04 Electronics Quiz | Basic Electronics Engineering MCQ | Advanced Level Practice Test
Electronics Quiz | Basic Electronics Engineering MCQ | Advanced Level Practice Test
1 >>A single-phase generator having a synchronous reactance of 5.5 and a resistance of 0.6 delivers a current of 100 A. Calculate the e.m.f. generated in the stator winding when the terminal voltage is 2000 V and the power factor of the load is 0.8 lagging. Sketch the phasor diagram. ?
2 >>A single-phase synchronous generator has a rated output of 500 kV A at a terminal voltage of 3300 V. The stator winding has a resistance of 0.6 and a synchronous reactance of 4 . Calculate the percentage voltage regulation at a power factor of: (a) unity; (b) 0.8 lagging; (c) 0.8 leading. Sketch the phasor diagram for each case. ?
3 >>A three-phase generating station contains a 50 MVA generator of 0.1 pu reactance and a 30 MVA generator of 0.08 pu reactance feeding the same 11 kV busbars. An outgoing 33 kV feeder having an impedance of (0.6 + j1.5) Ω/ph is supplied from these busbars through an 11/33 kV, 20 MVA transformer of reactance 0.1 pu. Calculate, by (a) the ohmic value method and (b) the pu method, the short-circuit current and MVA when a symmetrical three-phase short circuit occurs (i) on the feeder side of the transformer and (ii) at the far end of the feeder. Calculate also the busbar voltage during the short circuit for each case. ?
4 >>A factory is supplied from a substation containing two 500 kV A, 11 kV/400 V transformers connected in parallel, each having a reactance of 0.08 pu. A 750 kV A generator with 0.12 pu reactance also supplies the 400 V busbars which in turn supply a 400 V feeder having an impedance of (0.01 + j0.015) Ω/ph. Calculate the short-circuit current in the feeder and the voltage of the 400 V busbars when a symmetrical three-phase fault occurs at the far end of the feeder. That fault level of the 11 kV system is 100 MVA. ?
5 >>Explain how a rotating magnetic field may be produced by stationary coils carrying three-phase currents.Determine the efficiency and the output kilowatts of a three-phase, 400 V induction motor running on load with a fractional slip of 0.04 and taking a current of 50 A at a power factor of 0.86. When running light at 400 V, the motor has an input current of 15 A and the power taken is 2000 W, of which 650 W represent the friction, windage and rotor core loss. The resistance per phase of the stator winding (deltaconnected) is 0.5 Ω. ?
(A) 0.857 p.u., 25.57 kW
(B) 11 kW, 444 W
(C) 960 r/min, 844 W
(D) 27.5 per cent, 41 per cent
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6 >>If the star-connected rotor winding of a three-phase induction motor has a resistance of 0.01 Ω/ph and a standstill leakage reactance of 0.08 Ω/ph, what must be the value of the resistance per phase of a starter to give the maximum starting torque? What is the percentage slip when the starting resistance has been reduced to 0.02 Ω/ph, if the motor is still exerting its maximum torque? ?
(A) 0.07 Ù, 37.5 per cent
(B) 10.67 A, 0.376; 21.95 A, 0.303
(C) 9.36 mWb
(D) 11.33 A; 0.2, 40.8 A
7 >>A three-phase, 50 Hz induction motor with its rotor star-connected gives 500 V (r.m.s.) at standstill between the slip-rings on open circuit. Calculate the current and power factor at standstill when the rotor winding is joined to a star-connected external circuit, each phase of which has a resistance of 10 Ω and an inductance of 0.04 H. The resistance per phase of the rotor winding is 0.2 Ω and its inductance is 0.04 H. Also calculate the current and power factor when the slip-rings are short-circuited and the motor is running with a slip of 5 per cent. Assume the flux to remain constant. ?
(A) 0.07 Ù, 37.5 per cent
(B) 10.67 A, 0.376; 21.95 A, 0.303
(C) 9.36 mWb
(D) 11.33 A; 0.2, 40.8 A
8 >>A three-phase induction motor, at standstill, has a rotor voltage of 100 V between the slip-rings when they are open-circuited. The rotor winding is starconnected and has a leakage reactance of 1 Ω/ph at standstill and a resistance of 0.2 Ω/ph. Calculate: (a) the rotor current when the slip is 4 per cent and the rings are short-circuited; (b) the slip and the rotor current when the rotor is developing maximum torque. Assume the flux to remain constant. ?
9 >>Describe, in general terms, the principle of operation of a three-phase induction motor.The stator winding of a three-phase, eight-pole,50 Hz induction motor has 720 conductors, accommodated in 72 slots. Calculate the flux per pole of the rotating field in the airgap of the motor, needed to generate 230 V in each phase of the stator winding. ?
(A) 0.07 Ù, 37.5 per cent
(B) 10.67 A, 0.376; 21.95 A, 0.303
(C) 9.36 mWb
(D) 11.33 A; 0.2, 40.8 A
10 >>Explain the principle of action of a three-phase induction motor and the meaning of the term slip. How does slip vary with the load? A centre-zero d.c. galvanometer, suitably shunted, is connected in one lead of the rotor of a three-phase,six-pole, 50 Hz slip-ring induction motor and the pointer makes 85 complete oscillations per minute.What is the rotor speed? ?
(A) 971.7 r/min
(B) 1.585 Hz, 3.17 per cent
(C) 1470 r/min, 1 Hz
(D) 4.6 per cent, 954 r/min
11 >>Show how a rotating magnetic field can be produced by three-phase currents. A 14-pole, 50 Hz induction motor runs at 415 r/min. Deduce the frequency of the currents in the rotor winding and the slip. ?
(A) 971.7 r/min
(B) 1.585 Hz, 3.17 per cent
(C) 1470 r/min, 1 Hz
(D) 4.6 per cent, 954 r/min
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12 >>Explain how slip-frequency currents are set up in the rotor windings of a three-phase induction motor. A two-pole, three-phase, 50 Hz induction motor is running on load with a slip of 4 per cent. Calculate the actual speed and the synchronous speed of the machine. Sketch the speed/load characteristic for this type of machine and state with which kind of d.c. motor it compares. ?
(A) 2880 r/min, 3000 r/min
(B) 0.225 H, 223 Nm
(C) 71.1 A, 1920 V/ph
(D) 1200 kW, 0.388 leading
13 >>Two similar three-phase star-connected generators are connected in parallel. Each machine has a synchronous reactance of 4.5 Ω/ph and negligible resistance, and is excited to generate an e.m.f. of 1910 V/ph. The machines have a phase displacement of 30 electrical degrees relative to each other. Calculate: (a) the circulating current; (b) the terminal voltage/phase; (c) the active power supplied from one machine to the other. Sketch the phasor diagram for one phase. ?
(A) 111 A, 184.5 V, 615 kW
(B) 39.9 A, 3290 V, 131.3 kW
(C) 1650 V
(D) 12.5 per cent increase
14 >>Two single-phase generators are connected in parallel, and the excitation of each machine is such as to generate an open-circuit e.m.f. of 3500 V. The stator winding of each machine has a synchronous reactance of 30 ? and negligible resistance. If there is a phase displacement of 40 electrical degrees between the e.m.f.s, calculate: (a) the current circulating between the two machines; (b) the terminal voltage; (c) the power supplied from one machine to the other.Assume that there is no external load. Sketch the phasor diagram. ?
(A) 111 A, 184.5 V, 615 kW
(B) 39.9 A, 3290 V, 131.3 kW
(C) 1650 V
(D) 12.5 per cent increase
15 >>A 1500 kV A, 6.6 kV, three-phase, star-connected synchronous generator has a resistance of 0.5 Ω/ph and a synchronous reactance of 5 Ω/ph. Calculate the percentage change of voltage when the rated output of 1500 kV A at power factor 0.8 lagging is switched off. Assume the speed and the exciting current remain unaltered. ?
(A) 111 A, 184.5 V, 615 kW
(B) 39.9 A, 3290 V, 131.3 kW
(C) 1650 V
(D) 12.5 per cent increase
16 >>A three-phase, star-connected, 50 Hz generator has 96 conductors per phase and a flux per pole of 0.1 Wb. The stator winding has a synchronous reactance of 5 Ω/ph and negligible resistance. The distribution factor for the stator winding is 0.96. Calculate the terminal voltage when three non-inductive resistors, of 10 Ω/ph, are connected in star across the terminals. Sketch the phasor diagram for one phase. ?
(A) 111 A, 184.5 V, 615 kW
(B) 39.9 A, 3290 V, 131.3 kW
(C) 1650 V
(D) 12.5 per cent increase
17 >>A single-phase generator having a synchronous reactance of 5.5 Ω and a resistance of 0.6 Ω delivers a current of 100 A. Calculate the e.m.f. generated in the stator winding when the terminal voltage is 2000 V and the power factor of the load is 0.8 lagging. Sketch the phasor diagram. ?
(A) 150 Hz
(B) 4 poles
(C) 500 r/min
(D) 2412 V
18 >>The stator of an a.c. machine is wound for six poles, three-phase. If the supply frequency is 25 Hz, what is the value of the synchronous speed? ?
(A) 150 Hz
(B) 4 poles
(C) 500 r/min
(D) 2412 V
19 >>A star-connected balanced three-phase load of 30 Ω resistance per phase is supplied by a 400 V, threephase generator of efficiency 90 per cent. Calculate the power input to the generator. ?
(A) 6.4 kW
(B) 1.24
(C) 4.19 N m
(D) 8 poles, 31.2 mWb
20 >>The field-form of a synchronous machine taken from the pole centre line, in electrical degrees, is given below, the points being joined by straight lines.Determine the form factor of the e.m.f. generated in a full-pitch coil. Distance from pole centre (degrees) 0 20 45 60 75 90 105 120 135 Flux density (T) etc. ?
