Week-6 Challenge: EV Drivetrain

1) Which types of power converter circuits are employed in electric and hybrid electric vehicle ?

A) There are 4 main categories of power converter circuits which are used in 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 it to a DC one.Power generated from a plant is transmitted to our homes ata avoltage of 230 volts at afrequency of 50hz which can use to power our electrical applicances,but our applianced operate on aDC supply,not an AC one and so we need a converter for this application.Examples of AC to DC converters are phone charger,laptop charger etc. An AC powersupply has an alternating waveform which repeatedly reverses direction.An AC to DC converter will prevent to changing the current direction,so that it only flows undirectionally.In the circuit of an AC to DC converter ,there is a step down transformer, a diode,a capacitor anmd a low dropout voltage (LDO) regulator.

The step down transformer tskes the high level 230v from the mains supply and it steps it down to a voltage in to the lowerlevel.eg 12 or 13 v for 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 waveforms there will be positive and negative peaks at each half cycle like a sine wave. The diode will remove the neagtive peaks of the waveform so we can consider only as a positive peak.This is called rectifying. and this part of the circuit is referred to as a rectifier.this can be halfwave,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 use.

In the case of EVS nd HEVs , the battery pack which operates with a DC power supply is charged at a home orat a charging station both of which are supplied power in AC form from the elecrric grid, so the chargers for these vehicles are an examples of the AC to DC power converter

DC to AC converter :

This type of converter takes a DC supply in to AC one. power which is supplies from a battery or capacitor can have its form changed to these switche AC tpe for use for use in home or systems which have an AC load . for instance the power is generated by solar panels are stored in batteries in the form of DC and this can power a house which uses a AC power supply so that we can utilise this power from the mains.this type of converters is referred as an inverter and widely used in homes and in EV/HEVs.

The DC supply is changed to an AC waveform using an electronic switch,which is basicallya set of semiconductor transistor ie IGBTs,MOSFET.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 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 HEVs ,the battery pack will supply its power to the inverter .the inverter contains a transformer ehich consists ofa 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 windingwill induce an AC current in the secondary winding which the supplies this current to an load (eg:AC Traction motor

DC to DC converters:

This type of converter is similar to a transform in that it can change the voltage from its current level to another level.it can take an input supply of voltage and increases 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 toa particular component and for safety of the appliance and the user. If a component is supplied with too little or too much voltage,it wont operated due to lack of power or it will damaged,so it all depends on the load requirement.

Itworksby taking the DC supply and passing it through a switching element which convertsthe waveform to a square wave which is actulally AC in nature.  Then this AC passed through a filter which will turn it back in to a DC supply for use in a load in EV,HEVs and conventional vehicles. This type of converter is used for various purposes,E.g in the case of stepping down voltage12v to operate and wouldnot need the full voltage of the battery. In the case of stepping up the voltage,the motor of a car may require 500v to oprate,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 further to be divided in to the five types

. Buck – to step down voltage

. Boost – to step up the voltage

. Buck/Boost – This can do both step up and stepdown operation but is mainly used for reverse polarity

. Cuk – similar to the buck/boost type.

. charge pump – To step up/step down the voltage for lower power sources

The isolated type is classified in two types.

Flyback converter : similar to the buck boost converter except is uses a transformer instead of an indicator to store energy

Forward converter : This type will use a transformer to send energy between the input and output in a single step.

AC to AC converter :

This type of converter takes an 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 to maintain the depending on the application.This converter has many applications in our daily life eg. Lamp dimming,motor speed control.there are 4 main types of AC to AC converters.

CYCLO converters : this is also known as a frequency changer as it will change the frequency of input AC waveformand output AC waveform with another frequency but is can also change the magnitude of the power.This type of converters does not have any DC linkages and avoids any conversion between AC and DC to prevent losses and save costs.the input wave frequency is changing the firing angle applied to the thyristor.By switching the positive and negative limb thyristors, we can get an output waveform that has a stepped up/down frequency compared to the input.

AC to AC convert with DC link : As the name suggests.this type includes a DC link along with a rectifier and an inverter.The input AC waveform is first rectified by the rectifier to remove the negative portions of the AC wave and output only the positive portions. This create a DC waveform which is then stored in the DC link and is then converted back to AC waveform using the inverter.

Matrix converter : This type converts from AC to AC directly without using a DC linkages to reduce costs and losses that would otherwise be in occurred with a DC link storage element.It is used mainly to increases the reliability and efficiency of the system.Matrix converters are bidirectional switches (IGBT) to conduct current and block voltages of both polarities.

Hybrid matrix converter :This type is very similar to the matrix cinnverter in that they combine the AC/DC/AC design with the matrix design. It uses unidirectional switches and 2 converter stages,without the DC link to reduce the size and weight of the converter.

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%?

A) 

Firstly using the given information,we can determine the rolling resistance and drag forces

The rolling resistance formula is given by : Frr = μrr*mg

Where μrr is the coefficient of rolling resisitance and mg is the weight

The drag force formula is given by : Fd =1/2*Cd*ρ*A*v^2

Where Cd is the coefficient of drag

 ρ  is the air density in kg/m^3

A is the frontal cross sectional area in m^2

V is the velocity in m/s

Using these 2 formulas ,the rolling resistance and drag forces are

Frr = 88.29

Fd = 0.23228 v^2

Since we don’t know the velocity, we can only write an expression for the drag force in terms of velocity

Next we can equate the equations of tractive force using the summation of forces on the right hand side and the motor torque gear ratio  and the wheel radius on the left hand side

Fte = T*G/r

We know the summation 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 whch is equals to the 0.2794m.we don’t know the torque so we have to write the LHS in terms of T

T*1/0.2794 = 0.23228 (v^2 )+ 88.29

Since we used the wheel radius for the LHS we also know that velocity can be written as a product of angular speed and radius.Here v value is substituted in the given equation

V=r* ω

T*1/0.2794 = 0.23228*(0.2794^2* ω^2)+88.29

Simplifying this gives us an expression for the torque T shown below

T = 0.0051 ω^2+24.67

Next we know the torque speed relation for a motor we are provided the armature resistance voltage and kph values we also know the duty cycle is 70%

So the average voltage is given by the product of the battery voltage and the duty cycle is

Vo = Vdc

Vo = 72.07

The average voltage is 50.4v

We know the torque-speed relation is given by the equation

Tm = 70.56 – 0.98 ω

Now we have 2 expressions for torque that are both in the terms of the angular speed.The first expression is the torque of the vehicle and the second one is for motor.

Equating the 2 expressions we will get a quadratic equation which can be solved to find the angular speed

70.56-0.98 = 0.0051 ω^2 +24.67

0.0051ω^2+0.98 ω-45.89=0

We can solve the equation to find the roots of an equation by using the formula

Then the values are ω = 38.92 rad/sec or ω = - 231.08 rad/sec

Assuming the forward direction is the positive and that the vehicle s in steady speed moving in the forward direction we will accept the first solution and reject the second one because the vehicle speed does not to be negative value

 4)  Induction Versus DC brushless motors by Wally Rippel, Tesla

A)

The author writes this blog to entail the differences between the two most commonly used motor drives in the automobile industrythe dc brushless motor drive and the induction motor drive.Rippel goes to the detail about the construction working and cost between the two drive types.However with what he has written he has not arrived at the conclusion that one type is better than the other.In the last lines of the opening paragrasph ripple writes that there is no one best engine type and that is all a matter of personal preferencewhen it comes to choosing an engine. It is in the same regard that he ends the blog stating that there is no still no winner when it comes to the motor drive cycle selection either, inspite of both motor drive types having similarities they also have their benefits and drawbacks.

For DC brushless drive, we are told their construction implements 2 or more permanent magnets at the rotor to generate a DC magnetic field, it is also states a stator core(made up  of thin stacked laminations) it also uses an inverter for the current control meaning that it cannot run a DC power supply as the current direction has to be changed repeatedly in to maintain a constant torque.

For inductor drives there rotor has no magnets but instead stacked 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 not require an inverter to start the drive and generate the starting torque it has the ability to start under the mains load applied However DC brushes drives cannot do the same they cannot generate a starting torque when connected to a mains power supply.They require an inverter to do this.

Induction motor drives cannot run a DC power supply and are slowly dependent on a AC power supply also when they are connect to a 3phase mains 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 an 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 magnet and cannot changes the strengths of their magnetic fields.it would be desirable if they could because when maximum torque is needed by the rotor the magnetic strength should be also at maximum level.this allows for the inverter and motor currents 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 minimum to be reduced hysteresis and eddy current losses the permanent magnet are also quite expensive cost 50 per kilogram and their magnetic fields are difficult to control if ferro magnetic material gets close to enough the field.

On the other hand,induction drives donot 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 strength can be changed according to the requirement by the rotor. Theses losses do not increases if the size of the induction motor drive also increases but a DC brushless gets bigger its losses increases and their 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 rotors more easily and have higher peak point efficiency than induction drives,in addition the DC brushless DCmotor can operate up to a powerfactor approximately around 85%

In conclusion Rippel has elaborated the different aspects to compare the induction and DC brushless motor drives and has detailed the 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 a high performance with comparatively less costs then the induction motor drive would be better suited for those requirement.However if it is high peak point efficiency high power factor and easier control that a user seeks,then the DC brushless drives would be smarter choice, both types have their place in the automobile industry and will retain their relevance in their respective applications .Rippel believes that due to the increasing performance of vehicles one day both may be used by side under the hood of a vehicle in order to get the best of both drives This would result in a new kind of hybrid that combines the characteristics of an ordinary vehicle e.g sedan with that of a sports car