Question_Bank.pdf

---------- In an ideal transformer, A: windings have no resistance B: core has no losses C: core has infinite permeability D: all of the answers. ---------- Transformer cores are laminated in order to A: simplify its construction B: minimise eddy current loss C: reduce cost D: reduce hysteresis loss...

---------- In an ideal transformer, A: windings have no resistance B: core has no losses C: core has infinite permeability D: all of the answers. ---------- Transformer cores are laminated in order to A: simplify its construction B: minimise eddy current loss C: reduce cost D: reduce hysteresis loss ---------- A transformer has negative voltage regulation when its load power factor is A: zero B: unity C: leading D: lagging ---------- What is the function of a transformer? A: Transformer is used to step down or up the AC voltages and currents B: Transformer is used to step down or up the DC voltages and currents C: Transformer converts DC to AC voltages D: Transformer converts AC to DC voltages ---------- A 25 KVA transformer is constructed to a turns ratio of N1/N2 = 10. The impedance of primary winding is 3+j5 ohms and of secondary winding is 0.5+j0.8 ohms. What will be the impedance of transformer when referred to primary? A: 53 + 85j ohms B: 53j + 85 ohms C: Can’t be calculated D: 3.5 + 5.8j ohms ---------- Both resistances and leakage reactances of the transformer windings are __________ A: Series effects B: Parallel effects C: Series-parallel effects D: Cannot say ---------- In approximate equivalent circuit of the transformer _______________ A: All resistances and inductances are lumped before magnetizing branch B: All resistances and inductances are lumped after magnetizing branch C: Resistances and inductances aren’t changed D: Any of the above will work ---------- Silicon steel used in laminations, because it reduces ________________ A: Hysteresis loss B: Eddy current losses C: Copper losses D: Cannot be determined ---------- The iron loss of a transformer at 400 Hz is 10 W. Assuming that eddy current and hysteresis losses vary as the square of flux density, the iron loss of the transformer at rated voltage but at 50 Hz would be....... watt. A: 80 B: 640 C: 1.25 D: 100 ---------- In operating a 400 Hz transformer at 50 Hz A: only voltage is reduced in the same proportion as the frequency B: only kVA rating is reduced in the same proportion as the frequency C: both voltage and kVA rating are reduced in the same proportion as the frequency D: none of the above. ---------- A 200 kVA transformer has an iron loss of 1 kW and full-load Cu loss of 2kW. Its load kVA corresponding to maximum efficiency is....... kVA. A: 100 B: 141.4 C: 50 D: 200 ---------- If Cu loss of a transformer at 7/8th full load is 4900 W, then its full-load Cu loss would be.......watt. A: 5600 B: 6400 C: 375 D: 429 ---------- The ordinary efficiency of a given transformer is maximum when A: it runs at half full-load B: it runs at full-load C: its Cu loss equals iron loss D: it runs slightly overload. ---------- The output current corresponding to maximum efficiency for a transformer having core loss of 100 W and equivalent resistance referred to secondary of 0.25 Ω is....... ampere. A: 20 B: 25 C: 5 D: 400 ---------- The maximum efficiency of a 100-kVA transformer having iron loss of 900 kW and F.L. Cu loss of 1600 W occurs at....... kVA. A: 56.3 B: 133.3 C: 75 D: 177.7 ---------- The full-load copper loss of a transformer is 1600 W. At half-load, the copper loss will be _______ A: 1600 W B: 6400 W C: 400 W D: 800 W ---------- For a power transformer operating at full load it draws voltage and current equal to 200 V and 100 A respectively at 0.8 pf. Iron and copper losses are equal to 120 kW and 300kW. What is efficiency? A: 97.44% B: 99.12% C: 86.44% D: 96.44% ---------- Voltage regulation of transformer is given by _____________ A: V2-E2/E2 B: V2-E2/V2 C: E2-V2/V2 D: E2-V2/E2 ---------- To convert an ideal transformer into a practical transformer we add ____________ A: Primary winding resistance and secondary winding resistance B: Primary winding leakage reactance and secondary winding leakage reactance C: Primary winding resistance, leakage and secondary winding leakage reactance D: Cannot be determined ---------- Hysteresis loss and eddy current loss is directly proportional to __________ A: f and f2 B: f2 and f C: f and f D: f2 and f2 ---------- What will happen to hysteresis loss if voltage is doubled, load is doubled and frequency is halved? A: Will be twice B: Will be halved C: Will remain same D: Will be four times ---------- If the supply frequency to the transformer is increased, the iron loss will ___________ A: Not change B: Decrease C: Increase D: Cannot be determined ---------- The sole purpose of a commutator in a d.c. generator is to A: Increase output voltage B: Reduce sparking at brushes C: Reduce sparking at brushes D: Convert the induced a.c. into d.c. ---------- Lap winding is suitable for............ current,............ voltage d.c. generators A: High, low B: Low, high C: Low, low D: High, high ---------- As the load is increased, the speed of a d.c. shunt motor A: Increases proportionately B: Remains constant C: Increases slightly D: Reduces slightly ---------- When load is removed,.......... motor will run at the highest speed A: Shunt B: Cumulative compound C: Differential compound D: Series ---------- The series field of a short-shunt d.c. generator is excited by............ currents. A: shunt B: armature C: load D: external ---------- In a d.c. generator, the generated e.m.f. is directly proportional to the A: field current B: pole flux C: number of armature parallel paths D: number of dummy coils ---------- The commercial efficiency of a shunt generator is maximum when its variable loss equals............ loss. A: constant B: stray C: iron D: friction and windage ---------- As the load is increased, the speed of a d.c. shunt motor A: increases proportionately B: remains constant C: increases slightly D: reduces slightly ----------