MCQOPTIONS
Saved Bookmarks
MCQOPTIONS
This section includes 309 Mcqs, each offering curated multiple-choice questions to sharpen your Electrical Machines knowledge and support exam preparation. Choose a topic below to get started.
| 151. |
Two single-phase transformers T |
| A. | 614 |
| B. | 525 |
| C. | 555 |
| D. | None of these |
| Answer» D. None of these | |
| 152. |
A 220 V, 3-phase, 4-pole, 50 Hz inductor motor of wound rotor type is supplied at rated voltage and frequency. The stator resistance, magnetizing reactance, and core loss are negligible. The maximum torque produced by the rotor is 225% of full load torque and it occurs at 15% slip. The actual rotor resistance is 0.03 Ω/phase. The value of external resistance (in Ohm) which must be inserted in a rotor phase if the maximum torque is to occur at start is ___________. |
| A. | 17 |
| B. | 0.17 |
| C. | 1.7 |
| D. | All of the above |
| Answer» C. 1.7 | |
| 153. |
An 8-pole, 3-phase, 50 Hz induction motor is operating at a speed of 700 rpm. The frequency of the rotor current of the motor in Hz is _______ |
| A. | 30.03 |
| B. | 33.3 |
| C. | 3.33 |
| D. | None of the above |
| Answer» D. None of the above | |
| 154. |
A shunt-connected DC motor operates at its rated terminal voltage. Its no-load speed is 200 radians/ second. At its rated torque of 500 Nm, its speed is 180 radian/second. The motor is used to directly drive a load whose load torque T |
| A. | 179.86 |
| B. | 179 |
| C. | 86 |
| D. | 17 |
| Answer» B. 179 | |
| 155. |
A 15kW, 230 V dc shunt motor has armature ci r cuit r esi st ance of 0.4 Ω and field cir cuit resistance of 230 Ω. At no load and rated voltage, the motor runs at 1400 rpm and the line current drawn by the motor is 5 A. At full load, the motor draws a line current of 70A. Neglect armature reaction. The full load speed of the motor in rpm is ________. |
| A. | 540 |
| B. | 120 |
| C. | 1240 |
| D. | 140 |
| Answer» D. 140 | |
| 156. |
A star connected 400V, 50Hz, 4 pole synchronous machine gave the following open circuit and short circuit test results : |
| A. | 156 |
| B. | 15.06 |
| C. | 16 |
| D. | 5.06 |
| Answer» C. 16 | |
| 157. |
If height to width ratio of the window of core-type transformer is increased, then |
| A. | its leakage reactance and percentage voltage regulation will decrease |
| B. | its leakage reactance and percentage voltage regulation will increase |
| C. | its leakage reactance will be unaffected but total space in the window will increase to accommodate the windings better |
| D. | efficiency will increase and temperature rise will decrease |
| Answer» B. its leakage reactance and percentage voltage regulation will increase | |
| 158. |
With an armature voltage of 100V and rated field winding voltage, the speed of a separately excited DC motor driving a fan is 1000 rpm, and its armature current is 10A. The armature resistance is 1Ω. The load torque of the fan load i s proportional to the square of the rotor speed. Neglecting rotational losses, the value of the armature voltage (in Volt) which will reduce the rotor speed to 500 rpm is ___________. |
| A. | 47.5 |
| B. | 37.5 |
| C. | 45 |
| D. | 21 |
| Answer» B. 37.5 | |
| 159. |
A 3 phase, 400 V, 5 k W, star connected synchronous motor having an internal reactance of 10 W is operating at 50% load, unity pf. Now, the excitation is increased by 1%. What will be the new load in percent, if the power factor is to be kept same? Neglect all losses and consider |
| A. | 67.9% |
| B. | 56.9% |
| C. | 51% |
| D. | 50% |
| Answer» B. 56.9% | |
| 160. |
A 3 phase, 10 kW, 400 V, 4 pole, 50 Hz, star connected induction motor draws 20 A on full load. Its no load and blocked rotor test data are given below : |
| A. | 76% |
| B. | 81% |
| C. | 82.4% |
| D. | 85% |
| Answer» C. 82.4% | |
| 161. |
On the two sides of a star/delta transformer |
| A. | voltages and currents are both in phase |
| B. | voltages and currents both differ in phase by 30 |
| C. | voltages differ in phase by 30 but currents are in phase |
| D. | currents differ in phase by 30 but voltages are in phase |
| Answer» C. voltages differ in phase by 30 but currents are in phase | |
| 162. |
A /Y transformer has a phase-to-phase voltage transformation ratio of (delta phase): 1 (star phase). The line-to-line voltage ratio Y/ is given by |
| A. | <table><tr><td style="border-bottom:1px solid #000000;vertical-align:bottom;padding-bottom:2px;"><center> <span style=" text-decoration: overline;">3</span></center></td></tr><tr><td style="text-align: center;">a</td></tr></table> |
| B. | <table><tr><td style="border-bottom:1px solid #000000;vertical-align:bottom;padding-bottom:2px;"><center>a <span style=" text-decoration: overline;">3</span></center></td></tr><tr><td style="text-align: center;">1</td></tr></table> |
| C. | <table><tr><td style="border-bottom:1px solid #000000;vertical-align:bottom;padding-bottom:2px;"><center> <span style=" text-decoration: overline;">3</span></center></td></tr><tr><td style="text-align: center;">a</td></tr></table> |
| D. | a |
| Answer» D. a | |
| 163. |
A 2/1 ratio, two-winding transformer is connected as an auto transformer. Its kVA rating as an auto t r ansfor mer compared to at wo-winding transformer is |
| A. | same |
| B. | 1.5 times |
| C. | 2 times |
| D. | 3 times |
| Answer» E. | |
| 164. |
A transformer on no-load is switched on to a source of voltage. It will draw a current, which is |
| A. | the same as the steady-state magnetizing current. |
| B. | several times the steady-state magnetizing current depending upon the initial state of the residual flux in the transformer core. |
| C. | several times the steady-state magnetizing current independent of the initial state of the residual flux in the transformer core. |
| D. | twice the steady-state magnetizing current provided the core has no residual flux. |
| Answer» C. several times the steady-state magnetizing current independent of the initial state of the residual flux in the transformer core. | |
| 165. |
A transformer has maximum efficiency at full load. Compare its all-day efficiency when operated |
| A. | Efficiency will be the same in all the three cases. |
| B. | Efficiency will be highest in case (i) and lowest in case (ii). |
| C. | Efficiency will be lowest in case (i) and highest in case (iii). |
| D. | Efficiency will be highest in case (i) and lowest in case (iii). |
| Answer» E. | |
| 166. |
The applied voltage of a certain transformer is increase by 50% while the frequency is reduced to 50%. The maximum core flux density will |
| A. | become three times |
| B. | become1.5 times |
| C. | become half |
| D. | remain the same |
| Answer» B. become1.5 times | |
| 167. |
Building steel core out of stampings reduces eddy current loss because it |
| A. | increases core resistivity. |
| B. | increases the effective length of eddy current paths thereby increasing effective resistance to the flow of eddy currents. |
| C. | increases core permeability. |
| D. | reduces the effective length of eddy current path, thereby reducing effective resistance to the flow of eddy currents. |
| Answer» C. increases core permeability. | |
| 168. |
Transformer excited from a sinusoidal voltage source will draw a no-load current, which |
| A. | is sinusoidal and of the same frequency as the voltage source. |
| B. | comprises the fundamental frequency (same as that of the voltage source) and the peaking third harmonic. |
| C. | comprises the fundamental frequency and the depressing third harmonic. |
| D. | is sinusoidal of frequency three times that of the voltage source. |
| Answer» C. comprises the fundamental frequency and the depressing third harmonic. | |
| 169. |
For fixed primary voltage how does the secondary terminal voltage of a transformer change if the pf of inductive load is raised from 0.7 to 0.9 with load current remaining fixed? |
| A. | No change in voltage |
| B. | Voltage decreases |
| C. | Voltage rises |
| D. | Voltage will rise, go through a maximum and then decrease |
| Answer» D. Voltage will rise, go through a maximum and then decrease | |
| 170. |
Hysteresis losses are directly proportional to the |
| A. | square of frequency and flux density to the power 1.6. |
| B. | frequency and proportional to flux density to the power 1.6. |
| C. | the square of frequency and proportional to the square of flux density. |
| D. | frequency and proportional to flux density. |
| Answer» C. the square of frequency and proportional to the square of flux density. | |
| 171. |
Power input to a transformer on no load at rated voltage comprises predominantly |
| A. | copper loss |
| B. | hysteresis loss |
| C. | core loss |
| D. | eddy current loss |
| Answer» D. eddy current loss | |
| 172. |
Which of the following tests must be performed on a transformer to determine its leakage reactance? |
| A. | SC test only |
| B. | OC test only |
| C. | Both OC and SC tests |
| D. | test by an impedance bridge |
| Answer» D. test by an impedance bridge | |
| 173. |
A 2500/250 V, 25 kVA transformer is connected as an auto transformer to give 2300/2750 V. Its rating would be |
| A. | 250 kVA; 225 kVA is transformed conductively |
| B. | 250 kVA; 225 kVA is transformed inductively |
| C. | 275 kVA; 250 kVA is transformed inductively |
| D. | 275 kVA; 250 kVA is transformed conductively |
| Answer» E. | |
| 174. |
Two single-phase transformer rated 1000 kVA and 500 kVA have per unit leakage impedance of (0.02 + j0.06) and (0.025 + j0.08) respectively. Then, the largest kVA load delivered by the parallel combination of these two transformers without overloading any one is |
| A. | 1825.3 kVA |
| B. | 1377.44 kVA |
| C. | 1500 kVA |
| D. | 3202.74 kVA |
| Answer» C. 1500 kVA | |
| 175. |
In a transformer the phase angle between primary and secondary terminal voltage is |
| A. | 90 ; primary voltage leading the secondary voltage |
| B. | 90 ; primary voltage lagging the secondary voltage |
| C. | a few degrees, primary voltage leading the secondary voltage |
| D. | a few degrees, primary voltage lagging the secondary voltage |
| Answer» D. a few degrees, primary voltage lagging the secondary voltage | |
| 176. |
The efficiency of a 100 kVA transform is 0.98 at full as well as at half load. For this transformer at full load the copper loss is |
| A. | less than core loss |
| B. | equal to core loss |
| C. | more than core loss |
| D. | none of these |
| Answer» D. none of these | |
| 177. |
Voltage regulation of a large transformer is mainly influenced by |
| A. | no-load current and load power factor |
| B. | winding resistances and load power factor |
| C. | leakage fluxes and load power factor |
| D. | winding resistances and core loss |
| Answer» C. leakage fluxes and load power factor | |
| 178. |
The efficiency of a transformer at full-load 0.85 pf lag is 95%. Its efficiency at full-load 0.85 pf lead will be |
| A. | less than 95% |
| B. | more than 95% |
| C. | 95% |
| D. | 100% |
| Answer» D. 100% | |
| 179. |
In a Scott-connected transformer, the number of primary and teaser turns respectively are |
| A. | <table><tr><td rowspan="2"> N, </td><td style="border-bottom:1px solid #000000;vertical-align:bottom;padding-bottom:2px;"><center>2</center></td><td rowspan="2"> N </td></tr><tr><td style="text-align: center;"> <span style=" text-decoration: overline;">3</span></td></tr></table> |
| B. | <table><tr><td style="border-bottom:1px solid #000000;vertical-align:bottom;padding-bottom:2px;"><center>N</center></td><td rowspan="2">, N </td></tr><tr><td style="text-align: center;">2</td></tr></table> |
| C. | <table><tr><td style="border-bottom:1px solid #000000;vertical-align:bottom;padding-bottom:2px;"><center> <span style=" text-decoration: overline;">3</span></center></td><td rowspan="2"> N , N </td></tr><tr><td style="text-align: center;">2</td></tr></table> |
| D. | <table><tr><td rowspan="2"> N, </td><td style="border-bottom:1px solid #000000;vertical-align:bottom;padding-bottom:2px;"><center> <span style=" text-decoration: overline;">3</span></center></td><td rowspan="2"> N </td></tr><tr><td style="text-align: center;">2</td></tr></table> |
| Answer» E. | |
| 180. |
A 1 k VA, 230 V/ 100V, single phase, 50 H z transformer having negligible winding resistance and leak age induct ance i s oper at ing under saturation, while 250V, 50 Hz sinusoidal supply is connected to the high voltage winding. A resistive load is connected to the low voltage winding which draws rated current. Which one of the following quantities will not be sinusoidal? |
| A. | Voltage induced across the low voltage winding |
| B. | Core flux |
| C. | Load current |
| D. | Current drawn from the source |
| Answer» E. | |
| 181. |
A single-phase transformer is to be switched to the supply to have minimum inrush current. The switch should be closed at |
| A. | maximum supply voltage |
| B. | zero supply voltage |
| C. | <table><tr><td style="border-bottom:1px solid #000000;vertical-align:bottom;padding-bottom:2px;"><center>1</center></td><td rowspan="2"> maximum supply voltage </td></tr><tr><td style="text-align: center;"> <span style=" text-decoration: overline;">2</span></td></tr></table> |
| D. | <table><tr><td style="border-bottom:1px solid #000000;vertical-align:bottom;padding-bottom:2px;"><center>1</center></td><td rowspan="2"> maximum supply voltage </td></tr><tr><td style="text-align: center;">2</td></tr></table> |
| Answer» B. zero supply voltage | |
| 182. |
A 2400/240 V, 200 kVA single-phase transformer has a core loss of 1.8 kW at rated voltage. Its equivalent resistance is 1.1 percent. Then, the transformer efficiency at 0.9 pf and on full-load is |
| A. | 97.57% |
| B. | 98.05% |
| C. | 97.82% |
| D. | 96.56% |
| Answer» D. 96.56% | |
| 183. |
The hysteresis eddy current losses of a single phase transformer working on 200 V, 50 Hz supply are P |
| A. | 32, 36 |
| B. | 20, 36 |
| C. | 25, 20 |
| D. | 40, 80 |
| Answer» C. 25, 20 | |
| 184. |
Supply to one terminal of a delta-wye connection three-phase core type transformer which is on no-load, fails. Assuming magnetic circuit symmetry, voltages on the secondary side will be |
| A. | 230, 230, 115 |
| B. | 230, 115, 115 |
| C. | 345, 115, 115 |
| D. | 345, 0, 345 |
| Answer» B. 230, 115, 115 | |
| 185. |
Keeping in view the requirement of parallel operation, which of the 3-phase connections given below are possible? |
| A. | delta-delta to delta-star |
| B. | delta-delta to star-delta |
| C. | star-star to delta-delta |
| D. | delta-star to star-delta |
| Answer» D. delta-star to star-delta | |
| 186. |
Auto-transformer is used in transmission and distribution |
| A. | when operator is not available |
| B. | when iron losses are to be reduced |
| C. | when efficiency considerations can be ignored |
| D. | when the transformation ratio is small |
| Answer» E. | |
| 187. |
A voltage v = 400 sin 314.16 t is applied to a 1 phase transformer on no-load. If the no load current of the transformer is 2 sin (314.16 t 85 ), |
| A. | 141 90 |
| B. | 200 85 |
| C. | 200 85 |
| D. | 282 80 |
| Answer» D. 282 80 | |
| 188. |
A 220/440 V, 50 Hz, 5 kVA single phase transformer operates on 220 V, 40 Hz supply with secondary winding. Then |
| A. | the eddy current loss and hysteresis loss of the transformer decrease |
| B. | the eddy current loss and hysteresis loss of the transformer increase |
| C. | the hysteresis loss of the transformer increases while eddy current loss remains the same |
| D. | the hysteresis loss remains the same whereas eddy current loss decreases |
| Answer» B. the eddy current loss and hysteresis loss of the transformer increase | |
| 189. |
A separately excited 300 V DC shunt motor under no load runs at 900 rpm drawing an armature current of 2 A. The armature resistance is 0.5 Ω. and leakage inductance is 0.01 H. When loaded, the armature current is 15 A. Then the speed in rpm is _____. |
| A. | 450 rpm |
| B. | 880 rpm |
| C. | 240 rpm |
| D. | 580 rpm |
| Answer» C. 240 rpm | |
| 190. |
A coil would on a magnetic core is excited from an ac voltage source. The source voltage and its frequency are both doubled. The eddy current loss in the core will become |
| A. | half |
| B. | remains same |
| C. | double |
| D. | four times |
| Answer» E. | |
| 191. |
A synchronous generator is connected to an infinite bus with excitation voltage E |
| A. | 3 |
| B. | 1.12 |
| C. | 3.10 |
| D. | 0.10 to 0.12 |
| Answer» E. | |
| 192. |
A single phase induction motor draws 12 MW power at 0.6 lagging power. A capacitor is connected in parallel to the motor to improve the power factor of the combination of motor and capacitor to 0.8 lagging. Assuming that the real and reactive power drawn by the motor remains same as before, the reactive power delivered by the capacitor in MAR is __________. |
| A. | 6.97 |
| B. | 6.97 to 7.03 |
| C. | 7.03 |
| D. | 6 |
| Answer» C. 7.03 | |
| 193. |
A 3 phase, 4 pole, 400 V, 50 Hz, star connected induction motor has following circuit parameters |
| A. | 63.6 Nm |
| B. | 74.3 Nm |
| C. | 190.8 Nm |
| D. | 222.9 Nm |
| Answer» C. 190.8 Nm | |
| 194. |
The no-load speed of a 230 V separately excited dc motor is 1400 rpm. The armature resistance drop and the brush drop are neglected. The field current is kept constant at rated value. The torque of the motor in Nm for an armature current of 8 A is ____________. |
| A. | 5.5 Nm |
| B. | 10 Nm |
| C. | 12.5 Nm |
| D. | None of the above |
| Answer» D. None of the above | |
| 195. |
A non-salient pole synchronous generator having synchronous reactance of 0.8 pu is supplying 1 pu power to a unity power factor load at a terminal volt age of 1.1 pu. Neglecting the armature resistance, the angle of the voltage behind the synchronous reactance with respect to the angle of the terminal voltage in degrees is ________. |
| A. | 33.61 |
| B. | 13 |
| C. | 31 |
| D. | 41.34 |
| Answer» B. 13 | |
| 196. |
A 50H z gener at i ng uni t has H -constant of 2 MJ/MVA. The machine is initially operating in steady state at synchronous speed, and producing 1 pu of real power. The initial value of the rotor angle is 5 , when a bolted three phase to ground short circuit fault occurs at the terminal of the generator. Assuming the input mechanical power to remain at 1 pu, the value of in degrees, 0.02 second after the fault is ________. |
| A. | 5.1 |
| B. | 5.7 to 6.1 |
| C. | 6 |
| D. | None of the above |
| Answer» C. 6 | |
| 197. |
A three-phase, 11 kV, 50 Hz, 2 pole, star connected, cylindrical rotor synchronous motor is connected to an 11 kV, 50 Hz source, Its synchronous reactance is 50 Ω per phase, and i t s st at or resistance is negligible. The motor has a constant field excitation. At a particular load torque, its stator current is 100A at unity power factor. If the load torque is increased so that the stator current is 120 A, then the load angle (in degrees) at this node is _____________. |
| A. | 48 to 46 |
| B. | 41 |
| C. | 51 |
| D. | 132 |
| Answer» B. 41 | |
| 198. |
The speed of a 4-pole induction motor is controlled by varying the supply frequency while maintaining the ratio of supply voltage to supply frequency (V/f) constant. At rated frequency of 50 Hz and rated voltage of 400 V its speed is 1440 rpm. What is the speed at 30 Hz, if the load torque is constant? |
| A. | 882 rpm |
| B. | 864 rpm |
| C. | 840 rpm |
| D. | 828 rpm |
| Answer» C. 840 rpm | |
| 199. |
Two transformers are to be operated in parallel such that they share load in proportion to their kVA ratings. The rating of the first transformer is 500 kVA and its pu leakage impedance is 0.05 pu. I f the rating of second transformer is 250 kVA, then its pu leakage impedance is |
| A. | 0.20 |
| B. | 0.10 |
| C. | 0.05 |
| D. | 0.025 |
| Answer» C. 0.05 | |
| 200. |
A 220 V DC machine supplies 20 A at 200 V as a generator. The armature resistance is 0.2 ohm. If the machine is now operated as a motor at same terminal voltage and current but with the flux increased by 10%, then ratio of motor speed to generator speed is |
| A. | 0.87 |
| B. | 0.95 |
| C. | 0.96 |
| D. | 1.06 |
| Answer» B. 0.95 | |