Week-6 Challenge: EV Drivetrain

Q=1

EXLINATION

• 1. Power converter circuits :

  This technology deals with processing and controlling the flow of electrical energy in order to supply voltages and currents in a form that is optimally suited for the instruments.

As shown in the above block diagram, it consists of an electrical energy source, a power electronic circuit, a control circuit and an electric load. This convertor changes one form of electrical energy to other forms of electrical energy. The power electronic circuit performs both the power part and the control part. The power part transfers the energy from source to load and it consists of power electronic switches, transformers, electric choke, capacitors, fuses and sometimes resistors. The control circuit regulates the elements in the power part of the converter. This block is built with a complex low power electronic that consists of either digital or analog circuit assembly.

Depending upon the functions performed, the following are the 4 different classifications

a) AC to DC converter which is called "Rectifier" - It converts AC into Unipolar DC current.

b) DC to AC converter which is called "Inverter" - It converts DC into AC of desired voltage and frequency.

c) DC to DC converter which is called "Chopper" - It converts High voltage DC to low voltage DC (Step down) or Low voltage DC to High voltage DC (Step up)

d) AC to AC converter which is called "Cycloconverter" - It converts the AC of the desired frequency and desired voltage magnitude from the AC supply.

Except for the AC to AC converter the other 3 are predominantly used in both the HEV and EV.

AC to DC converter (Rectifier) this used for charging the traction battery. Because the source of current is AC and the voltage stored in the traction Battery is DC.

The DC current from the Battery is then transmitted to 2 different converters, DC-AC converter (Inverter) and DC-DC converter (Chopper).

The inverter is usually found between Battery and the DC induction motor, this motor is directly connected to the wheels and is responsible for the traction. Incase of Regenerative braking, this motor functions as a generator by converting the mechanical energy (from the wheels & axles) to electrical energy which is then stored in the traction battery.

DC-DC converter takes the DC source from the traction battery. The output from this DC-DC converter is used to power all the other components of the EV/HEV like HMI, Headlamps, AC, etc.

Q=2

To calculate:

EV steady state speed if duty cycle is 70%.

Given data:

Rated Armature voltage = 72V

Rated armature current = 400A.

RaRa = 0.5ΩΩ

KΦKΦ = 0.7 Volt second.

Chopper switching frequency = 400 Hz

Solution:

Rolling Resistace,Frr=μrr⋅m⋅gFrr=μrr⋅m⋅g.

Drag Force,Fad=12⋅ρ⋅A⋅v2⋅CdFad=12⋅ρ⋅A⋅v2⋅Cd

Tractive Effort,Fte=Frr+FadFte=Frr+Fad

Let,

μrr=0.009μrr=0.009

Cd=0.2Cd=0.2

m=1000kgm=1000kg

ρ=1.1614kg/m3ρ=1.1614kg/m3

A=2m2A=2m2

r=0.28mr=0.28m

Frr=0.009∗1000∗9.81Frr=0.009∗1000∗9.81 = 88.29 N

Fad=12∗1.1614∗2∗v2∗0.2Fad=12∗1.1614∗2∗v2∗0.2.

Fad=0.01821∗ω2Fad=0.01821∗ω2

Fte=88.29+0.01821∗ω2Fte=88.29+0.01821∗ω2

Also ,we know that

Fte=G.TrFte=G.Tr,where G = 1

T0.28=88.29+0.01821∗ω2T0.28=88.29+0.01821∗ω2

Tv=24.7+0.0051ω2Tv=24.7+0.0051ω2

This is the torque speed characterstics of the vehicle.

Now the angular velocity ωω of DC motor i given by

ω=VkΦ−T⋅RkΦ2ω=VkΦ-T⋅RkΦ2

So, T=V⋅kΦR−ω⋅kΦ2RT=V⋅kΦR-ω⋅kΦ2R

Now putting all the values the above equation

Since duty cycle is 70%,V = 72*0.7 = 50.4V

T=50.4∗0.70.5−ω∗0.720.5T=50.4∗0.70.5-ω∗0.720.5

Tm=70.56−0.98ωTm=70.56-0.98ω

At steady state,

Tv=TmTv=Tm

On equating both the sides we get,

ω=38.90rad/s

EV staedy state speed if duty cycle is 70% is 38.90 rad/s

MATLAB program for speed torque characterstics.

>> % given data
V = 72; % rated armature voltage
I = 400; % rated armature current
R = 0.5; % armature resistance
K_phi = 0.7; % constant factor
f = 400; % chopper switching frequency

omega = linspace(0,100,50);
% Speed-Torque characterstics equation
T_v = 24.7 + 0.0051*omega.^2;
V = 72*0.7;
T_m = 70.56 - 0.98*omega;
omega = (V/K_phi)-((R*T_m)/K_phi);

% plotting the result
plot(omega,T_v)
hold on
plot(omega,T_m)
axis([30 80 0 80])
xlabel('Speed(rad/s)')
ylabel('Torque(N-m)')
>> run

Results

Q=3

Let V = 50.4 Volts

KϕKϕ= 0.7

RaRa= 0.5Ω0.5Ω

and also let consider torque as a linearly increasing signal.

Model:-

RESULTS:>

Q=4

Wally Ripple is a long-time proponent of electric vehicles. Prior to joining Tesla Motors, he was an engineer at AeroVironment, where he helped develop the EV1 for General Motors and was featured in the documentary movie, Who Killed the Electric Car? Wally has also worked for the Jet Propulsion Laboratory on electric vehicle battery research, among other projects. In 1968, as a Caltech undergraduate student, he built an electric car (a converted 1958 Volkswagen microbus) and won the Great Transcontinental Electric Car Race against MIT.

In the beginning, the author describes the dominance of Internal Combustion Engines(ICE) in the world of automobile industry. He explains that all engines were similar and it was up to the user to select according to their requirement. Next, the author highlights topics related to EV technology especially related to DC Brushless Motor and Induction Motor.

According to him, DC Brush Motors is replaced by either DC Brushless Motor or Induction Motor. Also, the Lead Acid Battery is replaced by lithium-ion batteries for EVs.He also quoted that there will be the future of both the technology (DC Brushless Motor and Induction Motor) depends upon their demands and uses.

DC Brushless Motor:

With brushless machines, the rotor includes two or more permanent magnets that generate a DC magnetic field (as seen from the vantage point of the rotor). In turn, this magnetic field enters the stator core (a core made up of thin, stacked laminations) and interacts with currents flowing within the windings to produce a torque interaction between the rotor and stator. As the rotor rotates, it is necessary that the magnitude and polarity of the stator currents be continuously varied – and in just the right way - such that the torque remains constant and the conversion of electrical to mechanical energy is optimally efficient. The device that provides this current control is called an inverter. Without it, brushless motors are useless motors.

The author also said that a brushless DC motor produces no starting torque when directly connected to fixed frequency utility power. They really need the aid of an inverter whose “phase” is maintained in step with the angular position of the rotor.

Advantage:-

The advantage of DC Brushless Motor is that there is low heat generation in the rotor and its cooling capacity is better as compared to Induction Motor. Also, peak point efficiency is generally higher for this drive. The DC brushless drive can also operate at unity power factor, whereas the best power factor for the induction drive is about 85%. This means that the peak point energy efficiency for a DC brushless drive will typically be a few percentage points higher than for an induction drive.

Disadvantage:-

As Machine size grows, the magnetic losses increase proportionately and part load efficiency drops. Permanent magnets are expensive and difficult to handle due to very large forces that come into play when anything ferromagnetic gets close to them.

Induction Motor:

The technology behind the induction motor is that it basically uses a 3-phase induction motor which was invented by Nikola Tesla sometime before 1889. Unlike the DC brushless rotor, the induction rotor has no magnets – just stacked steel laminations with buried peripheral conductors that form a “shorted structure.” Currents flowing in the stator windings produce a rotating magnetic field that enters the rotor. In an Induction motor current interacts with the Magnetic field generated by the stator to produce a force which is required for rotor torque.

When a 3-phase induction motor is connected to utility type 3-phase power, torque is produced at the outset. The fact that induction motors are directly compatible with conventional utility power is the main reason for their success.

Advantage:-

The induction motor does not have any magnets and in this the B field is adjustable. The motor has the ability to start under load and there is no need of an inverter. In an induction motor B field is proportionate to V/f (Voltage to frequency). This technology is used to reduce the magnetic losses and conduction losses, due to which efficiency will remain at maximum peak. This all leads to increasing the performance, induction motor is very useful and efficient for high performance.

Disadvantage:-

An induction motor cannot operate from DC Voltage it always requires an AC voltage supply. In an induction motor, the Shaft speed is proportionate to line frequency. Hence, when used with utility power they are constant speed machines. When operated from utility power, they have limited starting torque and somewhat limited running peak torque capabilities, when compared to DC-type machines.

Inference:-

In the end, the author couldn't choose which drive is better than the other but makes it clear that DC Brushless motor drives will likely continue to dominate in the Hybrid and upcoming Plug-in hybrid markets and induction motor drives will likely maintain dominance for the high-performance pure electric.

Conclusion:

• We have discussed and studied the different types of power converter circuits employed in electric and hybrid electric vehicles.
• EV Steady-state speed of an EV Drivetrain was determined with the given input conditions.
• The Mathematical model for DC motor is developed.