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The Open University of Sri Lanka Department of Electrical and Computer Engineering ECX4238-Electrical Machines Academic Year- 2013/2014 Course Information

The course consists of

Three assignments #1, #2 and #3. The assignments should be submitted on or before due

dates as mentioned in the activity diary..

Laboratory work: Laboratory work is compulsory and you should get a minimum mark

of 40%. Dates of the lab classes are specified in the activity diary.

Continuous Assessment Test (CAT): There will be two CAT on the dates specified in

the activity diary which are of open book type.

Day schools: There are three day schools on the dates specified in the activity diary.

Although day schools are not compulsory, you are advised to attend to these day schools.

Final examination: This will be of closed book type. The question paper will be of

duration 3hours.

Eligibility Criteria: Average of the best five out of six activities 40%, AND Lab 40%. Assignment No 1 (To be submitted on or before as mentioned in the activity diary 2013/2014)

Q1

a. Compare the ONAF and OFAN cooling methods of a transformer.

b. A 100MVA, 230/115-kV - three-phase power transformer has a per-unit resistance of

0.02 pu and a per-unit reactance of 0.055 pu. The excitation branch elements are Rc = 110

pu and Xm= 20 pu.

i. If this transformer supplies a load of 80 MVA at 0.85 PF lagging, draw the phasor

diagram of one phase of the transformer.

ii. What is the voltage regulation of the transformer bank under these conditions?

iii. Sketch the equivalent circuit referred to the low-voltage side of one phase of this

transformer. Calculate all the transformer impedances referred to the low-voltage

side.

Q2

a. Group connection of a certain transformer is 41Dy-11. Explain what this term means

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b. Draw the vector diagrams and the winding connection of the three phase transformer

having vector symbol Dy11.

c. Three single-phase, 50 kVA, 2300/ 230 V, 60 Hz transformers are connected to form a 3-

phase, 4000V / 230-V transformer bank. The equivalent impedance of each transformer

referred to low-voltage is 0.012 + j 0.016 W. The 3-phase transformer supplies a 3-phase,

120 kVA, 230 V, 0.85 power-factor (lagging) load.

i. Draw a schematic diagram showing the transformer connection.

ii. Determine the winding currents of the transformer.

iii. Determine the primary voltage (line to line) required.

Q3

a. Derive an expression for load (kVA) on a transformer at maximum efficiency.

b. A 11000/230 V, 150 kVA single phase, 50 Hz transformer has core loss of 1.4kW and

full load Cu loss of 1.6 kW. Determine

i. The kVA load for maximum efficiency and maximum efficiency at unity power

factor.

ii. The efficiency at half load at 0.8 p.f leading c. A 30 kVA, 1000/250 V single-phase transformer is subjected to short circuit test. All the

equipments are connected to high voltage side and following readings are observed.

S.C. test: 80V, 30 A and 180 W

The maximum efficiency occurs at unity power factor and at 1.2 times full load current.

Determine full load efficiency at 0.8 power factor lagging. Also calculate the maximum

efficiency

Q4

a. What are the methods used for speed controlling of a DC shunt motor? Describe briefly

one of the methods.

b. A 7.460 kW, 230 V, 1000 rpm DC shunt motor has an armature resistance of 0.08 and shunt field current of 2A.The full load efficiency of the motor is 85%. By inserting a

resistance in the armature circuit the speed of the motor is reduced to 500 rpm, the load

torque remains constant. Find

i. The armature current

ii. External resistance

iii. Motor output power

iv. Overall efficiency

(Hint: Assume that all losses except copper losses vary directly with the speed of the motor)