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CONFIDENTIAL EE/APR 2007/KJK559

UNIVERSITI TEKNOLOGI MARAFINAL EXAMINATION

COURSECOURSE CODEEXAMINATIONTIME

: ELECTRICAL MACH INES & DRIVES: KJK559: APRIL 2007: 3 HOURS

INSTRUCTIONS TO CANDIDATES1 . This question paper cons ists of six (6) questions.2 . Answer FIVE (5) questions in Answer Booklet. Start each answer on a new page.3. Do not bring any material into the examination room unless permission is given by theinvigilator.4 . Please check to make sure that this examination pack consists o f:

i) the Qu estion Paperii) an Answ er Booklet - provided by the Faculty

DO NOT TURN THIS PAG E UNTIL YO U A RE TOL D TO D O S OThis examination paper consists of 5 printed pages

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CONFIDENTIAL 2 EE/APR 2007/KJK559

QUESTION 1a) A coil having a reactance of 6 Q and a resistance of 2 Q is conne cted in parallel with

a capacitive reactance of 5 Cl. If the single-phase supply voltage is 240 V, calculate;i) the active power supplied to the circuit,ii) the reactive power absorbed by the coil,iii) the reactive power generated by the capacitor,iv) the apparent power supp lied to the circuit. (6 marks)

b) A three-phase, 4 1 5 V, 50Hz substation supplies a factory consisting of the followingloads:i) Lighting loads of 15kW at unity power factor,ii) Air conditioning system loads of 4 0kW at 0.78 lagging power factor,iii) Motor loads with a total output of 260 hp at an efficiency of 95% and at 0.8lagging power factor.Determine the following:i) Active power absorbed by the motor loads.ii) Total active power taken from the substa tion.iii) Total reactive power absorbed by the loads .iv) The powe r factor at the subs tation.v) If a 3-phase capacitor bank rated at 180 KVAR is connected in parallel to thefactory, what is the new value of current taken from the substation and its newpower factor? (14 marks)

QUESTION 2a) A single-phase transformer rated at 400 kVA, 33 / 3.3 kV, 50 Hz, has an equivalentseries resistance of 30 Q and an equivalent series reactance of 125 Q referred to theprimary. The transformer is supplying full load at 3.3 kV and at a power factor of 0.8lagging. Calculate;

i) the voltage at the primary termina ls.ii) the voltage regulation at full load and at a power factor of 0.8 lagging ,iii) the efficiency of the transformer at full load and at a power factor of 0.8 lagging ,assuming the core losses is 3000 W. (10 marks)

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CONFIDEN TIAL 3 EE/APR 2007/KJK559

b) Three single-phase transformers each rated at 80 kVA, 2 300 / 2 30 V, 50 Hz, areconnected to form a 3-phase 3980 / 230 V transformer bank. The equivalentimpedance of each transformer referred to the low voltage side is (0.02 + 0.035) O.The 3-phase transformer supplies a 3-phase, 200kVA, 230 V, 0.8 lagging powerfactor load.i) Draw a schematic diagram showing the transformer connection.ii) Determ ine the load current, the phase currents in the wind ings , and the linecurrent at the primary side,iii) Determ ine the actual line voltage at the primary and the voltage regulation.

(10 marks)

QUESTION 3a) Describe with the help of diagrams , the two types of rotors of the 3-phase inductionmotor. State the advan tages of each type of rotor.

(8 marks)b) A 50 Hz three-phase induction motor having a synchronous speed of 1500 rpmdraws 1 20 kW from a 3-phase power supply. The copper losses of the stator totals3.4 kW and the iron losses amount to 3 kW.

Draw the complete power flow diagram of the motor.If the full load speed is 14 50 rpm , determine the following;i) The active power that crossed over to the rotor.ii) The rotor copper lossesiii) The mechan ical power developediv) The pow er output of the motor if the friction and winda ge losses are equal to 2.5kW .v) The induced torque at the rotorvi) The shaft torque or output torquevii) The efficiency of the motor. (12 marks)

QUESTION 4a) Sketch the schematic diagram of the following motors:

i) DC Shunt motorii) DC Series motor (4 m arks)b) Sketch the speed - torque curves of a DC shunt motor and a DC series motor. Explainbriefly the significant differences of the characteristic curves. (6 marks)

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CONFIDENTIAL 4 EE/APR 2007/KJK559

c) A DC shunt motor is connected to a 2 4 0 V DC supply. Its shaft is connected to amechanical load that causes the motor to take full load current from the supply, whichis 300 A.The speed of the motor at full load is 1400 rpm. Given that the armature resistance is0.03 Q and the field resistance is 120 Q, determine;i) the induced armature voltage EA if the rotational losses is 12 00 W.ii) the output power and output torque,iii) the efficiency of the motor. (10 marks)

QUESTION 5a) A 3-phase star-connected synchronous generator is rated at 10 MVA. The armatureresistance per phase is 0.2 D. and the synchronous reactance per phase is 1.2 Q. Ifthe line voltage at the terminals of the generator is 12.8 kV, calculate, at rated load,the excitation voltage EF and the voltage regulation at a power factor of:

i) 0.8 laggingii) 0.8 leadingDraw the phasor diagram for each of the above case, labeling the phasors properly.(12 marks)

b) A 3-phase, 2 4 -p ole , star-connected synchronous motor rated at 1000 kW, 3.3 kW,50 Hz, has a synchronous reactance per phase of 3.24 Q, the resistance beingnegligible.The motor is fed from a 3-phase supply at 3.3 kV. It is drawing rated power at 0.9power factor leading.i) What is the synchrono us speed of the motor?ii) Calculate the excitation voltage EFof the motor at the above conditions,iii) Determ ine the maximum power and torque the motor can deliver while itsexcitation remains constant. (8 marks)

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CONF IDENTIAL 5 EE/APR 2007/KJK559

QUESTION 6a) The input of a single-phase diode bridge rectifier is connected to a 2 40 V, 50 Hz, ac

power supply. A pure resistive load, which draws a dc current of 50 A, is connectedto the output of the rectifier. {Assume all diodes used in the rectifier are idealswitches).b) i) Draw the circuit schem atic of a single-phase diode bridge rectifier.ii) Calculate the dc voltage across the load, and the average dc current flowingthrough each diode.iii) Sketch the wave forms of the actual output voltage (V L(t)), and the average dcvoltage across the load (E d). (8 marks)

c) A 3-phase, 6-pulse thyristor converter produce s a variable outpu t voltage Ed, to aload represented by a resistor in series with an inductor. The line-to-line input voltageto the converter is 1 100 V, 60 Hz. When the delay angle is 45 , the load draws a dccurrent of 70 A.,Draw the schematic diagram of the 3-phase, 6-pulse thyristor converter with theinductive load, and calculate the following:i) the dc output voltage Edii) the active power supplied to the conve rteriii) the reactive power absorbed by the conve rteriv) the effective value of the input line current. (12 marks)

END OF QUESTION PAPER

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