# MCQ in AC Motors Part 8 | REE Board Exam

(Last Updated On: August 8, 2020) This is the Multiple Choice Questions Part 8 of the Series in AC Motors as one of the Electrical Engineering topic. In Preparation for the REE Board Exam make sure to expose yourself and familiarize in each and every questions compiled here taken from various sources including but not limited to past Board Exam Questions in Electrical Engineering field, Electrical Engineering Books, Journals and other Electrical Engineering References.

#### Continue Practice Exam Test Questions Part 8 of the Series

Choose the letter of the best answer in each questions.

351.  The arrangement required for producing a rotation equal to the sum or difference of the rotation of two shafts consist of the following coupled synchros.

A.  control transmitter

C.  control differential transmitter

D.  all of these

Solution:

352.  Which of the following motor would suit applications where constant speed is absolutely essential to ensure a consistent product?

A.  brushes dc motor

B.  disk motor

C.  permanent-magnet synchronous motor

D.  stepper motor

Solution:

353.  A switched reluctance motor differs from a VR stepper motor in the sense that it

A.  has rotor poles of ferromagnetic material

B.  rotates continuously

C.  is designed for open – loop operation only

D.  has lower efficiency

Solution:

354.  The electrical displacement between the two stator windings of a resolver is

A.  120°

B.  90°

C.  60°

D.  45°

Solution:

355.  Which of the following motor runs from a low dc supply and has permanently magnetized salient poles on its rotor?

A.  permanent magnet dc motor

B.  disk dc motor

C.  permanent magnet synchronous motor

D.  brushless dc motor

Solution:

356.  A dc servomotor is similar to a regular dc motor except that its design is modified to cope with

A.  electronic switching

B.  slow speeds

C.  static conditions

D.  both B and C

Solution:

357.  One of the basic requirements of a servomotor is that it must produce high torque at all

B.  frequencies

C.  speeds

D.  voltages

Solution:

358.  The most common two-phase ac servomotor differs from the standard ac induction motor because it has

A.  higher rotor resistance

B.  higher power rating

C.  motor stator windings

D.  greater inertia

Solution:

359.  Squirrel-cage induction motor is finding increasing application in high-power servo systems because new methods have been found to

A.  increased its rotor resistance

B.  control its torque

C.  decrease its inertia

D.  decouple its torque and flux

Solution:

360.  A six-pole, three-phase squirrel-cage induction motor is connected to a 60-cps supply. At full-load, the rotor’s induced emf makes 72 complete cycles in 1 minute. Find the rotor speed.

A.  1176 rpm

B.  1200 rpm

C.  1054 rpm

D.  1124 rpm

Solution:

361.  A 50 HP, 440 V, 3-phase, 60 Hz, 6-pole squirrel cage induction motor is operating at full-load and 0.8 pf. The full-load efficiency is 85% and the percentage slip is 5%. Determine the full-load torque.

A.  214.55 N-m

B.  206.72 N-m

C.  312.47 N-m

D.  323.24 N-m

Solution:

362.  A synchronous motor is drawing 60 kW is connected in parallel with a load drawing 250 kW at a lagging power factor of 0.8. If the combined load has a power factor of 0.9, at what power factor is the synchronous motor operating?

Solution:

363.  A three-phase Y-connected synchronous motor with a line-to-line voltage of 440 V and a synchronous speed of 900 rpm operates with a power of 9 kW and a lagging power factor of 0.8. The synchronous reactance per phase is 10 ohms. Determine the torque angle in electrical degrees.

A.  36.33°

B.  33.51°

C.  35.51°

D.  38.46°

Solution:

364.  A 10-hp, 230-V, three-phase Y-connected synchronous motor has a reactance of 3 ohms per phase and a negligible resistance. To what voltage must the motor is excited in order to deliver full load at unity power factor and 90% efficiency?

A.  262 V

B.  254 V

C.  248 V

D.  242 V

Solution:

365.  A three-phase, star connected synchronous motor takes 50 kW at 660 V, the power factor being 0.8 lagging. This machine has a synchronous reactance of 1 ohm per phase with a negligible resistance. If the emf is increased by 30%, the power taken remaining the same, find the new leading current.

A.  94.1 A

B.  85.2 A

C.  90.3 A

D.  88.6 A

Solution:

366.  A 6.6 kV, Y-connected synchronous motor has a synchronous reactance per phase of 10 ohms and a negligible resistance. For a certain load, the motor draws 990 kW at normal voltage. At this load, the induced emf per phase is 5160 V. Determine operating pf.

A.  0.478

B.  0.502

C.  0.462

D.  0.518

Solution:

367.  A 75-hp, 600-volt, 1000-rpm, three-phase Y-connected synchronous motor has an armature resistance of 0.05 ohm per phase and a leakage reactance of 0.45 ohm per phase. The efficiency at rated load, 80% pf leading is 92%. Determine mechanical power developed within the armature.

A.  64 kW

B.  62 kW

C.  60 kW

D.  67 kW

Solution:

368.  A 230 V, 60 Hz, 3-phase, Y-connected synchronous motor draws a current of 20 A and delivers 8 bhp. Armature resistance per phase is 0.5 Ω. If the iron and friction losses amount to 300 W, determine the operating power factor of the motor.

A.  0.809

B.  0.807

C.  0.866

D.  0.862

Solution:

369.  A 2200 V, 200 hp, 3-phase mesh connected synchronous motor is working on full load at an efficiency of 0.88 and 0.8 pf leading. The armature reactance per phase is 5 ohms. Determine the induced emf per phase. Neglect resistance.

A.  2377 V

B.  2345 V

C.  2380 V

D.  2307 V

Solution:

370.  A 500-hp, 2200 V, 3-phase, star-connected synchronous motor has a synchronous impedance of 0.3 + j3 Ω per phase. Determine the induced emf per phase if the motor works on full load with an efficiency of 94% and a power factor of 0.8 leading.

A.  1360 V

B.  1354 V

C.  1402 V

D.  1522 V

Solution:

371.  An alternator rated 530-kVA, is operating at 0.60 pf lagging. A synchronous condenser is in parallel to improve the pf to 90% lagging. Determine the operating power factor of the synchronous condenser. Assume the alternator is not supposed to be overloaded.

Solution:

372.  A 230-V, 60-Hz, single-phase synchronous motor is taking a current of 20 A at 0.866 lagging p.f. Effective armature resistance is 0.3 ohm. Iron and friction losses amount to 450 W. Determine the efficiency of the motor at this load. Neglect losses in the excitation.

A.  86.15%

B.  85.30%

C.  88.52%

D.  83.20%

Solution:

373.  A 15-hp, 230 V, single phase synchronous motor is taking a current of 50 A at a power factor of 0.85 leading. Effective resistance is 0.2 ohm. If this motor drives a mechanical load of 12 hp, determine its rotational losses.

A.  323 W

B.  340 W

C.  376 W

D.  314 W

Solution:

374.  A single-phase rotary converter has an output voltage of 110 volts. Determine the line current in the AC side when the machine delivers a DC output of 50 A. Assume the machine is operating with a pf of 0.85 and efficiency of 95%.

A.  70.71 A

B.  90.34 A

C.  87.57 A

D.  82.23 A

Solution:

375.  Three single-phase transformers connected delta-delta fed a three-phase rotary converter at 2,300 V. This converter supplies power to a DC load that draws 100 kW at 250 V. If the rotary converter operates at an efficiency of 95% at unity pf, calculate the current in the secondary windings of the transformer bank.

A.  397 A

B.  355 A

C.  229 A

D.  249 A

Solution:

376.  A 4-phase star connected rotary converter operating at 0.90 efficiency and 0.85 power factor is connected across a 220 V, 60 Hz mains and draws 50 A. Determine the DC load current.

A.  208.46 A

B.  220.54 A

C.  216.37 A

D.  212.25 A

Solution:

377.  The dc output of a six-phase star-connected converter is 500 kW at 400 V. If the pf and efficiency of the machine is 0.866 and 0.92 respectively, determine the ac current drawn from the supply.

A.  762 A

B.  740 A

C.  732 A

D.  729 A

Solution:

378.  A 600 kW, 6-phase, Y-connected, 18-pole, synchronous converter has simplex lap wound armature with 1000 conductors. The flux per pole is 60 mWb. The machine runs at 600 rpm at rated load. Determine the ac current in the slip rings. Assume the operating power factor and efficiency of the machine equal to 1.0 and 0.9 respectively.

A.  496.21 A

B.  488.42 A

C.  462.25 A

D.  472.07 A

Solution:

379.  A 4-pole 230-V, 60 Hz, three-phase induction motor directly drives a 6-pole alternator. Determine the output frequency. Assume a slip of 3% in the motor.

A.  90.0 Hz

B.  88.4 Hz

C.  86.6 Hz

D.  87.3 Hz

Solution:

380.  A three-phase rotary converter delivers 150 A at 500 V dc. Determine the current on the AC side if the operating power factor of the machine is 0.8 and the efficiency is 90%

A.  179 A

B.  157 A

C.  189 A

D.  196 A

Solution:

381.  An open delta bank is serving a three-phase rotary converter. The converter supplies 175 A of current to 1.4 ohm dc resistive load. If the machine draws an AC current of 100 A at 0.8 pf lagging, determine minimum size of transformer needed.

A.  15.0 kVA

B.  24.7 kVA

C.  25.9 kVA

D.  19.4 kVA

Solution:

382.  The percent slip of an induction motor is the ratio

A.  rotor copper loss / rotor power input

B.  stator copper loss / stator power input

C.  stator copper loss / rotor power output

D.  rotor copper loss / stator copper loss

Solution:

383.  An 80 kW, 440 volts, 3-phase Y connected, 60 Hz, 4-pole synchronous motor has a synchronous reactance of 3-ohm per phase. Calculate the torque in Newton-meter if it is operating at rated condition at 92% efficiency.

A.  714

B.  135

C.  571

D.  461

Solution:

384.  A 220 volts single phase induction motor draws current of 10 amps at 0.75 pf. A capacitor of 50 F is connected in order to improve the power factor. What is the new power factor?

B.  0.866 lagging

C.  0.95 lagging

Solution:

385.  A test is performed on a 230 volts, delta connected synchronous motor having 92% efficiency and 85% power factor. With 6 volts dc impressed across its terminals, the current reading is 90 amperes. Determine the ohmic resistance per phase.

A.    0.067 /phase

B.    0.44 /phase

C.  0.1 /phase

D.  0.15 /phase

Solution:

386.  A synchronous motor operates continuously on the following duty cycle: 50 hp for 10 sec, 100 hp for 10 sec, 150 hp for 5 sec, 120 hp for 20 sec, idling for 14 sec. What would be the size of motor to be used?

A.  90 hp

B.  100 hp

C.  150 hp

D.  120 hp

Solution:

387.  A polyphase motor may be protected against reversal of rotation by using a

A.    reverse speed relay

B.    reverse power relay

C.  reverse phase relay

D.  reverse current relay

Solution:

388.  A 1000 kVA, 11 kV, 3-phase, Y-connected synchronous motor has a resistance of 3 ohms and a reactance of 25 ohms per phase. Determine the induced emf when fully loaded at 0.8 power factor lagging.

A.  5521 V

B.  5362 V

C.  5475 V

D.  5582 V

Solution:

389.  If Ns is the synchronous speed and s the slip, then actual running speed of an induction motor will be

A.  Ns

B.  sNs

C.  (1 – s)Ns

D.  (Ns – 1)s

Solution:

390.  When the synchronous motor has a leading power factor, the generated emf is ____ than the applied voltage.

A.  equal

B.  greater

C.  lesser

D.  none of these

Solution:

391.  The word TEFC stands for

A.  totally enclosed frequency controlled

B.  torque, energy, frequency controlled

C.  totally enclosed fused cooled

D.  totally enclosed fan cooled

Solution:

392.  Synchronous motor are

A.  self-starting

B.  not self-starting

C.  essentially self-starting

D.  none of these

Solution:

393.  The motor in which stator and rotor magnetic field rotate at the same speed is

A.  induction motor

B.  universal motor

C.   reduction motor

D.  synchronous motor

Solution:

394.  An over-excited synchronous motor takes

A.  lagging current

D.  none of these

Solution:

395.  In a synchronous motor, which loss does not vary with load?

A.  copper losses

B.  hysteresis losses

C.  windage losses

D.  none of these

Solution:

396.  Squirrel cage induction motor differs from the slip ring type in that it has no

A.  slip rings

B.  windings on the stator

C.  rotor winding

D.  rotating part

Solution:

397.  A 3- synchronous motor runs at full-load speed of 1200 rpm. If the load is reduced to one-half of rated, calculate its speed regulation.

A.  100%

B.  50%

C.  0%

D.  86.6%

Solution:

398.  An industrial load is taking 500 kW at 0.6 lagging p.f. is desired to raise the p.f. to 0.866 lagging using a synchronous machine whose power input is 300 kW. Calculate the kVA rating of the motor required for such operation and the power factor at whuch the motor will operate.

Solution:

399.  A 3–phase, 440-volts, 50 Hz induction motor has 4% slip. What is the frequency of the rotor induced emf?

A.  200 Hz

B.  50 Hz

C.  2 Hz

D.  0.2 Hz

Solution:

400.  What is the ratio of maximum to full-load torque in a motor having a slip of 5% at full-load and 25% at maximum torque?

A.  15

B.  2.6

C.  3

D.  5.2

Solution:

401.  A 3Φ 6-pole induction motor has a nameplate speed of 1160 rpm, what is its full-load slip?

A.  2.58%

B.  3.3%

C.  3.59%

D.  1.4%

Solution:

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