is to generate variable DCV output from fixed DCV input. Basically instead of converting it steps up or down the output DC voltage from a fixed input DC voltage known as Buck/Boost .
Application:
- The main dc-dc converter changes dc power from an on-board 200-800V high voltage battery into lower dc voltages (48V or 12V) to power
headlights, interior lights, wiper and window motors, fans, pumps and many other systems within electric vehicles (EV) and hybrid electric vehicles
(HEV). This high voltage to low voltage (HV-LV) dc-dc converter is often referred to as an auxiliary dc-dc, or Auxiliary Power Module (APM). Isolation
is critical for separating the control systems from high-voltage domains. - A chopper uses high speed to connect and disconnect from a source load. A fixed DC voltage is applied intermittently to the source load by continuously triggering the
power switch ON/OFF. The period of time for which the power switch stays ON or OFF is referred to as the chopper’s ON and OFF state times, respectively.
Choppers are mostly applied in electric cars, conversion of wind and solar energy, and DC motor regulators.
ANS2.
Objective: 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,
- 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
Solution:
We have to calculate the speed if EV is in steady state and duty cycle is 70%.
Solution: At steady state torque of the car and the motor will be same. So,
Tm = Tv (m= motor , v= vehicle) ..................EQ1
Tv= 24.7+(0.0051)w^2...................................EQ2
average voltage= armature voltage *duty cycle
or, v= 72*0.7
or, v= 50.4
Tm =V (Kϕ)/Ra - (Kϕ)^2.w/Ra..........................EQ3
Thus from equation 1,2,3 it can be summerised that,
Tm = Tv
or, 24.7+(0.0051)w^2= V (Kϕ)/Ra - (Kϕ)^2.w/Ra
or, 24.7+(0.0051)w^2 = (50.4*0.7)0.5 - 0.7^2w/0.5
or, 24.7+(0.0051)w^2= 70.56-0.98w
or, 0.0051w^2 + 0.98w - 45.86=0
Now for a quadratic equation ,
where , a = 0.0051
b = 0.98
c = -45.86
by putting the above value we get , w= -80.62 . w= +38.92
by eleminating the negative value we get EV steady state speed w = 38.92 rad/sec
ANS3:
Objective: Develop a mathematical model of a DC Motor for the below equation using Simulink.
ω = V/Kϕ -Ra/Kϕ^2 .T
Simulation Blocks used:
- CONSTANT BLOCK(for assigning Kϕ,Ra,V)
- RAMP BLOCK (for assigning T)
- DIVIDE BLOCK
- SQUARE BLOCK
- ADD BLOCK
- SCOPE
Model Explanation: The model is easy enough to made by basics blocks mentioned above which represents the given equation ω = V/Kϕ - Ra/(Kϕ^2) .T.
- step1: Open Matlab , open simulink, create new blank model , save .
- step2: Using constant block to denote V , Kϕ , and Ra.
- step3: From previous questoin we assumed the values are 50.5, 0.7 , 0.5 respectively.
- step4: Run model.
Simulink Model:
Result:
ANS4:
Induction Versus DC brushless motors by Wally Rippel, Tesla
- There is no such thing called the best engine. According to the personal requirements, we need to adapt the engine type.
- There are 3 most common car battery types in India including Lead Acid Battery, Lithium-ion Battery, Nickel Metal Hydride Battery but LI-ion battery will suppress all other types of batteries.
- DC brush motors are replaced by DC Brushless or Induction Motors.
- Contactor Controllers are replaced by Modulating Inverters.
- The author discusses the two technologies such as: 1. DC Brushless Drives and 2. Induction Motor with it's feature of operation.
- He says brushless motors are useless motors without the inverter.
- He also mentioned that the 3-phase induction motor has great utility but with several limitations.
- The major limitation being, the Induction Motor can not operate from DC, and AC is a must.
- Further, he compares the future trends which may occur in DC Brushless as well as in Induction motor.
Conclusion:
He concludes that DC brushless drives will likely continue to dominate in the hybrid and coming plug-in hybrid markets, and that induction drives will likely maintain dominance for the high-performance pure electrics.