ECE3051 2014 Exam PDF

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ECE3051 - Semester 2, 2014 Office Use Only

Semester Two 2014 Examination Period Faculty of Engineering EXAM CODES:

ECE3051

TITLE OF PAPER:

Electrical Energy Systems

EXAM DURATION:

3 hours writing time

READING TIME:

10 minutes

THIS PAPER IS FOR STUDENTS STUDYING AT:  Berwick  Caulfield  Pharmacy

 Clayton  Malaysia  Gippsland  Peninsula  Other (specify)

 Off Campus Learning  Open Learning  Enhancement Studies  Sth Africa

Attempt all questions. During an exam, you must not have in your possession, a book, notes, paper, electronic device/s, calculator, pencil case, mobile phone or other material/item which has not been authorised for the exam or specifically permitted as noted below. Any material or item on your desk, chair or person will be deemed to be in your possession. You are reminded that possession of unauthorised materials, or attempting to cheat or cheating in an exam is a discipline offence under Part 7 of the Monash University (Council) Regulations.

No examination papers are to be removed from the room. AUTHORISED MATERIALS: CALCULATORS

 YES

 NO

OPEN BOOK

 YES

 NO

SPECIFICALLY PERMITTED ITEMS

 YES

 NO

All answers and solutions must be contained under each question within the provided spaces. Empty back pages can be used for working but their content will not be marked. Students must complete this section.

STUDENT ID

__ __ __ __ __ __ __ __

DESK NUMBER

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1

ECE3051 - Semester 2, 2014 Question 1. (10 marks) A 3-phase load connected in delta is supplied from a 3-phase 415-V feeder. The load impedances are as follows: ZAB = 120 Ω, ZBC = j 3⋅120 Ω, ZCA = -j 3⋅120 Ω. (a) Draw circuit diagram showing the load connections.

(1 mark)

(b) Calculate all three line currents in the feeder supplying the load. (3 marks)

IA = IB = IC =

2

ECE3051 - Semester 2, 2014 (c) Calculate the overall amount of active and reactive power supplied to the load.

(3 marks)

Pload = Qload =

(d) Can this load be considered as balanced or unbalanced? Circle the correct answer. balanced

(3 marks)

unbalanced

3

ECE3051 - Semester 2, 2014 Question 2. (10 marks) A 3-phase, 415 V, 50 Hz, 6-pole induction motor has returned the following no-load test results: • •

P = 0.5 kW, Q = 12.5 kVAr during the zero slip test P = 1.2 kW, Q = 13 kVAr during the small slip test

The same motor underwent a locked-rotor test giving the following readings: •

P = 1.5 kW, Q = 1.85 kVAr

where P and Q are the total active and reactive power supplied to the motor during these tests. (a) What is the amount of iron losses in the motor?

(1 mark)

P0 =

(b) What is the amount of rotational losses in the motor?

(1 mark)

Pr =

(c) When the same motor outputs its full rated power of 25 kW it runs with a 3% slip. What is the motor rated torque? (2 marks)

Tout =

(d) Calculate efficiency of this motor under the load condition in (c).

(2 marks)

η=

4

ECE3051 - Semester 2, 2014 (e) Calculate the motor power factor under the load condition in (c).

(2 marks)

PF =

(f) The same machine is made to run with slip s = 2. Circle the correct state of the machine operation. (2 marks) motoring

generating

plugging

5

ECE3051 - Semester 2, 2014 Question 3. (10 marks) A 3-phase, 50-Hz, 150 rpm, 5-MVA, 11-kV synchronous motor has a synchronous reactance of 24 Ω. The stator resistance is negligible. A test with the use of a stroboscope has revealed that the mechanical torque angle is equal to 2.4° when the motor outputs 4 megawatts. (a) Calculate the motor excitation voltage under the 4-MW output.

(2 mark)

E0 =

(b) Calculate the motor stator current and power factor under the load condition in (a).

(2 marks)

Is = PF =

(c) Adjust the motor excitation voltage such that it runs with unity power factor under the same load. (3 marks)

E0 @PF=1 =

6

ECE3051 - Semester 2, 2014 (d) Draw phasor diagram representing the motor operating condition described in (a). The diagram must include all voltages and currents of the machine. (3 marks)

7

ECE3051 - Semester 2, 2014 Question 4. (10 marks) (a) Three-phase transformer winding configurations are described in terms of vector group numbers. Explain the meaning of each term in the following group numbers: (i) Dy11; (ii) Ydy6 (3 marks)

(i) D–

(ii) Y–

y–

d–

11 –

y–

6–

(b) A distribution transformer has the high voltage winding connected in delta and the low voltage winding connected in star. Draw phasor diagrams representing all possible cases of phase angle displacements between HV side and LV side voltages of this transformer. (3 marks)

(c) In a Y-y connected, 3-phase transformer the presence of harmonics and the presence of unbalanced loads can have one effect on the transformer performance that is similar in both cases. Name the effect. (2 marks)

8

ECE3051 - Semester 2, 2014 (d) Sketch transformer windings configured in open-delta connected to a 3-phase supply.

(1 mark)

(e) When and why open-delta connection can be used in practice? Circle the correct answer.

(1 mark)

3-phase transformer bank

shell-type transformer

core-type transformer

9

ECE3051 - Semester 2, 2014 Question 5. (10 marks) The armature of a separately excited DC motor is supplied from a 250-V source with 360 A of current. At full load, the motor runs at 680 RPM. At no load, the motor speed increases to 750 RPM if the field current is unchanged. The machine rotational losses are 2.5 kW. Neglect the armature reaction throughout. Calculate the following: (a) The armature voltage of the motor at no load. Assume that at no load the voltage drop in the armature resistance is negligible. (2 mark)

Ea =

(b) The armature resistance of the motor.

(2 marks)

Ra =

(c) The mechanical power output and torque of the motor at full load.

(2 marks)

P out = T out =

10

ECE3051 - Semester 2, 2014 (d) The motor speed at full load must be increased to 750 RPM with the voltage supplied to the armature and the output power remaining unchanged. Is this done by an increase or a decrease of the field current? Circle the correct answer and calculate the relative change of field current required. (2 mark) increase

decrease

If (@750rpm) =

(e) Find armature current under the load condition in (d). Assume linear relationship between field current and EMF. Show the necessary calculation and discussion to prove the result. (2 marks)

Ia =

11

ECE3051 - Semester 2, 2014 Question 6. (10 marks) A single phase half wave SCR rectifier is fed from a 283V rms, 50Hz, AC source and connected to an RL load comprising of a 20 Ω resistor and a 36.8 mH inductor. The firing delay angle is ∝=45.2°. (a) Develop a numeric time varying expression that describes the current flow through the RL load, as a function of angular time ฀t. Determine numeric values for all constants as part of your solution. (4 marks)

(b) Determine the average DC output voltage that develops across the load.

(2 marks)

(c) If a freewheeling diode is added across the RL load, calculate the new average DC output voltage across the load for the same operating condition. (2 marks)

(d) Calculate the firing delay angle required to achieve an output DC voltage of 100V rms if a full wave SCR rectifier is now used to supply the same RL load. (2 marks)

12

ECE3051 - Semester 2, 2014 Question 7. (10 marks) A single phase full wave SCR rectifier shown below is supplied from a 100V peak, 50Hz, AC source. The load is an RL load with a resistance of 10 Ω and a small load inductance of 10mH (zero resistance). The firing delay angle ∝=90°.

(a) Provide the output voltage and output current waveforms for two cycles (up to 4π).

(4 marks)

(b) Is the current continuoes? What will be the range of delay angle to make the current continuous (2 marks)

(c) Write down expressions for CONTINUOUS load currents for one complete cycle (∝...