Week-6 Challenge: EV Drivetrain
1. Which types of power converter circuits are employed in electric and hybrid electric vehicle?
There are 4 main categories of power converter circuits which are used in clectric and hybrid vechiels.
a)
AC to DC Converter
Electric power is transported on wires either as a direct current (DC) flowing in one direction at a non-oscillating constant voltage, or as an alternating current (AC) flowing backwards and forwards due to an oscillating voltage. AC is the dominant method of transporting power because it offers several advantages over DC, including lower distribution costs and simple way of converting between voltage levels thanks to the invention of the transformer. AC power that is sent at high voltage over long distances and then converted down to a lower voltage is a more efficient and safer source of power in homes. Depending on the location, high voltage can range from 4kV (kilo-volts) up to 765kV. As a reminder, AC mains in homes range from 110V to 250V, depending on which part of the world you live in.
Converters steer an alternating current, as its voltage also alternates, into reactive impedance elements, such as inductors (L) and capacitors (C), where it is stored and integrated. This process separates the power associated with the positive and negative potentials. Filters are used to smooth out the energy stored, resulting in creation of a DC source for other circuits. This circuit can take many forms but always comprises of the same essential elements, and may have one or more stages of conversion. The converter depicted in figure 1 is called a ‘forward converter”, which is a higher efficiency than a slightly simpler architecture; a ‘flyback converter’.
The simplest AC/DC converters comprise of a transformer following the input filtering, which then passes onto a rectifier to produce DC. In this case, rectification occurs after the transformer because transformers do not pass DC. However, many AC/DC converters use more sophisticated, multi-stage conversion topologies as depicted in figure 1 due to advantages of smaller transformer requirements and lower noise referred back to the mains power supply.
b) DC TO AC convertor (inverter)
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. In a solar power system, for example, the power stored by batteries charged by solar panels is converted to standard AC power by the inverter, which provides the power to plug-in outlets and other standard 120-volt devices.
An inverter serves the same kind of function in a hybrid or EV car, and the theory of operation is relatively simple. DC power, from a hybrid battery, for example, is fed to the primary winding in a transformer within the inverter housing. Through an electronic switch (generally a set of semiconductor transistors), the direction of the flow of current is continuously and regularly flip-flopped (the electrical charge travels into the primary winding, then abruptly reverses and flows back out). The in/outflow of electricity produces AC current in the transformer's secondary winding circuit. Ultimately, this induced alternating current electricity provides power for an AC load—for example, an electric vehicle's (EV) electric traction motor.
A rectifier is a similar device to an inverter except that it does the opposite, converting AC power to DC power.
c) DC TO DC conveter
A DC-to-DC converter is an elctronic circuit or electromechanical device that converts a source of dircet current (DC) from one voltage. level to another. It is a type of electric power converter. Power levels range from very low (small batteries) to very high (high-voltage power transmission).
Practical electronic converters use switching techniques. Switched-mode DC-to-DC converters convert one DC voltage level to another, which may be higher or lower, by storing the input energy temporarily and then releasing that energy to the output at a different voltage. The storage may be in either magnetic field storage components (inductors, transformers) or electric field storage components (capacitors). This conversion method can increase or decrease voltage. Switching conversion is often more power-efficient (typical efficiency is 75% to 98%) than linear voltage regulation, which dissipates unwanted power as heat. Fast semiconductor device rise and fall times are required for efficiency; however, these fast transitions combine with layout parasitic effects to make circuit design challenging.The higher efficiency of a switched-mode converter reduces the heatsinking needed, and increases battery endurance of portable equipment. Efficiency has improved since the late 1980s due to the use of power FETs, which are able to switch more efficiently with lower switching losses at higher frequencies than power bipolar tansistors , and use less complex drive circuitry.
Most DC-to-DC converters are designed to move power in only one direction, from dedicated input to output. However, all switching regulator topologies can be made bidirectional and able to move power in either direction by replacing all diodes with independently controlled active rectification. A bidirectional converter is useful, for example, in applications requiring regerative braking of vehicles, where power is supplied to the wheels while driving, but supplied by the wheels when braking.
Although they require few components, switching converters are electronically complex. Like all high-frequency circuits, their components must be carefully specified and physically arranged to achieve stable operation and to keep switching noise at acceptable levels. Their cost is higher than linear regulators in voltage-dropping applications, but their cost has been decreasing with advances in chip design.
d). AC to AC converters
AC to AC converters is used for converting the AC waveforms with one particular frequency and magnitude to AC waveform with another frequency at another magnitude. This conversion is mainly required in case of speed controlling of machines, for low frequency and variable voltage magnitude applications as well. We know that there are different types of loads that work with different types of power supplies like single-phase, three-phase supply, and the supplies can be differentiated based on the voltage and frequency range also.
Types of AC to AC aonverters
The AC to AC converters can be classified into different types:
- Cycl converters
- AC to AC Converters with DC link
- Matrix Converters
- Hybrid Matrix Converters
1. Cycloconverters
cycloconverters are majorly called as frequency changers that convert the AC power with one input frequency to AC power with a different output frequency and can be used for changing the magnitude of the AC power also. Cycloconverters are preferred for avoiding DC links and to avoid many stages like AC to DC to AC which is not economical and causes more losses. The cost of the DC link required will vary according to the ratings of the supply power being used.
2. AC to AC Converters with a DC Link
AC to AC converters with a DC link generally consists of a rectifier, DC link, and inverter as in this process the AC is converted into DC by using the rectifier. After being converted into DC, the DC link is used to store DC power, and then again it is converted into AC by using the inverter. AC to AC converter circuit with a DC link is shown in the figure.
3. Matrix Converters
Matrix converters are used for converting AC to AC directly without using any DC link for increasing the reliability and efficiency of the system by reducing the cost and losses of the DC-link storage element.
Matrix converter consists of the bidirectional switches that practically don’t exist at present but can be realized by using the IGBTs, and these are capable of conducting current and blocking voltage of both polarities.
4. Hybrid Matrix Converter
The matrix converters that convert AC/DC/AC are termed as hybrid matric converters, and similar to the matrix converters, these hybrid converters also don’t use any capacitor or inductor or DC link.
These are classified into two types based on the number of stages they take for conversion, if the voltage and current both are converted in a single stage, then that converter can be called as a Hybrid Direct Matrix Converter.
If the voltage and current are converted in two different stages, then that converter can be called a Hybrid Indirect Matrix Converter.
2)
An Electric Vehicle's Drivetrain 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 is2'. 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 V-s
Chopper Switching frequency= 400 Hz
The vehicle characteristics are given by the equation
What is EV steady state speed if duty cycle is 70%?
using the above information, we can determine the rolling resistance and drag forces.
thr rolling resistance is given by : Frr=Urr . mg
where Urr is the coefficient of rolling resistance and m is the weight.
the drag force formula is given by: Fd=1/2 . Cd . pA . V^2
where Cd is coefficeient of drag
p is density (kg/m^3)
A is teh frontal cross sectionals area (m^2)
V is velocity (m/s)
using the above formulas , the rolling resistance and drag forces were found to be :
Frr=88.29 and Fd=0.23228 v^2 since velocity is still unknown
next we can find tractive force using the formula
Fte = T.g/r
we know the sum of forces,so we can place that on the RHS.In the given data, gear ratio is 1, and the wheel radius is 11 inches which is equal to 0.02794m. we do not know the torque so we will write the LHS in terms on T.
`T . 1/0.2794 =0.23228 v^2 + 88.29`
we know that
v=r.w
using this `(T.1)/0.2794=0.23228*(0.2794^2.omega^2)+88.29`
simplifying we get
`T=0.0051omega^2+24.67`
next using the torque speed relation for a motor.we have been provided with aramture resistance, voltage and kphi values.We also know the duty cycle is 70%.
so the average voltage is give by
Vo = V . Dc
Vo = 72 . (0.7)
the average voltage is 50.4v
we know the torque-speed relation is goven by :
`T_m=(V_0.(k.phi))/R_a-(omega(k.phi)^2)/R_a`
`T_m=((50.4).(0.7))/0.5-omega.(0.7)^2/R_a`
simplifying, we will get this expression
Tm = 70.56 - 0.98w lets say this is equation 2
now equating the 2 equations we get
70.56-0.98w = 0.0051w^2 + 24.67
0.0051w^2 + 0.98w - 45.89 = 0
we can solve this quadratic equation using the quadratic formula:
`x=(-b+-sqrt(b^2)-4ac)/(2a)`
`x=(-0.98+-sqrt(0.98^2)-4.(0.0051).(-45.89))/(2(0.0051)`
simplifying
`x=(-0.98+-1.377)/0.0102`
so using the 2 soultions for x(angular speed)
w=38.92 rad/s or w = -231.08 rad/s
assuming the forward dircetion is positive and the vehicle is moving in a steady speed in forward direction, we will take teh first soultiona dn discaard the second solution.
3)
Refer to the blog on below topic:
Induction Versus DC brushless motors by Wally Rippel, Tesla
Explain in brief about author’s perspective.
in this blog the author mainly talks about the differnence between two most commonly used motor drive, he goes in detail about the construction,workign and cost between the two types.even after doin such extensive reserach he is yet to conclude which is the better motor drive, he says There is no one best engine type, rather there are different types of engines to suit personal requirements, such as price and performance.
the author next talks in detail about the two drives first DC brushless drive adn then talks about induction motor drives, he says With brushless machines, the rotor includes two or more permanent magnets that generate a DC magnetic field. In turn, this magnetic field enters the stator core flowing within windings to produce a torque interaction between the rotor and stator.The device that provides this current control is called an inverter.the 3-phase induction motor and the DC brushless motor are virtually identical. Both have three sets of “distributed windings” that are inserted within the stator core. The essential difference between the two machines is with the rotor.
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 turn, the frequency of this magnetic field as “seen” by the rotor is equal to the difference between the applied electrical frequency and the rotational “frequency” of the rotor itself. Accordingly, an induced voltage exists across the shorted structure that is proportionate to this speed difference between the rotor and electrical frequency.
In contrast, induction machines have no magnets and B fields are “adjustable,” since B is proportionate to V/f (voltage to frequency). This means that at light loads the inverter can reduce voltage such that magnetic losses are reduced and efficiency is maximized. Thus, the induction machine when operated with a smart inverter has an advantage over a DC brushless machine – magnetic and conduction losses can be traded such that efficiency is optimized. This advantage becomes increasingly important as performance is increased. With DC brushless, as machine size grows, the magnetic losses increase proportionately and part load efficiency drops. With induction, as machine size grows, losses do not necessarily grow.
is the end the author ripple concludes and elabroates the differnet asspect to compare the induction and DC brushless motor drives and has detailed advantages and disadvantages of both types.as mentioned earlier,at the end of the day it depends on the preference of the user .if a user is seeking high performance with comparetively lesser costs, then the induction drive would be better suited.however, if it high peak point efficiency , high power factor and easier control that a user seeks, then DC brushless drive would be the smarter choice.Both types have their palce in teh automobile industry and will retain their relevance in their respective applications.ripple belives that due to teh increasing performance of vehicles m one day both may be used side by side under the hood of the vehicle in order to ge the best of both sides.
the authors final conclusion is 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.