Week-6 Challenge: EV Drivetrain

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

ans. There are four main categories of power converter circuits they are

• AC to DC converter
• DC to AC converter
• DC to DC converter
• AC to AC converter

Out of these four only three are employed in electric and hybrid electric vehicle , they are

• AC to DC converter
• DC to AC converter
• DC to DC converter

AC to DC converter

This type of converter takes an AC power supply and converts it to a DC. Power generated from a power plant is transmitted to our homes at a voltage of 230V at a frequency of 50 Hz which we can use to power our electrical appliances. But our appliances operate 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 charger , laptop charger etc. An AC power supply has an alternating waveform which repeatedly reverses direction.An AC to DC converter will prevent the current 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 drop out voltage(LDO) regulator.

The step down transformer takes the high level 230V from the mains supply and steps it down to a lower level eg. 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.. Power which is supplied from a battery or capacitor can have its from changed to the AC type for use in homes or system 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 which uses an AC power supply so that we can utilise this power from the mains. This type of converter is refferred 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 eg, IGBTs, 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 which consists of a 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 eg. 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 a 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 supplied 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 in to a DC supply for use in a load. In EVs , HEVs and conventional vehicles, this type of converter is used for various purposes, eg in the case of stepping down the voltage, the car radio may require a stepped down voltage of 12 v 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 Busk/Boost type
• Charge pump - to step up/step down the voltage for low power sources

The isolated type can be subdivided into two 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.

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%?

ans.

Battery pack voltage = 72 V

Rated armature voltage = 72 V

Rated armature current = 400 A

Armature Resistance = 0.5ohm

Kw = 0.7

Chopper frequency = 400Hz

duty cycle = 70 %

Torque speed characteristics equation is given by,

Tv = 24.7 + 0.0051w^2

Tm:Tv = 1:1

Average voltage at 70% duty cycle = 0.7*72 = 50.4 V

w = (V/Kw)-(Ra/Kw^2)*Tm

w = (50.4/0.7)-(0.5/0.7^2)*Tm

w=72-1.0204Tm

Tm = 70.5605-0.98w

Tv = Tm

24.7 + 0.0051w^2 = 70.5605 - 0.98w

0.0051w^2 + 0.98w - 45.8605 = 0

by solving the quadratic equation,

w= 38.915 and -231.072

Hence w = 38.915 rad/sec

speed at rpm = 38.915*60/(2*3.14) = 371.79 rpm

Therefore Speed in rpm = 371 rpm

So the steady state EV speed is 371 rpm

3. Develop a mathematical model of a DC Motor for the below equation using Simulink.

ω">ω= V/Kϕ">ϕ -Ra/Kϕ">ϕ^2 .T

ans.

procedure

• Add the required blocks to the model
• Connect the blocks as required for the model
• for voltage V the constant input of 400 is given
• Kw is the constant value of 0.5
• Ra is the constant resistance value of 0.1
• Torque is given as the constant ramp input
• torque value is given to the X axis of XY graph
• Speed value is given to y axis of XY graph
• Run the model for 300 seconds

This is the following model

The mathematical model of the given equation using simulink is developed and torque speed characteristics is plotted in the output.

This is the following output achieved using simulink model

To refer to the matlab model i have attached torquespeed.slx file.

4. Refer to the blog on below topic:

Induction Versus DC brushless motors by Wally Rippel, Tesla

Explain in brief about the author’s perspective.

ans. In this Blog Wally Rippel, Tesla discussed about the various types of motor that he came across from 1960s. Back then the motor used in the electrical vehicle is brushed DC motor with contractor which is no more in use in this field. Nowadays it is replaced with Brushless DC motor and 3 phase induction motor. By using the contactors and brushed DC motor the losses are high.

In both brushless DC motor and 3 phase induction motor , the stator is same.Only difference is the type of rotor used. In the brushless DC motor, the rotor is made of the two or more permanent magnet. This permanent magnet produces the magnetic field which comes in contact with the stator windings. When the supply is given to the stator, the torque is produced in the rotor which makes the motor run. To maintain the torque constant, the current should be varied continuously. So that inverter is used. This type of motor performs well at peak speed. The torque values can be varied by changing the value of B. But it is not possible to change the value of B in a permanent magnet.

Here comes the induction motor which was invented by Nikola Tesla. The stator is same as we discussed before but the rotor is made of laminated steels and the conductors are arranged in the slot. When the supply is given to the stator, it produces rotating magnetic field. This comes in contact with the rotor. The frequency of the rmf is equal to the frequency of the supply. So that voltage is induced in the rotor and value is equal to difference between the value of supply frequency and speed of rotor itself. The ends of rotor are short circuited by the end rings, so the current flows through the rotor.Due to effect of rmf, the torque is produced in the rotor.

Hence the three phase motor is run directly from the utility power source. But the limitation is it runs on line voltage but not on DC voltage.Hence the inverter is added to the circuit. By using the switching operations in the inverter the speed can be varied and the value of B also can be reduced because there is no permanent magnet.

The brushless DC motor operation at the peak performance is efficient. The induction motor may be less efficient at peak. But average efficiency of the induction motor is better than the brushless DC motor