Week-6 Challenge: EV Drivetrain
Question 1: Which types of power converter circuits are employed in electric and hybrid electric vehicle?
Solution:
Power converters are used in electric/hybrid vehicles to change the form and magnitude of voltage and current fed from and to the vehicle.
DC/DC converter - This converts one value of DC voltage into another value of DC voltage ( increase/ decrease). The DC/ DC converters used in Electric vehicle for the following purposes:
- Providing low voltage (12V or 14V) for low voltage loads.
- Boosting the voltage level of the battery such that it matches the level of the DC bus voltage for driving the AC motor inverter
- Motor propulsion if the traction motor is a DC motor.
AC/AC converter - Regulated AC output from AC input. The above converters either increase the magnitude of the output voltage ( boost converters) or reduce the output voltage ( buck converter). This converter can convert from a fixed ac input voltage into variable AC output voltage. The output voltage is controlled by varying firing angles of TRIAC (Triode for Alternating currents) it is a bidiractional triode thryster. These types of converters are known as AC voltage regulators.
AC/DC converter(Rectifier) - These convert AC voltage into regulated DC voltage. Rectifier is a device that converts AC power to the DC power used in an electric vehicle motor. It can also increase or decrease the power or torque of the motor by adjusting the amplitude of the signal. Used to get back regen power from vehicle to the battery.
DC/AC converter (Inverter) - These convert DC voltage into regulated AC voltage. Form and magnitude are regulated. Usually emplyed when an AC motor is used to drive the vehicle. In the case of EVs and HEVs, the battery pack will supply its power to the inverter. The inverter contains a transformer which consists of a primary and secondary winding. The current from the battery is first supplied to the primary winding where the electronic switches will repeatedly reverse the direction of the current to create an AC waveform. This current will return the way it came from. The AC in the primary winding will induce an AC in the secondary winding which then supplies this current to an AC load e.g. the AC traction motor.
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Question 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%?
Solution:
Rated Armature voltage (V)= 72 V
Rated armature current= 400 A
Ra= 0.5Ω,
k*phi= 0.7 Volt second
Chopper Switching frequency= 400 Hz
Duty cycle (dc)= 70% = 0.7
The Average Voltage is given by the product of the battery voltage and the duty cycle,
Vs=V*dc
Vs=72*0.7
Vs=50.4 V
The Average voltage is 50.4 V.
The Vehicle Characteristics are given in the form of the equation,
Tv=24.7+(0.0051)*w^2 ................Equation(1)
The derived formula for the motor speed characteristics
T=(Vs*K*phi)/Ra-(((K*phi)^2)*w)/Ra)*************Equaation (2)
By substituting the Values,
T=(50.4*0.7)/0.5-(((0.7^2)*w)/0.5)
T=70.56-0.98*w ..........Equation(3)
Compare the Equations (1) and (3), we get
70.56-0.98*w=24.7+(0.0051)*w^2
0.0051*w^2+0.98*w-45.86=0
By using the Quadratic Equation Formula we get
w=(-b+-(Sqrt(b^2+4*a*c))/2*a
w=(-0.98+-(Sqrt(4*0.0051-(-45.86))/2*0.0051
w=(-0.98+-1.3770)/0.0102
We get two solutions for this angular speed,
w=38.92 rad/sec and w=-231.07 rad/sec
We can consider only the positive value,
Angular speed = 38.92rad/sec.
Speed In RPM= 371.6
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Question 3:
3. Develop a mathematical model of a DC Motor for the below equation using Simulink.
Solution:
Values from the above question are taken as constants vaalue and ramp of torque with constsnat slope is taken to visualize the results
Simulink Model
Equation is designed as per the equation given
Results:
The speed toreque cjaracteristics is shown in the results
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Question 4:
Refer to the blog on below topic:
Induction Versus DC brushless motors by Wally Rippel, Tesla
Explain in brief about author’s perspective.
Solution:
Wally Ripple is a long-time proponent of electric vehicles. before 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.
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.
Conclusion:-
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.
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