Coursework - EEE229 Electrical Energy and Conversion
(2023-24)
Total score: 100 (Part I and Part II)
Please carefully read the submission details below as well as the introductory notes in each part of the assignment.
Submission details: The assignment contains two parts, Part I for Semester I and Part II for Semester 2. Please complete your answers and present them in the form. of a report by using a word-processed document (or LaTex/Overleaf) and provide detailed solutions showing the individual steps and justification to reach your results.
You will need to submit each part separately, which means two submissions in two different links. So please ensure that you make two individual submissions, one submission for Part I and one for Part II. Combined submissions of both parts into 1 document will not be considered for marking. Please save each of your documents into an individual PDF file and submit these via the BlackBoard EEE229 site in the corresponding submission links under the ‘Coursework Assignment 2024’ content folder. Please do not zip your PDF files.
The deadline for submission is 23:59 on Monday, 20th May 2024. Grades and feedback will be released a few weeks after the deadline.
The questions in Part I contain data which are shown in RED CAPITAL TEXT, which is specific to each candidate. You can find the particular data for you to use based on your registration number in each question by Appendix A of this problem sheet.
Part I: Semester 1 (score 50)
Introductory notes: This assignment contains several elements that have not been explicitly covered in the lecture notes. These, quite deliberately, require you to access manufacturer datasheets or research other sources of information to complete the question(s). This additional material is not examinable in the main examination – only material covered in the Lectures, Tutorials, Slides, and Lecture Notes is game for an exam question.
You should provide all workings and explanations in your solutions – these will variously count towards achieving full marks in a particular question. Answer all questions.
QUESTION 1
Induction machine
A three-phase induction machine is designed to operate at a near constant speed for the compressor of an environmental control system of an aircraft. The power system to which it is connected is a standard 400Hz, 115 Vrms (phase voltage) sinusoidal AC supply.
The nominal speed for the compressor is NOMINAL_SPEED (i.e. the synchronous speed with zero slip). During manufacture, the stator resistance is measured to be R1. During a locked-rotor test at 400Hz after assembly, the measured input power per phase is LR_INPUT_POWER and the measured phase current is LR_CURRENT when operated at standstill with a reduced stator voltage of LR_VOLTAGE. The machine has a core loss at 3% slip of CORE_LOSS. The magnetising reactance, xm , is j30Ω .
a) Calculate the number of poles for the design to achieve the specified nominal speed. [2]
b) Using the full approximate equivalent circuit, calculate the torque produced by the machine for a slip of 3%. [5]
c) Calculate the total input current at this operating point. [5]
d) Calculate the efficiency at this operating point. [3]
QUESTION 2
Brushed PMDC machine
A permanent magnet brushed DC machine is used to drive a geared mechanism in an aerospace valve actuator. The machine in an aerospace grade brushed DC motor manufactured by Maxon (www.maxonmotor.com). The motor is part of their RE Programme range of high performance machines as has part number DC_PART_NO. The specification for this machine contains all the information you need to answer these questions (take care over units).
a) Calculate the total friction and drag torque at the no-load speed (i.e. when there is no external load applied to the machine). [5]
b) Starting from the equivalent circuit parameters, calculate the no-load speed taking into account the friction and drag torque calculated in part (a) and compare this with the value in the specification sheet. [5]
c) Plot the torque (y-axis) versus speed (x-axis) characteristic for this machine design at its nominal voltage, taking care to include calculations of any key operating points (Note this will not match exactly the experimental measurements in the published specification, but should be a value close to it). [5]
d) Using the value of the maximum continuous current for the machine (also referred to as the nominal current in the specification sheet), calculate the machine torque and speed when drawing this current. [5]
QUESTION 3
Simulation activity: electromechanical analysis of separately excited DC machine
Open MATLAB and then open Simulink. Create a new blank Simulink model, go to the Library Browser, search for the Simscape Library and find the DC Machine block (figure below).
The DC Machine block has one mechanical input, that can either be the torque of the load [Nm] or the angular velocity of the rotor [rad/s]. You can change the mechanical input by double-clicking on the DC Machine block and selecting the appropriate option under the preferences tab.
Under the preferences tab, you can also select the type of the DC machine under consideration. The DC Machine block can implement either a wound-field or a permanent magnet DC machine. In this case, we will study a separately excited DC machine, so select the appropriate type to account for the separately excited machine.
The output of the DC Machine block is a vector (m) that contains four signals. You can demultiplex these signals by using either the Demux (left figure below) or the Bus Selector (right figure below) blocks that are provided by the Simulink Library.
The four signals contained in the output vector (m) are:
1. The mechanical (angular) speed of the rotor.
2. The armature current.
3. The field current.
4. The electromagnetic torque produced by the machine.
From the Library Browser, locate the DC Voltage Source block (left figure below) and the Ground Connection block (right figure below) and use them to supply the armature winding (A+ and A-) and the field winding (F+ and F-) of the DC machine with power.
Use the Constant block to set the load torque at a fixed value and make the connection with the DC Machine block. The sign convention for the mechanical torque is: when the speed is positive, a positive torque signal indicates motor mode and a negative signal indicates generator mode.
Use the Scope block (left figure below) to create graph windows that will allow you to observe the four signals contained inside the output vector (m).
Do not forget to place the powergui block (right figure above) anywhere inside your Simulink model before running the simulation.
ACTIVITY 1: SPEED CONTROL THROUGH THE ARMATURE VOLTAGE [5]
From the parameters of the DC Machine block, change the preset model of the DC machine to:
01: 5HP 240V 1750RPM FIELD: 300V
Hint: Make sure that the other blocks contain the nominal (rated) values of the preset model.
Select the mechanical input as Torque TL and set the load torque as a constant of 10 [Nm].
Set the armature voltage at 100 [V], 160 [V], 240 [V] and 300 [V], and check the rotor speed and the armature current using the Scope blocks.
a. What do you observe?
b. How do your observations agree with the mathematical model of the separately excited DC machine?
In your answers, provide simulation graphs, comments, and mathematical formulas wherever necessary.
ACTIVITY 2: SPEED CONTROL THROUGH FIELD WEAKENING [5]
Use KVL for the armature circuit and modify the Simulink model so that you can
observe the Back-EMF as well.
Hint: Change the parameter measurement of the DC Voltage Source block that supplies the armature winding into Voltage. Also, use the Multimeter block.
Keep the armature voltage at 240 [V], set the field winding voltage at 300 [V], 260 [V], 220 [V], 200 [V], 50 [V], 40 [V], 30[V], and 20 [V], and check the rotor speed, the field current and the Back EMF produced, considering firstly the case of a load torque of 10 [Nm] and secondly the case of no load at all.
a. What do you observe in each case?
b. How do your observations agree with the mathematical model of the separately excited DC machine?
In your answers, provide simulation graphs, comments, and mathematical formulas wherever necessary.
ACTIVITY 3: DIRECTING ROTATION [5]
Hint: Remember the sign convention regarding the torque and the rotor speed.
Keep the load torque at 10 [Nm]. Change the polarity of the armature voltage by setting it at -240 [V] and observe the rotor speed and the armature current.
a. How does the DC machine behave (motor & generator)?
b. What happens and why?
Change the load torque, by setting it at -10 [Nm], while keeping the armature voltage at -240 [V]. Observe the rotor speed and the armature current.
a. How does the DC machine behave (motor or generator)?
b. What happens and why?
Set the armature voltage at +240 [V] and then change the polarity of the field voltage, by setting it at -300 [V]. Observe the rotor speed and the armature current when the load torque is -10 [Nm] and 10 [Nm].
a. How does the DC machine behave in each case (motor & generator)? b. What happens and why?
Keep the load torque at 10 [Nm]. Set the armature voltage at -240 [V] and set the field voltage at -300 [V]. Compare the direction of the rotation with the case where the armature voltage is +240 [V] and the field voltage is +300 [V].
a. How does the DC machine behave in each case (motor or generator)? b. What happens and why?
In your answers, provide simulation graphs, comments, and mathematical formulas wherever necessary.
Part II: Semester 2 (score 50)
Introductory notes: The following questions contain elements that were studied in several lectures in Semester 2, and motor equations (torque and power) have been also studied in Semester 1. For this section, we encourage you to use mathematics
software packages such as Wolfram Mathematica or Maple to facilitate your analysis, whose license is available for all students.
Similar to Part 1, supplementary information is provided in Table 1 of the Appendix B at the end of the document. Please use the relevant data for your registration number.
Question 1:
Consider the DC Motor control set-up in Fig. 1.
Figure 1- H-bridge drive system for DC motor
Considering bipolar switching, compute the average value of the voltage across the terminals of the DC motor va as a function of the duty cycle 。. Justify your answer using a diagram of the circuit and the step-by-step averaging procedure. [4]
Question 2:
a)
For the circuit shown inFigure 2, calculate the electromagnetic torque Te, the rotor speed 幼r and vf, when Ia = 5mA, and with the H-bridge driven with bipolar switching and a duty cycle = 80% . [9]
Figure 2
b)
For the circuit shown inFigure 3, calculate the electromagnetic torque Te and the rotor speed 幼r when Lf = 20H, and with the H-bridge driven with bipolar switching and a duty cycle = 70%. [9]
Figure 3
c)
For the circuit inFigure 4calculate the electromagnetic torque Te and the rotor speed 幼r when Lf = 20H, and with the H-bridge driven with bipolar switching and a duty cycle = 90%. [9]
Figure 4
d)
For the three cases above, assuming ideal switches and a switching frequency of 20kHz, compute:
1. Input and output power
2. The duty cycle and therefore the required Ton of each switch when the machine is rotating at 300 RPM (in the separately excited case, assume vf is constant. Hint: Ia will change) [14]
Question 3
Based on your findings in the previous section, select a suitable BJT from Farnell (https://uk.farnell.com/) for use in all 3 H-bridge circuits and explain your choice. [5]