EMT226C-S2-2021
MAIN
TEST/ASSIGNMENT
___________________
STUDENT NUMBER
___________________________________
SURNAME AND INITIALS OF STUDENT
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SIGNATURE OF STUDENT
INSTRUCTIONS TO CANDIDATES
NO GROUP WORK ALLOWED
SEE PAGE 3 FOR MORE INSTRUCTIONS
INSTRUCTIONAL PROGRAMME
CODE
Bachelors in Engineering Technology in Electrical Engineering
SUBJECT:
ELECTRICAL MACHINES
TECHNOLOGY II
QUESTION
SUBJECT CODE:
EMT226C
PROBLEM 1
EXAMINER:
MR. R. PRETORIUS
DATE OF
ASSESSMENT:
22 OCTOBER 2021
DATE OF RELEASE:
SUBMISSION ON
ETHUTO
EXAMINER
(B_ETE)
MODERATOR
PROBLEM 2
Assignment
will
be
released on 22 OCTOBER
2021 at 08:00.
The submissions and
answers on eThuto must
be done before 16:00 on
22 October 2021.
CALCULATOR:
NON-PROGRAMMABLE
TOTAL:
65
TOTAL
SIGNATURE
___________________________
Examiner
Page 1 of 6
EMT226C-S2-2021
Copy of student card or Id
DECLARATION OF OWN WORK
Student Number:
Surname & Initials:
Program:
Bachelors in Engineering Technology in Electrical Engineering
(B_ETE)
Subject Name:
Electrical Machines Technology II
E
Subject Code:
M
T
2
2
6
C
Main Test/Assignment
Due date:
Lecturer:
2
0
2
1
1
0
2
2
R. Pretorius
Declaration of own work
By submitting my work on this platform, I declare that:
 I have completed this test/assignment in accordance with CUT’s Code of Academic
Integrity;
 All the work included in this test/assignment is my own work; and
 I did not consult any person or means other than that allowed by test/assignment
instructions.
________________________________________
Signature of Student
_________________
Date
Page 2 of 6
EMT226C-S2-2021
Instructions
Main Test/Assignment
Direct current Machines
65
Problem 1- Direct Current Machines (Problems 1.1– 1.10)
Problem 2- Single Phase Transformers (Problems 2.1–2.8)
35
30
Step 1:
Download the Main Test/Assignment paper.
Answer all the theory questions and do all the calculation problems.
Please use two decimal places in all calculations, unless instructed
otherwise. Show all the calculation steps.
The theory problems and calculation problems must be done in your own
handwriting and not typed.
You also need to write neatly so that it is legible.
Mark the questions clearly.
Step 2:
After completion of step 1, do the on-line test.
Open the test: Electrical Machines II-ELM216A-S2-2021-Main
Test/Assignment-On-line test.
Problems 1 – 2:
For the theory question just choose the correct answers and for the
calculation problems just fill in your final answers in the blank box.
Please use a full stop (.) and not a comma (,) when entering values
with decimals. For example: 35.67 or -36.87.
Enter only the value and not the unit of measure, for example: 12.5
and not 12.5 V.
Step 3:
Submit your pdf-file on eThuto.
Make sure you fill in all your details on each of the pages of the pdf-file that
you are going to submit on eThuto. (Student number, Student initials and
Surname)
Please use your student number as name of pdf-file that you are
going to upload
The pdf-file consist of the following:
 Cover page (Page 1 of this document) Fill in all your details (Student
number, Student initials and Surname and Signature)
 Declaration form (Page 2 of this document) Fill in all your details and
sign the declaration form. You can write the declaration over in your
own handwriting and sign it.
 Copy of student card or Id (Page 2 of this document)
 Problems 1 – 2: Theory and Calculation Problems.
Please make sure that you know how to upload a PDF-file on eThuto
Take note:
Multiple
Attempts
The marks of the on-line test are taken as your final mark of the test.
Please work accurately and make sure of each answer you give.
If your evidence (pdf-file with signed declaration) is not uploaded before
the deadline, you will receive zero for the test.
In normal test conditions (sit down), you have two hours to write this paper.
This means that there are six hours left to do the on-line test as well as to
upload your pdf file.
This Assignment/Assessment allows only one attempt.
Page 3 of 6
EMT226C-S2-2021
Problem 1
Direct Current Machines
1.1 Which of the following statements is false?
[a]
[b]
[c]
[d]
[b]
[c]
[d]
(1)
A DC motor converts electrical energy to mechanical energy.
The efficiency of a DC motor is the ratio input power to output power.
A DC generator converts mechanical power to electrical power.
The efficiency of a DC generator is the ratio output power to input power.
1.2 Which of the following statements is false?
[a]
[35]
(1)
A commutator is necessary as part of a DC motor to keep the armature
rotating in the same direction.
A commutator is necessary as part of a DC generator to produce
unidirectional voltage at the terminals of the generator.
The field winding of a DC machine is housed in slots on the armature.
The brushes of a DC machine are usually made of carbon and do not rotate
with the armature.
1.3 If the speed of a DC machine is doubled and the flux remains constant, the
generated Emf ……………...
(1)
[a]
[b]
[c]
remains the same.
is doubled.
is halved.
1.4 Which of the following statements is false for a series-wound DC motor?
(1)
[a] The speed decreases with increase of resistance in the armature circuit.
[b] The speed increases as the flux decreases.
[c] The speed can be controlled by a diverter.
[d] The speed can be controlled by a shunt field regulator.
1.5 In DC generators iron losses are made up of:
[a]
[b]
[c]
[d]
hysteresis and friction losses
hysteresis, eddy current and brush contact losses
hysteresis and eddy current losses
hysteresis, eddy current and copper losses
1.6 A commutator is a device fitted to a generator. Its function is:
[a]
[b]
[c]
[d]
(1)
(1)
To prevent sparking when the load changes.
To convert the AC generated into a DC output.
To convey the current to and from the windings.
To generate a direct current.
Page 4 of 6
EMT226C-S2-2021
1.7 The shaft torque of a diesel motor driving a 100 V DC shunt-wound generator is
25 Nm. The armature current of the generator is 16 A at this value of torque. If
the shunt field regulator is adjusted so that the flux is reduced by 15%, the torque
increases to 35Nm. Determine the armature current at this new value of torque.
(2)
1.8 A 150 V DC motor draws a current of 25 A when running at 1200 r/min. If the
torque on the shaft of the motor is 24.88 Nm, determine the following:
(a) The % efficiency of the motor.
(b) The power loss in the motor in W.
(4)
(2)
1.9 A series motor has an armature resistance of 0.2 ohm and a series field
resistance of 0.3 ohm. It is connected to a 240 V supply and at a particular load
runs at 1440 r/min when drawing 15 A from the supply. Determine the following:
(a) The generated Emf at this load.
(2)
(b) The speed in r/min of the motor when the load is changed such that the
current is increased and this causes a doubling of the flux.
(5)
1.10 A 7.46 kW, 220 V, 900 r/min shunt motor has a full-load efficiency of 88%, an
armature resistance of 0.08 ohm and shunt filed current of 2 A. Determine the
following:
(a) The total losses of the motor in W.
(2)
(b) The total copper losses in the motor in W.
(4)
(c) The combined core losses, stray load losses and rotational losses of the
motor in W.
(2)
If the speed of this motor is reduced to 450 r/min by inserting a resistance in the
armature circuit, the load torque remaining constant, determine the following:
(d) The value of the additional resistance inserted in the armature circuit in ohm.
(6)
Problem 2
Single phase transformers
2.1 Which of the following statements is false?
[a]
[b]
[c]
[d]
[30]
(1)
In an ideal transformer, the volts per turn are constant for a given value of
primary voltage.
In a single-phase transformer, the hysteresis loss is proportional to
frequency.
A transformer whose secondary current is greater than the primary current
is a step-up transformer.
In transformers, eddy current loss is reduced by laminating the core.
Page 5 of 6
EMT226C-S2-2021
2.2 An ideal transformer has a turns ratio of 1:5 and is supplied at 200 V when the
primary current is 3 A. Which of the following statements is false?
(2)
[a]
[b]
[c]
[d]
[e]
The turns ratio indicates a step-up transformer.
The secondary voltage is 40 V.
The secondary current is 0.6 A.
The transformer rating is 0.6 kVA.
The secondary voltage is 1 kV.
2.3 The core of a transformer is laminated to:
[a]
[b]
[c]
[d]
(1)
Limit hysteresis loss.
Reduce the inductance of the windings.
Reduce the effects of the eddy current loss.
Prevent eddy currents from occurring.
2.4 The primary reason why the open-circuit test is performed on the low voltage
winding of the transformer is that it …………….
(1)
[a]
[b]
[c]
[d]
draw sufficiently large no-load current for convenient reading.
requires least voltage to perform the test.
needs minimum power input.
involves less core loss.
2.5 A transformer has negative voltage regulation when its load power factor is
………
(1)
[a] zero
[b] unity
[c] leading
[d] lagging
2.6 A single-phase transformer has a turns-ratio of 144/432 and operates at a
maximum flux of 7.5 mWb at 50 Hz. When working on no load, it takes 0.24 kVA
at a power factor of 0.26 lagging from the supply. Determine the following:
(a)
(b)
(b)
The no-load current.
The magnetizing current.
The core loss current.
(4)
(2)
(2)
2.7 A transformer with turns-ratio 8:1, has the resistances of the primary and
secondary windings as 0.85 Ω and 0.012 Ω respectively, and the leakage
reactances as 4.8 Ω and 0.07 Ω respectively. Determine the voltage to be applied
to the primary to obtain a current of 150 A in the secondary when the secondary
terminals are short-circuited. Ignore the magnetizing current.
(8)
2.8 The primary and secondary windings of a 500-kVA, 11-kV/415-V, single-phase
transformer have resistances of 0.42 Ω and 0.0019 Ω, respectively. Its core
losses are 2.9 kW. Assuming the power factor to be 0.8 leading, determine its
% efficiency on half load.
(8)
TOTAL 65
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