Week-6 Challenge: EV Drivetrain
EV Drive train
1.Which types of power converters circuits are employed in electric and hybrid electric vehicle?
There are 4 main categories of power converter circuits which are used to electric and hybrid electric vehicles. They are listed and elaborated upon below
AC to DC Converter:
This type of converter takes an AC power supply and converts to a DC one. Power generated from a power plant is transmitted to our home at a voltage of 230v at a frequency of 50HZ which we can use to power our electrical appliances it operated on DC supply, an AC supply has an alternating waveform which repeatedly reverse direction. An AC to DC converter will prevent the current changing direction, so that it only flows unidirectional in the circuit of AC to DC converter, there is a step down transformer, a diode, a capacitor and low drop output voltage (LDO) regulator.
Example: Phone charger, Laptop charger etc.
DC to AC converter:
This type of converter takes a DC supply and converts it to one. Power which is from a battery or capacitor can have its from changed to AC type for use in homes or systems which have an AC load. The power generated by solar panels is stored in batteries in the DC form and this can power a house which is uses an AC power supply that we can utilize this power from the mains. This type of converter is referred to as ‘inverter’ and is widely in used in home and EV/ HEV.
The DC supply is changed to an AC waveform using electronic switch, which is basically a set of semiconductor transistors. These switches are controlled by a controller to either be in a open or closed position to direct current flow in a circuit. The controller sends a signal to switches to open/close even decide for the same.
In case of EV/HEV, the battery pack will supply its power to the inverter. The inverter contains the primary and secondary winding. The current from the battery is first supplied to primary winding where the electronic switches will repeatedly reverse direction of current to create AC waveform.
DC to DC Converter:
This type of converter is similar to transformer that can change the voltage from its current level to another level. It can take an input supply of voltage and increases it or do the exact opposite and decrease it similar to a step up or step down transformer, this kind of converter is called as ‘chopper’. The purpose of this type of converter used for to provide necessary level of power to a particular component and for safety to appliances and the user.
It works by taking DC supply and passing through a switching element which converts the waveform to a square wave which is actually AC in nature; this AC is passes through a filter which will turn it back into a DC supply for use in a load. In EV/HEV and conventional vehicles, this type of converter is used for various purposes. E.g. in case of stepping down the voltage, the car radio may require a step-down voltage of 12v to operate and would not need the full voltage of car battery. In case of stepping up voltage, the motor of car may require 500v to operate, but the vehicle may need more voltage than battery.
AC to AC converter:
This type of inverter takes as input AC waveform with a particular voltage and frequency and outputs a different AC waveform with another voltage and frequency. The output voltage/frequency can be changed or maintained depending on the application, this converter has many applications in our daily life. E.g. lamp dimming, motor speed control. These are mainly 4 types of AC to AC converters:
Cycloconverter
This type is also known as frequency changer as it will change the frequency of AC input waveform and output of AC waveform with another frequency but it can also change the magnitude of the power. This type of converter doesn’t have any DC linkages and avoids any conversion between AC and DC to prevent losses and save costs. The input waves frequency is changed by changing the firing angle applied to thyristor, by switching the positive and negative limb thyristors, we can get an output waveform that has a steeped up/ down frequency compared to the input.
AC to AC converter with DC link:
As the name suggest, this type includes a DC link along with a reflector and an inverter. The input AC waveform is first rectified by the reflector to remove the negative portions of the AC wave an output only the positive portions. It performs DC waveforms which is stored in DC link and then converted back to an AC waveform using the inverter.
Matrix Converter:
It converts from AC to AC directly without using DC linkage to reduce cost and losses that would otherwise be incurred with a DC link storage element. It is used mainly to increase reliability and efficiency of system. Matrix converter bi-directional switches to conduct current and block voltages of both polarities.
Hybrid Matrix Converter:
This type is very similar to matrix converter in that they combine the AC/DC/AC design with matrix design. It uses uni-directional switches and 2 converter stages, without the DC link to reduce the size and weight of the converter.
- The EV power train with 72v battery pack below diagram? The duty ratio for acceleration operation ‘d1’, and breaking operation ‘d2’ the other parameters of electric vehicle is given below?
M= 1000bkg, cd=0.2, A= 2m^2, co= 0.009, c1 = 1, = 1.1614kg/m^3,
Radius of wheel = 11 inches.
Motor and controller parameters:
Rated Armature voltage = 72v
Rated Armature Current = 400A
Ra = 0.5ohm, ko= 0.7, volt/sec
Chopper switching frequency = 400hz
The vehicle speed-torque characteristics are given below:
TV = 24.7 + (0.0051) ^2
Firstly, using the given information, we can determine the rolling resistance and drag forces,
The rolling resistance formula given by Frr = urr.mg
Where urr is coefficient of rolling resistance and ‘mg’ is weight
The drag force formula given by Fd = ½.cd.A.^2
Where, cd coefficient of drag
is air density in kg/m^3
A is frontal crossectional area in m^2
V is velocity m/s
Using these 2 formulas the rolling resistances and drag forces were found to be Frr = urr.mg
Frr = 88.29
Drag Force: Fd = ½.cd.A.v^2
= ½.0.2*1.1614kg/m^3*2m^2*(0.7)^2
= ½*0.2*1.1614*2*1.4
Fd = 0.23228^2
Since, we don’t know the velocity, we can only write the expression for drag force in terms of velocity
We can equate equation for Tractive force using summation of forces on the right hand side, and motor torque, gear ratio and wheel radius on left hand side
Fte = T.G/r
We know that summation of forces, in given data, gear ratio is 1,and wheel radius 11 inches, which is equal to 0.2794m……
T*1 0.2794 = Fd +Frr
= 0.23228^2+ 88.29
We used wheel radius for LHS, we also know that the velocity can be written as a product of angular speed and radius
v= r*w
T.1/0.2794 = 0.23228. (0.2794.^2)+88.29
The expression for torque T,
T = 0.0051^2+ 24.67
Next, we know that torque-speed relation for a motor. We are provided the armature resistance, voltage and kphi values, we know that duty cycle is 70%, so the average voltage is given by product of battery voltage and duty cycle
vo = v.dc
o = 72* 0.7
The average voltage = 50.4
We know that torque-speed relation is given by
Tm = vo. - `vo.(ko)/Ra - w(k.o)^2/Ra`
Tm = `50.4*(0.7)/0.5-w(0.7)^2/Ra`
= – `35.28/0.5-w(1.4)/Ra`
Tm = `70.56-0.98w`
Now, we have 2 expressions for torque that are both in terms of angular speed the first expression is torque of vehicle, and second one is motor.
Equating 2 expressions, we will get a quadratic equation which can be solved in angular speed.
70.56-0.98 = 0.0051 ^2+24.67
We can solve this equation in quadratic form
X= -b+b^2-4ac
Entering all values in to formula
X = `0.98+(0.98^2-4(0.0051)(-45.89)/2*0.0051
X = -0.98`+1.377/0.0102`
Two expressions for angular speed
(or)
`w1= 38.92rad/sec (or) w2= -231.08rad/sec`
- Refer to the blog on below topic:
Introduction verses DC brushes motors by Wally ripple, Tesla
Explain in brief about author’s perspective
Answer: The Author writes this blog to entire the difference between two most commonly used motor drives in the automobiles industry. The DC brushless motor drive and the induction motor drive. Ripple goes in to detail about the difference in construction, working, and cost between the two drive types. However, with that written not arrived at the conclusion that one type is better than other. In the last lines of the opening paragraph, ripple writes that “there is no one best engine type” and that it is a matter of For personal preference when it comes to choosing an engine. It is an same regard that ends blog starting there is ;no winner’ when it comes to motor drive selection, in spite of both motor drive types having similarities , they also have their benefits and drawbacks.
DC brushless drives, we are told that their construction implements 2 or more permanent magnet at the rotor to generate a DC magnetic field. It also consists of a stator core (made up of thin, stacked laminations) it also uses an inverter for the current control, it means that cannot run a DC power supply as the current direction has to be changed repeatedly in to maintain a constant torque. For, induction drives, there rotor has no magnets, but instead stacked steel laminations with buried peripheral conductors which form a’ shorted structure’. It is the current in the stator windings that generates a rotating magnetic field that interacts with the rotor. Induction motor can be used directly with power from the mains and doesn’t require an inverter to start the drive and generate the starting torque. It has an ability to start under the mains load applied. However DC brushless drives cannot do the same. They cannot generate a starting torque when connected to a mains power supply. They require an inverter.
Induction motor drives cannot run on a DC power supply and are solely dependent on a AC power supply. Also, when they are connected to a 3 phase main supply, the rotor rotates at a constant speed and also they have a limited starting torque as well as peak torque. In order to rectify this, an inverter can be used to operate the induction drive from a DC power supply or a battery also, if the inverter frequency is varied, then the constant speed is no longer an issue it can be varied.
DC brushless drives use permanent magnets, and cannot change the strength of their magnetic fields it would be desirable if they could, because when the maximum torque is needed by the rotor, the magnetic field strength, should also be at a maximum level. For the inverter and motor currents to be at minimum, thereby reducing the power losses by heat dissipation. Similarly when the torque required is minimum, the magnetic field strength should also be minimum to reduce hysteresis and eddy current losses. The permanent magnet used are also quite expensive costing %50 per kilogram and their magnetic fields are difficult to control if ferromagnetic material gets close enough to the field. The induction drive do not have permanent magnets and so they can vary their magnetic field strength. If used with a smart inverter, they can be operated with minimum hysteresis and eddy current losses and maximum efficiency as the magnetic field strength can be changed according to requirement by the rotor. These losses do not increase if the size of the induction drive also increase but as a brushless drive gets bigger, it losses increase and their efficiency decreases. So induction drives have a cost advantage compared to DC brushless drives gets bigger, its losses increase and efficiency is decreases. Although DC brushless drives generate less heat at the rotors. So the rotors more easily and have higher point efficiency than induction drives. In addition, the DC brush less motor can operate at unity power factor 1, where the induction drives can operates to a power factor.