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UNIVERSITY OF BRISTOL

MAY/JUNE 2021 (Assessment Period 1)

FACULTY OF ENGINEERING
Third Year Examination for the Degrees of Bachelor and Master of Engineering
Combined Assessment for the Examination of

EENG30013 POWER ELECTRONICS,
MACHINES AND DRIVES

TIME ALLOWED:
7 days
(measured from the point at which
the assessment becomes available on Blackboard)

This paper contains 3 related questions.
Answer ALL questions.
The total for the paper is 60 marks.

PLEASE WRITE YOUR 7 DIGIT STUDENT NUMBER (NOT CANDIDATE
NUMBER) ON THE SUBMISSION FILE. YOUR STUDENT NUMBER CAN BE
FOUND ON YOUR UCARD.

Other Instructions
Read the paper through thoroughly before starting the assessment.


Make sure you know where the assessment point in Blackboard is before
starting the questions, it can be accessed through the link below.

https://bit.ly/3yMtS8P

You will need to be logged in to Blackboard for the link to work.



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Design of a power electronic converter and controller
for the field winding (not the armature) of a DC generator
based upon a wound-field brushed DC machine,
to a specification representative of a typical aircraft generator
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.

Fig Q1
The DC supply, !", 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 in Table 1.
Table 1: Machine Parameters
Parameter Symbol Value Units
Field winding resistance ! 75 × 10"# Ω
Field winding inductance ! 7.5 × 10"# H
Armature winding resistance $ 0.5 Ω
Armature winding inductance $ 100 × 10"% H
Rated armature voltage (at terminals) &'$&() 270 V
Rated (continuous) armature current $'$&() 100 A
Maximum (Peak) armature current $*$+ 160 A
Rated rotor speed/angular velocity , 12,000 rev/min , 1,257 rad/s
Rated Electro-magnetic torque (* 21.5 Nm
Total mass moment of inertia of Generator and engine rotor &-&$. 10.01 /
Viscous friction of generator and engine rotor &-&$. 2 × 10"/


DC Machine
DC Field
Converter
And Controller
DC Supply DC Field
Winding
DC Armature
Winding
Mechanical
Input Power
Electrical Output
Power
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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, # as an input and measure the
following variables as outputs:
• Generator terminal voltage (at
point of regulation), $ (V)
• Field Current, # (A)
• Armature Current, % (A)
• Field Flux, Ψ#
• Electro-magnetic torque, &' (Nm)
• Rotor Speed, ( (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
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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





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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


Question paper continues on the next page

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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 (%) 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).
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A suggested route (not every step is necessarily essential in every case) through the design
process is:
(A) Plot a graph of field voltage, #, against generator terminal (or armature) voltage, $, for
the generator in open-circuit condition (with no load connected),
(B) Use this graph to determine the field voltage, # (and current, #) 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, $ against Generator (or
armature) current, ).
(D) Construct an appropriate model of your converter and check its operation in Simulink
(E) Build an approximate state space model (if required) of your converter and the
appropriate load and validate that it runs and is correct.
(F) Linearization around an operating point (if this is needed)
(G) Decide on a suitable feedback controller to use the field voltage input, # to control the
generator terminal (or armature) voltage, $.
(H) Decide on a suitable design criteria for that controller
(I) Design, implement and test the controller
(J) Demonstrate clearly that your controller meets the required specification
Note: At very low values of duty cycle (where discontinuous conduction will occur) any state
space model you create will 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.
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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.
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• 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

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Assessment Marks Scale Being Used
A single mark will be determined for the combined assessment using the 21-point standard
marking scale (shown in Table 2 below) and with reference to the Intended Learning
Outcomes for the units and scaled to give the final marks.
Table 2: University of Bristol 21-point standard marking scale


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