Week-6 Challenge: EV Drivetrain
Question1:
Which types of power converter circuits are employed in Electric and Hybrid electric vehicles?
- AC to DC converter,
- DC to AC converter
- DC to DC converter
- AC to AC converter
AC to DC converter
This type of converter takes an AC power supply and converts it to a DC one. Power generated from a power plant is transmitted to our homes at a voltage of 230V at a frequency of 50Hz which we can use to power our electrical appliances. But our applianced operates on a DC supply, not an AC one and so we need a converter for this application. Examples of Ac to DC converters are phone chargers, laptop chargers etc. An AC power supply has an alternating waveform that repeatedly reverses direction. An AC-to-DC converter will prevent the current from changing direction so that it only flows unidirectionally. In the circuit of an AC-to-DC converter, there is a step-down transformer, a diode, a capacitor, and a low dropout voltage (LDO) regulator.
The step-down transformer takes the high-level 230V from the mains supply and steps it down to a lower level e.g. 12 or 13V for our use, but it is still in AC form. Then, a diode or group of diodes is implemented to restrict the current to flow in just one direction. In an AC waveform, there will be positive and negative peaks at each half cycle like a sine wave. The diode(s) will remove the negative parts of the waveform so that there are positive peaks only. This is called \' rectifying\' and this part of the circuit is referred to as a \'rectifier\'. This can be a half wave, full wave or a bridge rectifier. Then, the rectified output voltage waveform is smoothened by a filter capacitor. Finally, the LDO regulator will regulate the output for our use.
In the case of EVs or HEVs, the battery pack which operates with a DC power supply is charged at home or at a charging station both of which are supplied power in AC form from the electric grid. So, the chargers for these vehicles are an example of the AC to DC power converter.
DC to AC converter
This type of converter takes a DC supply and converts it to an AC one. The power that is supplied from a battery or capacitor can have its form changed to the AC type for use in homes or systems which have an AC load. For instance, the power generated by solar panels are stored in batteries in the DC form and this can power a house that uses an AC power supply so that we can utilize this power from the mains. This type of converter is referred to as an \'inverter\' and is widely used in homes and in EV/HEVs.
The DC supply is changed to an AC waveform using an electronic switch, which is basically a set of semiconductor transistors e.g. IGBTs, and MOSFETs. These switches are controlled by a controller to either be in an open or closed position to direct the flow of current in a circuit. The controller sends signals to the switches to tell it to open/close, and even decides the frequency for the same. By doing this operation, the DC current mimics an AC current and can be fed to the load which requires it.
In the case of EVs and HEVs, the battery pack will supply its power to the inverter. The inverter contains a transformer that consists of the 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 back the way it came from. The AC current in the primary winding will induce an AC current in the secondary winding which then supplies this current to an AC load e.g. the AC traction motor.
DC to DC converter
This type of converter is similar to a transformer in that it can change the voltage from its current level to another level. It can take an input supply of voltage and increase it or do the exact opposite and decrease it similar to a step-up or step down transformer. This kind of converter is called a \'chopper\'. The purpose of using this type of converter is to provide the necessary level of power to a particular component and for the safety of the appliance and the user. If a component if suppled with too little or too much voltage, it won\'t operate due to lack of power or it will get damaged, so it all depends on the load requirement.
It works by taking the DC supply and passing it through a switching element which converts the waveform to a square wave which is actually AC in nature. Then this AC is passed through a filter which will turn it back into a DC supply for use in a load. In EVs, HEVs, and conventional vehicles, this type of converter is used for various purposes. E.g. In the case of stepping down the voltage, the car radio may require a stepped-down voltage of 12V to operate and would not need the full voltage of the car battery. In the case of stepping up the voltage, the motor of a car may require 500V to operate, but the vehicle may need more voltage than the battery pack can provide.
There are 2 main types of DC-to-DC converters which are isolated and non-isolated type. The non-isolated type can be subdivided to 5 types:
Buck - to step down the voltage
Boost - to step up the voltage
Buck/Boost - this can do both step-up and step-down operations but is mainly used to reverse polarity
Cuk - similar to the Buck/Boost type
Charge Pump - to step up/step down the voltage for low power sources
The isolated type can be subdivided into 2 types:
Flyback converter - similar to the Buck-Boost converter except it uses a transformer instead of an inductor to store energy
Forward converter - this type will use a transformer to send energy between the input and output in a single step.
The figure illustrates on a typical EV-- usage of converters.
- Non-isolated converters:
The non-isolated converters type is generally used where the voltage needs to be stepped up or down by a relatively small ratio (less than 4:1). And when there is no problem with the output and input having no dielectric isolation. There are five main types of converters in this non-isolated group, usually called the buck, boost, buck-boost, Cuk, and charge-pump converters. The buck converter is used for voltage step-down, while the boost converter is used for voltage step-up. The buck-boost and Cuk converters can be used for either step-down or step-up. The charge-pump converter is used for either voltage step-up or voltage inversion, but only in relatively low-power applications.
- Isolated converters:
Usually, in this type of converter, a high-frequency transformer is used. In applications where the output needs to be completely isolated from the input, an isolated converter is necessary. There are many types of converters in this group such as Half-Bridge, Full-Bridge, Fly-back, Forward and Push-Pull DC/DC converters. All of these converters can be used as bi-directional converters and the ratio of stepping down or stepping up the voltage is high.
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AC to AC converter:
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, and three phase supplies,and the supplies can be differentiated based on the voltage and frequency range also..
We require a particular voltage and particular frequency for operating some special devices or machines ..for speed control of induction motors,, AC to AC converters is used majorly for obtaining a desired AC power supply from the actual power supply ,we need some converters called AC to AC converters
2. An Electric Vehicle's powertrain with 72V battery pack in shown in the diagram below. The duty ratio for the 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:
Comparing the given Speed torque characteristics with the general derived result which is: T = V*k/R - (k^2)*w/R
// T = Armature torque, V = Armature voltage, R = Armature resistance, w = Speed in rad/sec //
Thus, 24.7 + (0.0051)w^2 = V*k/R - (k^2)*w/R
Substituting the given values of voltage, resistance, constant in above equation. Here voltage will be 0.7*72 as we have to consider 70% duty cycle.
24.7 + (0.0051)w^2 = 72*0.7*0.7/0.5 - (0.7*0.7)/0.5*w
0.0051w^2 + 0.98w - 45.86 = 0
w^2 + 192.15w - 8992.15 = 0
Solving this quadratic equation as ax^2 + bx + c = 0 with solutions as
Therefore, from the above equation, two values of speed is obtain w1 = 38.91 rad/sec and w2 = -231.07 rad/sec
As the value of speed cannot be in negative 38.91 is the correct solution.
So the steady state speed of EV with duty cycle 70% is 38.91 rad/sec
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- Develop a mathematical model of a DC Motor for the below equation using Simulink.
ω = V/K -Ra/K^2 *T
Assuming values of parameters such as V, Kɸ, Ra and Ta
V = 50.4, Kɸ = 0.7, Ra = 0.5, T = ramp signal
All these values are considered from the above-solved problem and model is prepared below with its result.
Model:
Result:
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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:
In the above topic, the author Walley Rippel initially described how the substitution of DC brushless motor and Induction motor in the future can also be possible as evolution is going on as from his past experience nothing had stayed permanent. He has given examples of Lead batteries replaced by Lithium, and DC motors replaced by DC brushless or Induction motors.
Further discussing each motor in detail starting with DC brushless motor he said:
DC brushless machine has one or more permanent magnets in the rotor structure, which generates DC magnetic field and this field is linked with the currents carrying windings in the stator body. To get output torque must be constant which is taken care by continuously varying the magnitude and polarity of the stator current.
Constant torque brings out the application for DC brushless motors and this control is able to achieve by the help of an inverter.
Induction motor:
The construction of the rotor is different from than DC brushless motor as this motor does not include permanent magnets, it has stacked steel laminations with shorted structure. Although the stator has the same three sets of the distributed winding present.
In this motor, AC supply has given to the stator which creates a rotating magnetic field and is linked to rotor to produce torque.
Induced voltage exists across the shorted structure that is proportionate to the speed difference between the rotor and electrical frequency.
No inverter is required for the Induction machine, unlike DC brushless machine which has brought much worth to Tesla.
Comparison:
A few comparisons are already stated above regarding construction besides that the Induction motor can run only on AC power, and shaft speed is proportionate to line frequency hence Induction machine operates at a constant speed.
An induction motor has the ability to start under no load, also compatible with conventional utility power. Whereas DC machine requires Inverter
Induction machines have limited starting and running torque capabilities compared to DC machines.
By adding an inverter to an Induction machine can eliminate the two drawbacks of it which are constant speed characteristics and can power up from DC supply or battery but still torque performance is low although with these it can contribute to applications in Electric vehicles.
Both of these machines use 3-phase modulating inverters, controls of inverter code is different.
The major difference is that heat generated in DC brushless machines is less, cooling of the rotor is quick and peak point efficiency is higher.
Induction machine operates at 85% of the power factor and cannot operate at unity power factor, unlike DC machines. This means peak point efficiency is high in DC brushless machines compared to Induction.
In ideal brushless machines, magnetic fields produced by permanent magnets are adjustable, by keeping the magnetic field at the maximum inverter, and rotor currents are maintained at the lowest possible value. Ideally, the magnetic field is adjusted such that eddy, hysteresis, and I^2R losses are minimized. But there is no easy way of changing B value with permanent magnets.
In Induction machines, magnetic fields are adjustable as the B value is proportional to V/f. This improves efficiency.
Average efficiency can be said better with Induction machines when the load is more, as for DC machines with an increase in load size of machine growth and losses are also increased which is not happening much compared to Induction.
Induction machines also have a cost advantage over PM rotors as well as higher rating drives. But achieving stability over torque speed curve and temperature is much more difficult with complex control laws.
Conclusion:
In this week's challenge, we concluded about 4 topics
First, we have seen the types of converters used in HEV and EV and also learned very basic about all the converters.
We have calculated the steady state speed of EVs having some duty cycle.
Derived a mathematical model of a DC motor by assuming values.
At last, explained in brief the article written by author Walley Ripple in which comparison of DC brushless and Induction machine has made.
The worked-out model is attached for your perusal.