Coursework Task Sheet

Coursework Task Sheet

Submission arrangements
All coursework must be submitted through Moodle website  by 11:55 pm on the specified date

Assessment Criteria
The work will be assessed by comparison with an approved marking scheme. Fully correct solutions will receive the number of marks indicated. Marks will be reduced for errors in proportion to their severity.  You need to indicate the units for all electrical quantities. You need to clearly present the equations used for calculations and the comparison results.

Q1A.  Single-Phase AC Circuits

A single-phase AC circuit with resistance of R Ohm in series with inductance of L Henry is connected across the supply of 200 V (RMS), f Hertz. The value of R and L are given in Table 1(a) against the last digit of your student ID. The value of frequency f is given in Table 1(b) against the last but one digit of your student ID.

(i) Simulate the circuit and measure the current flow from the source using the Multisim software.

Calculate the current values and verify the calculated values.

(3 Marks)

(ii) Measure & calculate the real power & reactive power supplied by the source and power factor of the circuit.
(4 Marks)

(iii) Calculate the value of capacitance required, by reactive compensation method, to bring the power factor of the circuit to 0.95. Implement this in the Multisim and measure the power factor.                                                                                                                        (3 Marks)

Table 1(i) R and L values for Q1

Last Digit of

Student ID    Resistance R (Ohm)    Inductance L (H)
0    20    0.06
1    10    0.12
2    15    0.15
3    9    0.10
4    12    0.16
5    10    0.15
6    12    0.16
7    15    0.18
8    10    0.14
9    6    0.10

Table 1(ii) Frequency value for Q1

Last but one digit of

Student ID    Frequency f (Hz)
0     200
1    100
2    50
3    150
4    100
5    75
6    50
7    60
8    120
9     20

Q1B. A single-phase AC circuit with resistance (R) of 4 Ohm in series with inductance (L) and capacitance ( C ) is connected across the supply of 220 V (RMS) with variable frequency.  The value of L and C are given in Table 2 against the last digit of your student ID.  Calculate
i) The resonance frequency & the voltage across the coil & capacitance                              (3 marks)                                                                                                                   ii) Simulate the circuit in Multisim and plot VR, VL & VC vs frequency within the frequency ranges: fr ± 800 Hz where fr is the resonance frequency.    Also explain the simulated graphs with the theoretical values.                                                                                      (7 marks)
Last Digit of

Student ID    Inductance:  L (mH)    Capacitance:  C (µf)
0    10    5
1    12    4
2    15    6
3     18    10
4     12    8
5    20    12
6    12    20
7    14    10
8    18    8
9    12    10
Table 2:  L and C values for Q2

Q2A. Three-phase AC Circuits

A balanced star connected load consisting per phase impedance of Z ohm. The value of Z is given in Table 3 against the last digit of your student ID number. The load is connected to a balanced 3-phase supply of 410V (Line Voltage), 50Hz, star connected, using three-phase 4-wire system. The phase sequence is ABC. In Multisim:
(i) Simulate the three-phase circuit and measure the magnitude of line current and phase current.

Check your answers by calculation.

(5 Marks)

(ii) Measure the total real power consumed by the load and power factor of the circuit, using a suitable wattmeter method. Check your answer by a calculation. From the measurements of real power and power factor values, calculate the reactive power flow in the circuit.
(5 Marks)

(iii) Measure the total real power consumed by the load; if the impedance of the load in phase B is changed to 10-j5? (keeping all other load impedance values same as in part (a)). Check your answer by calculation. For this loading condition, determine the reactive power flow in the circuit.
(5 Marks)

Table 3 Load Impedance (Z) values for Q3
Last digit of Student ID    ZR (Ohm)
0    10-j8
1    12+j10
2    12-j10
3    10+j5
4    12+j8
5    12+j10
6    9+j6
7    15-j12
8    10-j8
9    12+j7

Q2B. Star-Delta Connection

(i) In a three-phase AC circuits, capacitor banks can be used to   improve the power factor of the circuit. Explain, whether capacitor banks should be connected in star or delta. Your explanation should include relative advantages and disadvantages of both methods, with appropriate equations.                                                                                              (5 Marks)

(ii) A three phase water heater has been installed in an industrial installation and it has three
Elements rated at 240 V each with a resistance of 10 O. The elements could be connected
Either in star or delta.  Calculate the difference in the power consumed by the water heater
for both cases.                                                                                                            (5 marks)

Question 3.

(a) What are hysteresis and eddy current losses? Also explains how to minimize the iron losses in the
Transformer ?
(8 Marks)
(b) How to identify better magnetic material from the B-H characteristics of several materials?
(4 Marks)
(c) Briefly explain what information of the core is necessary to solve linear and non-linear magnetic circuit.  (3 marks)

Important: Your contribution to each answer is important, you must balance view points and data from all independent sources of information i.e. books, journal publications and website (Wikipedia should not be used!!!).  In text citation and referencing, all sources including diagrams must be referenced using the CU Harvard system.

Q4. Transformer Open & Short Circuit Test

IMPORTANT:  Student is responsible to collect the data from Lab assistant (Mr. Alex
Beaumont) including the physical signature. This information is essential to ensure the originality
of your results. If the original signature is not included in this paper the results are considered not
valid.

Open Circuit Test Data

Low-voltage winding voltage
(V)
Ammeter
(A)
Wattmeter
(W)    High-Voltage Winding Voltage (V)
15    0.030    0.3    31
25    0.041    0.7    51
35    0.052    1.3    71
45    0.063    2.1    91
55    0.073    2.9    110
65    0.085    4.0    130
75    0.100    5.2    151
85    0.117    6.4    170
95    0.139    8.0    191
105    0.168    9.7    211
115    0.216    11.7    231
120    0.246    12.7    240

Short Circuit Test Data

High-voltage winding current
(A)
Voltage
(V)
Wattmeter
(W)    Low-Voltage Winding Current
(A)
0.390    1.2    0.3    0.772
0.884    2.8    1.6    1.761
1.350    4.2    3.7    2.700
1.850    5.9    7.0    3.740
2.350    7.4    11.3    4.744
2.850    8.9    16..3    5.679
3.350    10.5    22.8    6.735
3.850    12.1    29.9    7.695
4.200    13.2    36    8.435

STUDENT INFORMATION

Family Name

Forename

ID Number (from your student card) 5955668

Lab ASSISTANT

Family Name:
Forename:
Signature :     Date:

Based  on  the  test  results,  you  need  to  answer  the  following  questions  and prepare
the final report. Your report should include any calculations and relevant formula used to
answer these questions.

1. Does the transformer draws current from the source, even there is no-load connected to
it ? If yes, explain why?
(2 Mark)

2.  Determine the magnetising and working component of the no-load current and draw the equivalent circuit and phase diagram of the open circuit test?
(5 Mark)

3.  Determine the copper loss of the transformer when it supplies 50% and 80% of full-load.
(2 Mark)

4.  Determine the efficiency of the transformer at full-load and half-load for 0.85 power factor lagging conditions.
(2 Mark)

5.  Determine the loading condition at which the efficiency of the transformer would be the maximum?
(2 Mark)

6.  Explain the benefits of testing transformer by indirect methods, such as open circuit
and short circuit tests.
(2 Mark)

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