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.
| 101. |
The power input to a 415 V, 50 Hz, 6 pole, 3-phase induction motor running at 975 rpm is 40 kW. If startor losses are 1 kW and friction and windage losses total 2 kW, then efficiency of the motor is |
| A. | 92.5% |
| B. | 91% |
| C. | 90% |
| D. | 88% |
| Answer» B. 91% | |
| 102. |
Assume that startor impedance of an induction motor is negligible. The frequency of voltage applied to the motor is increased keeping V/f constant. The maximum motor torque will |
| A. | increase and will occur at smaller values of slip |
| B. | increases and will occur at larger values of slip |
| C. | remain constant but will occur at smaller values of slip |
| D. | remains constant but will occur at larger values of slip |
| Answer» D. remains constant but will occur at larger values of slip | |
| 103. |
If a squirrel-cage induction motor having a rated slip of 2% on full-load has a starting torque of 0.5 full-load torque, then starting current is |
| A. | equal to full-load current |
| B. | twice full-load current |
| C. | four times full-load current |
| D. | five times full-load current |
| Answer» E. | |
| 104. |
The speed of an induction motor is controlled by varying supply frequency keeping V/f constant breakdown torque |
| A. | and magnetizing current would both remain constant |
| B. | would remain constant but magnetizing current would increase |
| C. | would decrease but magnetizing current would remain constant |
| D. | and magnetizing current would both decrease. |
| Answer» B. would remain constant but magnetizing current would increase | |
| 105. |
A 3-phase induction motor has a full-load slip of 3 percent at normal voltage. Which of the following will be the value or slip of the motor if it develops same torque theoretically while operating at 110 percent of its normal voltage? |
| A. | 2.48% |
| B. | 0.248% |
| C. | 0.483% |
| D. | 4.83% |
| Answer» B. 0.248% | |
| 106. |
A synchronous motor with negligible armature resistance runs at a load angle of 20 at the rated frequency. If supply frequency is increased by 10%, keeping other parameters constant, the new load angle will be |
| A. | 16 |
| B. | 18 |
| C. | 20 |
| D. | 22 |
| Answer» C. 20 | |
| 107. |
If an induction motor when started on load does not accelerate up to full speed but runs at 1/7th of the rated speed, then it is called |
| A. | locking |
| B. | plugging |
| C. | crawling |
| D. | clogging |
| Answer» D. clogging | |
| 108. |
An induction motor is fed from a balanced threephase supply at rated voltage and frequency through a bank of three single phase transformers connected in delta-delta. If one unit of the bank develops fault and is removed, then |
| A. | single phasing will occur and the machine fails to start |
| B. | single phasing will not occur but the motor terminal voltages will become unbalanced and machine can be loaded to the extent of 57.7% of its rating |
| C. | machine can be loaded to the extent of 57.7% of its rating with balanced supply at its terminals. |
| D. | machine can be loaded to the extent of 66.66% with balanced supply at its terminals. |
| Answer» E. | |
| 109. |
If rotor of an induction motor is assumed to be purely resistive, then angle between resultant flux density wave and rotor mmf wave is |
| A. | dependent upon the load |
| B. | 180 |
| C. | 90 |
| D. | 45 |
| Answer» D. 45 | |
| 110. |
Unbalanced supply voltage given to a 3-phase, delta-connected induction motor will cause |
| A. | zero sequence currents |
| B. | less heating of the motor |
| C. | negative sequence components current |
| D. | all of these |
| Answer» D. all of these | |
| 111. |
In three phase slip ring induction motor input is 440 V and the voltage at the slip rings is 110 V. If an input of 110 V is given to the slip rings, the startor voltage would be |
| A. | 500 V |
| B. | nearly equal to 440 V |
| C. | nearly equal to 110 V |
| D. | zero |
| Answer» C. nearly equal to 110 V | |
| 112. |
In a shaded-pole induction motor, the rotor runs from the |
| A. | shaded portion to the unshaded portion of the pole while the flux in the former leads that of the latter |
| B. | shaded portion to the unshaded portion of the pole while the flux in the former lags that in the latter |
| C. | unshaded portion to the shaded portion while the fluxing the former leads that in the latter |
| D. | unshaded portion to the shaded portion while the flux in the former lags that in the latter |
| Answer» E. | |
| 113. |
A single-phase startor winding when excited with ac voltage produces |
| A. | a single rotating field rotating at synchronous speed. |
| B. | two rotating fields rotating at synchronous speed in opposite directions. |
| C. | two rotating fields rotating in the same direction but at different speeds ' |
| D. | two rotating fields rotating in opposite directions but with difference speeds. |
| Answer» C. two rotating fields rotating in the same direction but at different speeds ' | |
| 114. |
Approximate value of the efficiency of a threephase induction motor running at a slip s is given by |
| A. | <table><tr><td style="border-bottom:1px solid #000000;vertical-align:bottom;padding-bottom:2px;"><center>1</center></td></tr><tr><td style="text-align: center;">1 + s</td></tr></table> |
| B. | <table><tr><td style="border-bottom:1px solid #000000;vertical-align:bottom;padding-bottom:2px;"><center>s</center></td></tr><tr><td style="text-align: center;">1 + s</td></tr></table> |
| C. | <table><tr><td style="border-bottom:1px solid #000000;vertical-align:bottom;padding-bottom:2px;"><center>1 s</center></td></tr><tr><td style="text-align: center;">1 + s</td></tr></table> |
| D. | <table><tr><td style="border-bottom:1px solid #000000;vertical-align:bottom;padding-bottom:2px;"><center>s</center></td></tr><tr><td style="text-align: center;">1 s</td></tr></table> |
| Answer» D. <table><tr><td style="border-bottom:1px solid #000000;vertical-align:bottom;padding-bottom:2px;"><center>s</center></td></tr><tr><td style="text-align: center;">1 s</td></tr></table> | |
| 115. |
Emf induced in the rotor of a 3-phase induction motor is proportional to the |
| A. | relative velocity between startor field and rotor conductor |
| B. | voltage applied to the startor |
| C. | slip |
| D. | all of these |
| Answer» B. voltage applied to the startor | |
| 116. |
The shunt resistance circuit component obtained by no-load test of an induction motor is representation of |
| A. | windage and friction loss only |
| B. | core loss only |
| C. | core loss and windage and friction loss |
| D. | copper loss only |
| Answer» D. copper loss only | |
| 117. |
In a 3-phase squirr el cage induction motor, skewing of rotor slots reduces |
| A. | parasitic torque and noise but increases pullout torque |
| B. | parasitic torque and noise but increases starting torque |
| C. | noise but increases pull out torque and parasitic torque |
| D. | parasitic torque, noise pullout torque and starting torque |
| Answer» E. | |
| 118. |
An induction motor and synchronous motor are connected to a common feeder line. To operate the feeder line at unity pf, synchronous motor should be |
| A. | under-excited |
| B. | over-excited |
| C. | normally excited |
| D. | disconnected from the common terminals |
| Answer» C. normally excited | |
| 119. |
Mmf produced by the current of a 3-phase induction motor |
| A. | rotates at the speed of rotor in the air gap |
| B. | is stand still with respect to startor mmf |
| C. | rotates at slip speed with respect to startor mmf |
| D. | rotates at synchronous speed with respect to rotor |
| Answer» C. rotates at slip speed with respect to startor mmf | |
| 120. |
If rotor power factor of a 3-phase induction motor is 0.866, then spat ial displacement bet ween startor magnetic field and rotor magnetic field will be |
| A. | 30 |
| B. | 90 |
| C. | 120 |
| D. | 150 |
| Answer» D. 150 | |
| 121. |
A differentially compounded motor under highover-load conditions will behave like a/ an |
| A. | shunt motor |
| B. | series motor |
| C. | cummulative compound motor |
| D. | ac synchronous motor |
| Answer» C. cummulative compound motor | |
| 122. |
6-pole lap-wound dc armature with 720 conductors draws 50 A from the mains. Its armature reaction per pole is |
| A. | 1000 AT peak, triangular in wave shape. |
| B. | 500 AT peak, sinusoidal in wave shape. |
| C. | 1000 AT peak, sinusoidal in wave shape. |
| D. | 500 AT peak, triangular in wave shape. |
| Answer» C. 1000 AT peak, sinusoidal in wave shape. | |
| 123. |
Interpoles in a dc motor must be |
| A. | series excited and should have polarity opposite to that of the next main pole in the direction of rotation of armature. |
| B. | series excited had should have the same polarity as that of the next main pole in the direction of rotation of the armature. |
| C. | shunt excited and should have polarity opposite to that of the next main pole in the direction of rotation of the armature. |
| D. | shunt excited and should have t he same polarity as the next main pole in the direction of rotation of the armature. |
| Answer» B. series excited had should have the same polarity as that of the next main pole in the direction of rotation of the armature. | |
| 124. |
What would be observed if a dc shunt motor is started with an open-circuited field? |
| A. | The motor does not pick up speed but draws only a small current |
| B. | The motor does not pick up speed but draws a large current. |
| C. | The motor picks up fast and acquires full speed while drawing large current |
| D. | The motor picks up fast and acquires full speed while drawing small current. |
| Answer» C. The motor picks up fast and acquires full speed while drawing large current | |
| 125. |
Open slots are used in dc machine armature because |
| A. | of the ease with which winding can be placed inside the slots |
| B. | it increases induced emf per coil |
| C. | it reduces armature voltage drop |
| D. | it reduces coil reactance emf and hence aids in commutation. |
| Answer» E. | |
| 126. |
The magnetic core of a certain material is operated at constant flux density. If core losses measured at 50 Hz and 75 Hz are respectively 2000 W and 3375 W, the hysteresis and eddy current losses of the core at 100 Hz would respectively be |
| A. | 2500 W, 2000 W |
| B. | 2000 W, 2500 W |
| C. | 3000 W, 2000 W |
| D. | 2000 W, 3000 W |
| Answer» D. 2000 W, 3000 W | |
| 127. |
The developed electromagnetic force and/or torque in electro-mechanical energy conversion systems act in a direction that tends to |
| A. | increase stored energy at constant m.m.f. |
| B. | decrease stored energy at constant flux |
| C. | decrease co-energy at constant m.m.f |
| D. | decrease stored energy at constant m.m.f. |
| Answer» C. decrease co-energy at constant m.m.f | |
| 128. |
An iron-cored choke with 1 mm air-gap length, draws 1 A when fed from a constant voltage ac source of 220 V. If the length of air-gap is increased to 2 mm, the current drawn by the choke would |
| A. | become nearly one half |
| B. | remain nearly the same |
| C. | become nearly double |
| D. | become nearly zero |
| Answer» D. become nearly zero | |
| 129. |
In a transformer fed fr om a fundament al frequency voltage source, the source of harmonics is the |
| A. | overload |
| B. | poor insulation |
| C. | iron loss |
| D. | saturation of core |
| Answer» E. | |
| 130. |
To prevent maloperation of differentially connected relay while energising a transformer, the relay restraining coil is based with |
| A. | second harmonic current |
| B. | third harmonic current |
| C. | fifth harmonic current |
| D. | seventh harmonic current |
| Answer» B. third harmonic current | |
| 131. |
If the frequency of input voltage of a transformer is increased keeping the magnitude of voltage unchanged, then |
| A. | both hysteresis loss and eddy current loss in the core will increase |
| B. | hysteresis loss will increase but eddy current loss will decrease |
| C. | hysteresis loss will decrease but eddy current will increase |
| D. | hysteresis loss will decrease but eddy current will remain unchanged. |
| Answer» E. | |
| 132. |
For successful parallel operation of two singlephase transformers, the most essential condition is that their |
| A. | percentage impedance are equal |
| B. | polarities are properly connected |
| C. | turns ratios are exactly equal |
| D. | kVA ratings are equal. |
| Answer» E. | |
| 133. |
Can a 50 Hz transformer be used for 25 Hz, if the input voltage is maintained constant at the rated vale corresponding to 50 Hz? |
| A. | Yes. Since the voltage is constant, current levels will not changed. |
| B. | No. Flux will be doubled which will drive the care to excessive saturation |
| C. | No. Owing to decreased reactance of transformer, input current will be doubled atload |
| D. | Yes. At constant voltage, insulation will not be overstressed. |
| Answer» C. No. Owing to decreased reactance of transformer, input current will be doubled atload | |
| 134. |
While performing the open-circuit and shortcircuit tests on a transformer to determine its parameters the status of the low voltage (L.V.) and high voltage (H.V.) winding will be such that |
| A. | in O.C., LV is open and in SC, H.V. is shorted |
| B. | in O.C., HV is open and in SC, L.V. is shorted |
| C. | in O.C., LV is open and in SC, L.V. is shorted |
| D. | in O.C., HV is open and in SC, H.V. is shorted |
| Answer» C. in O.C., LV is open and in SC, L.V. is shorted | |
| 135. |
Short-circuit test is performed on a transformer with a certain impressed voltage at rated frequency. If the short-circuit test is now performed with the same magnitude of impressed voltage, but at a frequency higher than the rated frequency, then |
| A. | the magnitude of current and power factor will both increase |
| B. | the magnitude of current will decrease but the power factor will increase |
| C. | the magnitude of current will increase, but power factor will decrease |
| D. | the magnitude of current as well as the power factor will decrease. |
| Answer» E. | |
| 136. |
A transformer has a core loss of 61 W and copper loss of 144 W. When it is carrying 20% over load current, the load at which this transformer will operates at the maximum efficiency |
| A. | 80% |
| B. | 66% |
| C. | 120% |
| D. | 44% |
| Answer» B. 66% | |
| 137. |
Three single phase transformer each wit h a 10 kVA rating are connected in a closed delta arrangement. If one transformer is taken out, the output capacity of the system will be |
| A. | 20 kVA |
| B. | 8.66 kVA |
| C. | 17.39 kVA |
| D. | 10 kVA |
| Answer» D. 10 kVA | |
| 138. |
A 50 Hz transformer having equal hysteresis and eddy current losses at rated excitation is operated at 45 Hz at 90% of its rated voltage compared to rated operating point, the core losses under this condition will be |
| A. | reduce by 10% |
| B. | reduce by 19% |
| C. | reduce by 14.5% |
| D. | remain unchanged |
| Answer» D. remain unchanged | |
| 139. |
In a 4: 1 turn ratio transformer, the leakage reactance as referred to the secondary side is 0.05 pu. The leakage reactance as referred to primary is |
| A. | 0.05 ohm |
| B. | 0.05 pu |
| C. | 0.8 pu |
| D. | 0.2 pu |
| Answer» C. 0.8 pu | |
| 140. |
A transformer has a voltage regulation of 3% at full load, 0.8 power factor lagging. At full load unit power factor, the regulation will be |
| A. | 3% |
| B. | negative |
| C. | less than 3% |
| D. | more than 3% |
| Answer» C. less than 3% | |
| 141. |
A 2-phase, 4-pole permanent magnet stepper motor has a step of |
| A. | 90 |
| B. | 45 |
| C. | 30 |
| D. | 22 |
| Answer» E. | |
| 142. |
A 400/200 V transformer has a PU impedance of 0.05. The HV side voltage required to circulate full load current during short circuit test is |
| A. | 40 V |
| B. | 20 V |
| C. | 10 V |
| D. | 5 V |
| Answer» C. 10 V | |
| 143. |
A 200/100 V 50 Hz transformer is to be excited at 40 Hz from the 100 V side. For the exciting current to remain the same, the applied voltage should be |
| A. | 150 V |
| B. | 125 V |
| C. | 100 V |
| D. | 80 V |
| Answer» E. | |
| 144. |
In an autotransformer of voltage ratio V |
| A. | <table><tr><td style="border-bottom:1px solid #000000;vertical-align:bottom;padding-bottom:2px;"><center>V<sub>1</sub></center></td></tr><tr><td style="text-align: center;">V<sub>2</sub> + V<sub>2</sub></td></tr></table> |
| B. | <table><tr><td style="border-bottom:1px solid #000000;vertical-align:bottom;padding-bottom:2px;"><center>V<sub>2</sub></center></td></tr><tr><td style="text-align: center;">V<sub>1</sub></td></tr></table> |
| C. | <table><tr><td style="border-bottom:1px solid #000000;vertical-align:bottom;padding-bottom:2px;"><center>V<sub>1</sub> V<sub>2</sub></center></td></tr><tr><td style="text-align: center;">V<sub>1</sub> + V<sub>2</sub></td></tr></table> |
| D. | <table><tr><td style="border-bottom:1px solid #000000;vertical-align:bottom;padding-bottom:2px;"><center>V<sub>1</sub> V<sub>2</sub></center></td></tr><tr><td style="text-align: center;">V<sub>1</sub></td></tr></table> |
| Answer» E. | |
| 145. |
If a sinusoidal voltage source is connected to a power transformer, its no-load current would be |
| A. | sinusoidal and lagging the voltage by 90 |
| B. | sinusoidal and lagging the voltage by less than 90 |
| C. | rich in third harmonic and its fundamental would lag the voltage by 90 |
| D. | rich in third harmonic and its fundamental would lag the voltage by less than 90 |
| Answer» D. rich in third harmonic and its fundamental would lag the voltage by less than 90 | |
| 146. |
Full-load voltage regulation of a power transformer is zero when power factor of the load is near |
| A. | unity and leading |
| B. | zero and leading |
| C. | zero and lagging |
| D. | unity and lagging |
| Answer» B. zero and leading | |
| 147. |
A DC motor has the following specifications: 10hp, 37.5 A, 230V; flud/pole = 0.01Wb, number of poles = 4, number of conductors = 666, number of parallel paths = 2. Armature resistance = 0.267 &ohm. The armature reaction is negligible and rotational losses are 600W. The motor operates from a 230V DC supply. I f the motor runs at 1000 rpm, the output torque produced in (in Nm) is _________. |
| A. | 42 |
| B. | 57 to 58 |
| C. | 51 |
| D. | 15 |
| Answer» C. 51 | |
| 148. |
The torque of a reluctance motor can be effectively increased by |
| A. | increasing reluctance of the magnetic circuit along the direct axis |
| B. | decreasing reluctance of magnetic circuit along the quadrature |
| C. | increasing ratio of t he quadrature axis reluctance to the direct axis reluctance |
| D. | decreasing ratio of t he quadrature axis reluctance to direct axis reluctance |
| Answer» C. increasing ratio of t he quadrature axis reluctance to the direct axis reluctance | |
| 149. |
A separately excited DC motor runs at 1000 rpm on no load when its armature terminals are connected to a 200V DC source and the rated voltage is applied to t he field winding. The armature resistance of this motor is 1Ω. The no-load armature current is negligible. With the motor developing its full load torque, the armature voltage is set so that the rotor speed is 500 rpm. When the load torque is reduced to 50% of the full load value under the same armature voltage conditions, the speed rises to 520 rpm. Neglecting the rotational losses, the full load armature current (in Ampere) is _______. |
| A. | 8 |
| B. | 56 |
| C. | 25 |
| D. | 15 |
| Answer» B. 56 | |
| 150. |
A 200/400V, 50 Hz, two-winding transformer is rated at 20 kVA. Its windings are connected as an auto-transformer of rating 200/600V. A resistive load of 12 Ω is connected to the high voltage (600V) side of the auto-transformer. The value of equivalent load resistance (in Ohm) as seen from low voltage side is _________. |
| A. | 1.3 to 1.4 |
| B. | 4 |
| C. | 6.5 |
| D. | 1.2 |
| Answer» B. 4 | |