General Design Problem

You are required to design a converter and controller for the field (not the armature) of a medium sized aircraft DC generator (Fig Q1) based upon a wound field, brushed DC machine.

Mechanical Input Power

Electrical Output Power

Fig Q1

The DC supply, VIN, for the converter will be taken from the available low voltage avionics

supply on the aircraft and will vary between 22 and 29V DC.

The parameters that define the DC machine are given inTable 1.

Table 1: Machine Parameters

Provided models

You have been provided with two SIMULINK models.

1.   A model of a power converter (part01_model.slx)

2.   A model of a wound field DC generator (part02_model.slx.

The  machine  model allows you to supply field voltage, Vf  as  an  input and  measure the following variables as outputs:

•   Generator terminal voltage (at point of regulation), Vt (V)

•   Field Current, If (A)

• Armature Current, Ia (A)

•   Field Flux, Ψf

•   Electro-magnetic torque, Tem (Nm)

•   Rotor Speed, NR (RPM)

You should also use these to test and present the output of your converter and controller design in your solution.

Engine and Generator Rotor Dynamics:

Note that for completeness, the simulation includes a model of the mechanical dynamics of the generator and the engine to which it is connected and a controller to control it, hence you may see some fluctuations in the speed of the generator rotor.

These will normally be small and stable and can be ignored.  Should large fluctuations in the rotor  speed  (or  angular velocity)  occur then you  have  probably  implemented  your  own controller or driven the model in a way that destabilises this control.  This is not a fault with the model, more an indication of something that you have done incorrectly.

Part 1: Converter Design

You are required to design a Buck Converter Switched-mode power supply, as in Fig Q2, to deliver the required field voltage of about 3V to 4V from an input of 28 VDC and load current in range of 40 to 80 A. A SIMULINK model supplied as part of this assessment is a model of the  buck  converter  topology  shown  in Fig Q2 and  should  be  used  to  demonstrate  the effectiveness of your converter performance. The key element here is to use the available datasheets to select the appropriate semiconductor device among the options available. You need to fully justify your selection.

Fig Q2

Q1       Using the datasheets available at the following location:

https://bit.ly/3oRJLWH

you are required to perform the relevant design calculations and select appropriate components in order to ensure that the converter works as intended.

(A) Determine values for the duty cycle, filter inductor (L), and the filter capacitor (C). For the purposes of calculations, you can assume the switch, transformer, diodes and filter components are ideal and have negligible     resistance.

(B) Estimate the range of duty cycles needed to deliver the required output current and voltage.

(C) Select the transistor switch and diode from the datasheet choices. Justify your selection.

(D) Specify the size of heatsink needed for the transistor switch (a range of heatsink options are given). Justify your selection.

(E)  Using MATLAB/Simulink model (part01_model.slx) provided verify your design.

(F)  Comment on your design and any suggest any improvements that could be made.

(G) Comment on design rules you have to follow in designing a converter.

(H) Comment on the difference that a Silicon and a Silicon Carbide power semiconductor device can make in operation of the converter.

Choice of heatsinks:

(datasheets can also be found on blackboard on above link)

•   Extrusion 63730

•   Board Cooling – Channel 5770

•   287-1ABH

•   Board Cooling – Channel 5900

Choice of Power Transistors:

(datasheets can also be found on blackboard on above link)

•   APT50GP60J

•   IXDN 55N120 D1

•   C2M1000170J

•   C3M0025065D

•   C3M0021120K

Choice of Diodes:

(datasheets can also be found on blackboard on above

link)

•   DSEI2x101-06A

•   STTH200L06TV

•   APT2X61DC60J

•   SS150TI60110

Part 2: Controller design and validation

Using  the  converter  that  you  have  designed  in  part  1  of  this  assessment,  design  and demonstrate a feedback controller that can be used to control the field automatically to maintain 270V at the terminals over the full range of operating conditions set out in Table 1.

Fig Q3 is a figure extracted from  MIL-STD-704F, the standard  used to  regulate electrical generating equipment on civil aircraft.

Fig Q3: Envelope of normal voltage transient for 270 volts DC system.

Your converter and controller for the field winding of the generator will need to be able to:

1.   maintain the steady-state voltage at the armature terminals of the generator at the required nominal condition of 270V DC over the operating range of the generator (including the maximum condition) within the limits specified in the relevant standard (250V to 280V).

2.   Keep the voltage ripple at the armature terminal voltage (Va ) below 6V

3.   Keep the armature terminal voltage between the limits (bold lines) shown in Fig Q3 in response to at least step changes in load from light load (10% rated armature current) to fully rated conditions (100% rated armature current).

A suggested route (not every step is necessarily essential in every case) through the design process is:

(A) Plot a graph of field voltage, Vf, against generator terminal (or armature) voltage, Vt, for the generator in open-circuit condition (with no load connected),

(B) Use this graph to determine the field voltage, Vf (and current, If ) required to achieve the required  generator  terminal  voltage   in   an   open-circuit   condition   (with   no   load connected)

(C) Set the field voltage to the value you determined in part (B) and then use this value to plot  a  graph  of  generator  terminal  (or  armature)  voltage,  Vt  against  Generator  (or armature) current, IA .

(D) Construct an appropriate model of your converter and check its operation in Simulink     (E)  Decide on a suitable feedback controller to use the field voltage input, Vf to control the

generator terminal (or armature) voltage, Vt .

(F)  Decide on a suitable design criteria for that controller

(G) Design, implement and test the controller

(H) Demonstrate clearly that your controller meets the required specification

Note: At very  low values  of  duty cycle  (where discontinuous conduction will  occur) any average model you create may not accurately represent true behaviour of the converter. However, in this exercise it is not expected that discontinuous conduction will occur at the rated operating point.

Part 3: System Analysis

In most applications, power transmission and distribution is via AC.   Why is this the case? Using example of 50Hz utility such as those in UK, discuss the stability implications of AC distribution and suggest where and why might you use DC transmission for small parts of the network.

Assessment requirement

The aim of this assessment is for you to demonstrate your understanding of and ability to use the techniques and principles taught in EENG30013: Power Electronics, Machines and Drives. As such you should be aware that there is no “right” answer to the problems here and that you are being assessed in how you approach and synthesise the design and how you analyse and demonstrate the suitability of your design solution to the problem you have been set.

Submission Requirements

The minimum submission requirements for this exercise are:

1.   A SIMULINK simulation file that implements and demonstrates the performance of your controller/analysis

2.   A single pdf (guide length 5 to 10 pages) containing a design narrative and analysis and that includes screenshots of simulation models and scope figures that:

-     justifies the design route that you have taken for the converter and the controller design,

-     demonstrates the performance of the converter and the controller that you have designed (with and without the controller),

-     discusses the limitations of your converter and controller and the reasons for these limitations,

-     the key design choices and implications if you were to discretise the controller that you have chosen,

-     anything that you tried that didn’t work as expected (and why),

3.   The outcomes of design parameters for your converter and controller in one of the following ways (examples of how to do this are given at the location below):

https://bit.ly/2QOOEDg

-     A MATLAB “ .mat” data file containing variables/parameters used in your model,

-     A clearly referenced table of these variables/parameters in your written narrative (submitted as pdf),

-     Variables/parameters embedded directly into the SIMULINK simulation in some way .

You are able to submit supplementary SIMULINK simulation files that support the various stages in your design process but should only submit a single narrative/report.  If you decide to submit multiple simulation files, label them clearly and refer to them directly in your pdf.

IMPORTANT POINT – Plagiarism & Cheating

This assessment is open-book and use of external resources is allowed.

Plagiarism  (submitting  and  claiming  others’  work  as  your  own)   is  against  university regulations,  and  carries  severe  penalties,  such  receiving  a  mark  of  zero  (not  the  worst penalty!). Be aware that 'cheating' and 'helping someone else to cheat' is penalised in the same way. Please don't fall into this trap.

Some examples of plagiarism during a timed assessment:

• Collusion:  Discussing,  in  person,  over  the  phone,  email,  instant  messaging,  or websites, the substance of your question or answer with others. This includes friends, fellow  students, family  members,  private tutors,  and  internet forum  participants. Asking others to review the question or your answer, or to provide academic advice for improvement prior to submission.

• Copying: Copying and pasting any material (text, images, coding, calculations) from other sources, including teaching material and shared revision notes, solution papers, example answers, directly into your answer without appropriate acknowledgement.

• Contract cheating: Paying (with money, goods or services) another person or company to complete the assessment for you.

• Not protecting your work: Being the source for other peoples' plagiarism, e.g. sharing your screen with others, or not locking your laptop, where others could access your work, and any other kind of deliberate or inadvertent 'helping', during the exam.

We  will  be  using  the  normal  plagiarism  checking  processes  as  we  would  a  coursework assessment to ensure that any plagiarism/cheating is fairly penalised in accordance with the University’s regulations.

Remember,  use  citations  (or  references)  and  a  bibliography  state  clearly  ALL  resources (including copied ideas from books, papers, other people, etc) you have used (excluding the material provided as part of this unit on Blackboard).

If you do not follow this advice and we find material that is duplicate online or in other students’ work we will treat the case as a case of cheating /plagiarism.

Submission Point

Submit your assessment files to the Assessment point in Blackboard called “Exam” at the following location on the Power Electronics, Machines and Drives unit (EENG30013):

https://bit.ly/3yMtS8P

Advice:-

-     Avoid photographing and including long mathematical derivations in your report as the aim of the exercise is to describe, demonstrate and analyse the outcomes and limitations of the design you produce. The main reason for choosing to include these is if you suspect your analysis is wrong and can’t find a way to make your design function in the simulation and would like to demonstrate your approach.

-     Avoid   copying   and   pasting   code   snippets  for  the   same   reason   unless  you  are demonstrating an ability to use the code to optimise the process in a novel manner (e.g. using code to automatically run multiple scenarios).  Even then exercise judgement as it is the outcomes not the volume of work that you have done that is being tested.

-     Keep your narrative simple and write as you go, don’t leave it until the end.

-     Make it clear if you are simply copying and pasting other’s materials (only requirement for referencing)

Assessment Marking scheme is on the next page