Week-6 Challenge: EV Drivetrain

SOLUTION 1.

THERE ARE FOUR MAIN CATEGORIES OF POWER CONVERTER CIRCUITS ARE EMPLOYED IN ELECTRIC & HYBRID ELECTRIC VEHICLES ARE AS FOLLOWS:-

AC TO DC CONVERTER:-

AC to DC Converters is one of the most important elements in power electronics. The electrical circuits that transform alternating current (AC) input into direct current (DC) output are known as AC-DC converters. They are used in power electronic applications where the power input is a 50 Hz or 60 Hz sine-wave AC voltage that requires power conversion for a DC output.

The process of conversion of AC current to DC current is known as rectification. The rectifier converts the AC supply into the DC supply at the load end connection. Similarly, transformers are normally used to adjust the AC source to reduce the voltage level to have a better operation range for DC supply.

Example of AC-DC converter is phone charger and laptop charger etc. In the case of EVs or HEV's the battery pack that 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 charger for these vehicles are an Example of AC to DC power converters.

DC TO AC CONVERTER:-

This type of converter takes a DC supply and converts it to an AC one. The power which 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 is stored in the batteries in the DC form and this can power a house which 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 & in EV/HEV's.

The DC supply is changed to an AC waveform using an electronic switch, which is basically a set of semiconductor transistors,eg IGBT's, MOSFET's. These switches are controlled by the 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 decide 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 EV's and HEV's the battery pack will supply its power to the inverter.The inverter contains a transformer that consists of primary and secondary windings.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. 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 eg. the AC traction motor.

DC TO DC CONVERTER:-

This type of converter is similar to the transformer in which 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 the step-up or step-down transformer. This kind of converter is called "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 is supplied with too little or too much voltage, it won't operate due to lack of power or it will damage. 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 EV's and HEV's and conventional types of vehicles this type of converter is used for various purposes.Eg in the case of stepping down the voltage as the car radio may require low voltage up to 12V to operate and it will not need the full voltage of the car battery. In the case of stepping up the voltage, the motor of the car may require 500V to operate but the vehicle may need more voltage than the battery pack can provide.

THERE ARE TWO MAIN TYPES OF DC-DC CONVERTERS WHICH ARE

1)ISOLATED TYPE

2)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 buckBoost 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 & output in a single step.

AC TO AC CONVERTER:-

This type of converter takes an input AC waveform with a particular voltage & frequency and output a different AC waveform with another voltage & frequency. The output voltage and/or frequency can be changed or maintained depending on the application. This converter has many applications in our daily life. Eg lamp dimming, motor speed control.

• CYCLOCONVERTER:-

This type is also known as a frequency changer as it will change the frequency of an input AC waveform and output an AC waveform with another frequency but it can also change the magnitude of the power.

SOLUTION 2.

PARAMETERS GIVEN IN QUESTION AS INPUT:-

Battery packed voltage (V)=72V

Rated Armature Current=400A

Armature Resistance (Ra)=0.5

Motor Constant (K)=0.7

Chopper Switching Frequency=400Hz

Duty cycle=70%

SOLUTION:-

Vehicle Output voltage(V)=Battery voltage(V)*Duty cycle

=72*0.70

=50.4V

So, vehicle voltage is 50.4V

Now equation for vehicle torque is:-

Tv=Vn.(Kϕ/Ra)-((kϕ)^2/Ra).ω-------------(1)

where,

Tv= Vehicle Torque(N-m)

Vn=Vehicle Voltage(Volts)

ω=steady state speed of vehicle(rps)

Kϕ=Motor constant

Ra=Armature Resistance

Substituting all the values in equation (1), we get

Tv=((50.4*0.7)/0.5)-(((0.7)^2)/0.5)*ω

Tv=(70.56)-(0.98ω^2)------------(2)

Now, the vehicle speed torque characteristics is given as,

Tv=24.7-0.0051ω^2---------------(3)

At steady state,

Eqn(2)=Eqn(3)---------------------(4)

24.7-0.0051ω^2=70.56-0.98ω^2

0.0051ω^2+0.98ω-45.85=0-------(5)

To solve the above Eqn we have to apply the Quadratic Equation Formula

x=(-b±√((b^2)-(4ac)))/2a

x=-0.98±√((0.98^2)-(4*0.0051*-45.85)))/2(0.0051)

ω=38.92 rad/sec

So, the steady-state speed of the vehicle is 38.92 rad/sec.

SOLUTION 3.

Let us draw a model for the above equation

let V=50.4Volts

kϕ=0.7

Ra=0.5ohm

and also let's consider torque as a linearly increasing signal.

MODEL:-

RESULT:-

SOLUTION 4.

The author writes this blog to show the difference between the two most commonly used motor drives in the automobile industry:

1)DC Brushless Motor Drive

2)Induction Motor Drive

Rippel goes into detail about the difference in construction, working & cost between the two drive types. However, with what he has written, he has not arrived at any conclusion that one type is superior to the other type.

At the End, Rippel writes that"there is no one best engine type" and that it is all a matter of personal preference when it comes to choosing an engine.

In spite of both motor drives types having similarities, they also have their benefits and drawbacks.

For DC Brushless drives, we are told that their construction implements two or more permanent magnets 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, meaning that it cannot run on a DC power supply as the current direction has to be changed repeatedly to maintain a constant torque.

For induction drives, their 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.

An induction motor can be used directly with power from the mains and does not require an inverter to start the drive and generate the starting torque. It has the ability to start under the main load applied. However, DC brushless drives cannot do the same. They cannot generate the starting torque when connected to the main power supply. They require an inverter to do this.

Induction motor drivers cannot run on a DC power supply and are solely dependent on an AC power supply and are totally dependent on AC power supply. Also when they are connected to a 3ϕ 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 drivers 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. This allows for the inverter and motor current to be at a minimum thereby reducing the power loss by heat dissipation. Similarly when the torque required is minimum the magnetic field strength should also be minimized to reduce hysteresis & eddy current losses. The permanent magnet used are also quite expensive.

On the other hand, induction drives 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 the requirement of the rotor. These losses do not increase if size of the induction drive also increases but as a DC brushless Drive gets bigger, its losses increase and its efficiency decreases.

So Induction drives have a cost advantage compared to DC brushless drives. Although, DC Brushless drives generate less heat at the rotors. So rotor more easily and have a higher peak point efficiency than induction drives.In addition, the DC brushless motor can operate at unity power factor i.e 1 whereas induction drives can operate upto a power factor of approximately 85%.

So,At the end, if a user is seeking high performance with comparatively lesser cost, then the induction drive would be better suited for those requirements. However, if it is high peak point efficiency, high power factor and easier control that a user seeks, then the DC brushless drive would be the smarter choice.

Both types of motors have their place in the automobile industry and will retain their relevance in their respective applications.