You dont have javascript enabled! Please enable it! MCQ in AC Motors Part 7 | REE Board Exam

# MCQ in AC Motors Part 7 | REE Board Exam

This is the Multiple Choice Questions Part 7 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 7 of the Series

Choose the letter of the best answer in each questions.

301.ย  If the field of a synchronous motor is under-excited, the power factor will be

A.ย  lagging

C.ย  unity

D.ย  more than unity

Solution:

302.ย  Ignoring the effects of armature reaction, if excitation of a synchronous motor running with constant load is decreased from its normal value it leads to

A.ย  increase in Ia but decrease in Eb

B.ย  increase in Eb but decrease in Ia

C.ย  increase in both Ia and p.f. which is lagging

D.ย  increase in both Ia and Eb

Solution:

303.ย  A synchronous motor connected to infinite busbars at constant full-load, 100% excitation and unity p.f. On changing the excitation only, the armature current will have

C.ย  lagging p.f. with over-excitation

D.ย  no change of p.f.

Solution:

304.ย  The V-curves of a synchronous motor show relationship between

A.ย  excitation current and back emf

B.ย  field current and p.f.

C.ย  dc field current and ac armature current

D.ย  armature current and supply voltage

Solution:

305.ย  When a load on a synchronous motor is increased, its armature currents is increased provided it is

A.ย  normally excited

B.ย  over-excited

C.ย  under-excited

D.ย  all of these

Solution:

306.ย  If main field current of a salient-pole synchronous motor fed from an infinite bus and running at no-load is reduced to zero, it would

A.ย  come to a stop

B.ย  continue running at synchronous speed

C.ย  run at sub-synchronous speed

D.ย  run at super-synchronous speed

Solution:

307.ย  In a synchronous machine when the rotor speed becomes more than the synchronous speed during hunting, the damping bars develop

A.ย  synchronous motor torque

B.ย  d.c. motor torque

C.ย  induction motor torque

D.ย  induction generator torque

Solution:

308.ย  In a synchronous motor, the rotor Cu losses are met by

A.ย  motor input

B.ย  armature input

C.ย  supply lines

D.ย  d.c. source

Solution:

309.ย  A synchronous machine is called a doubly-excited machine because

A.ย  it can be overexcited

B.ย  it has two sets of rotor poles

C.ย  both its rotor and stator are excited

D.ย  it needs twice the normal exciting current

Solution:

310.ย  Synchronous capacitor is

A.ย  an ordinary static capacitor bank

B.ย  an over-excited synchronous motor driving mechanical load

C.ย  an over-excited synchronous motor running when mechanical load

D.ย  none of these

Solution:

Unsolved problems

311.ย  REE Board Exam September 2004

What is the percent slip of the synchronous motor when running?

A.ย  3

B.ย  2.5

C.ย  1

D.ย  0

Solution:

312.ย  The speed regulation of as synchronous motor is

A.ย  unity

B.ย  zero

C.ย  infinity

D.ย  always less than one

Solution:

313.ย  Synchronous motors are to be used in situation where

B.ย  the load is required to be driven at very high speeds

C.ย  the load is to be driven at constant speed

D.ย  the starting torque requirement of the load is very high

Solution:

SYNCHRONOUS CONVERTERS

314.ย  EE Board Exam April 1994

In a three phase synchronous converter, the ratio of the AC voltage to DC voltage is

A.ย  0.612

B.ย  1.0

C.ย  0.50

D.ย  0.707

Solution:

315.ย  EE Board Exam April 1992

A 500 kW, 600-V DC, 12-phase synchronous converter operates as a direct converter at a full load efficiency of 92% and a power factor of 0.93. Calculate the AC voltage between slip rings and the AC current drawn from a 12-phase transformer fed supply.

A.ย  112 V, 229 A

B.ย  112 V, 215 A

C.ย  110 V, 229 A

D.ย  110 V, 215 A

Solution:

316.ย  EE Board Exam April 1992

A generator is rated 600 kVA, 240 V, 60 cycles, 3-phase, 6 poles and wye-connected. What will be the speed of the driving pulley if the driven and driving pulleys are 1 ft and 2 ft in diameter respectively?

A.ย  2400 rpm

B.ย  1200 rpm

C.ย  600 rpm

D.ย  900 rpm

Solution:

317.ย  REE Board Exam October 1998

The pulley of an old gen-set has a diameter of 20 inches. The belt exerts a pull of 353 pounds on the pulley. The gen-set runs at 900 rpm. What is the approximate kW rating of the genset?

A.ย  75

B.ย  250

C.ย  200

D.ย  37.5

Solution:

318.ย  The speed of a motor-generator set consisting of a 6-pole induction motor and a 4-pole dc generator fed from a 3-phase.

A.ย  1000

B.ย  600

C.ย  1500

D.ย  3000

Solution:

319.ย  A rotary converter generally

A.ย  combines the functions of an induction motor and a d.c. generator

B.ย  has a set of slip โ rings at both ends

C.ย  has one armature and two fields

D.ย  is a synchronous motor and a d.c. generator combined

Solution:

320.ย  The a.c. line current at slip-rings in a 6-phase, 6-ring, rotary converter having 100% efficiency and unity p.f. is ____ times the d.c. current.

A.ย  0.943

B.ย  0.472

C.ย  0.236

D.ย  1.414

Solution:

321.ย  A 3-phase supply can be converted into a 6-phase supply by joining the secondaries of the 3-phase transformer in

A.ย  double delta

B.ย  double star

C.ย  diametrical

D.ย  any of these

Solution:

322.ย  One unique advantage of employing induction regulator method for controlling the dc output voltage of a rotary converter is that it

A.ย  is extremely simple and relatively cheaper

B.ย  responds instantaneously to changes in load

C.ย  can be used for inverters

D.ย  gives voltage changes in exact jumps

Solution:

323.ย  The most commonly used connection for joining the six secondaries of a transformer used for 3-phase to 6-phase conversion is

A.ย  diametrical

B.ย  zig-zag

C.ย  double star

D.ย  double-delta

Solution:

324.ย  The function of a cycloconverter is to convert

A.ย  ac power into dc power

B.ย  direct current into alternating current

C.ย  high ac frequency directly to low ac frequency

D.ย  a sine wave into a rectangular wave

Solution:

325.ย  Major disadvantage of using three sets of SCRs for variable voltage speed control of a SCIM is the

A.ย  considerable I2R loss

B.ย  poor power factor

C.ย  long delay of thyristor firing pulses

D.ย  necessity of using a processor

Solution:

326.ย  In the current fed frequency converter arrangement for controlling the speed of an individual SCIM, the direction of rotation can be reversed by

A.ย  changing the output frequency of the inverter

B.ย  altering the phase sequence of pulses that trigger the d.c link current

C.ย  interchanging any two line leads

D.ย  reversing the d.c link current

Solution:

327.ย  In the chopper uses speed control method for a WRIM the motor speed inversely depends on

A.ย  fixed resistor across the rectifier

B.ย  chopper switching frequency

C.ย  chopper ON time TON

D.ย  both B and C

Solution:

328.ย  In the synchronous motor drive using current fed dc link

A.ย  converter-2 functions as a self commutated inverter

B.ย  converter-1 works as uncontrolled rectifier

C.ย  converter-3 is a controlled rectifier

D.ย  gate triggering of converter 2 is done at motor frequency

Solution:

329.ย  In the three cycloconverter drive of a synchronous motor

A.ย  each cycloconverter produces a 3-phase output

B.ย  all cycloconverters act as voltage sources

C.ย  a 3-phase controlled rectifier produces field-exciting current

D.ย  air-gap flux is kept constant by controlling stator currents only

Solution:

SPECIAL MACHINES

330.ย  A single โ stack, 4 โ phase, 6 โ pole VR stepper motor will have a step angle of

A.ย  15ยฐ

B.ย  30ยฐ

C.ย  45ยฐ

D.ย  90ยฐ

Solution:

331.ย  In a three-stack 12/8 pole VR motor, the rotor pole pitch is

A.ย  15ยฐ

B.ย  30ยฐ

C.ย  45ยฐ

D.ย  60ยฐ

Solution:

332.ย  A three-stack VR stepper motor has a step angle of 10ยฐ. What is the number of rotor teeth in each stack?

A.ย  36ยฐ

B.ย  24ยฐ

C.ย  18ยฐ

D.ย  12ยฐ

Solution:

333.ย  If a hybrid stepper motor has a rotor pitch of 36ยฐ and a step angle of 9ยฐ, the number of its phases must be

A.ย  4

B.ย  2

C.ย  3

D.ย  6

Solution:

334.ย  What is the step angle of a permanent-magnet stepper motor having 8 stator poles and 4 rotor poles?

A.ย  60ยฐ

B.ย  45ยฐ

C.ย  30ยฐ

D.ย  15ยฐ

Solution:

335.ย  A stepping motor is a ____ device.

A.ย  mechanical

B.ย  electrical

C.ย  analogue

D.ย  incremental

Solution:

336.ย  Operation of stepping motors at high speed is referred to as

A.ย  fast forward

B.ย  slewing

C.ย  inching

D.ย  jogging

Solution:

337.ย  Which of the following phase switching sequence represents half-step operation of a VR stepper motor?

A.ย  A, B, C, A โฆโฆโฆ

B.ย  A, C, B, A โฆโฆโฆ.

C.ย  AB, BC, CA, AB โฆโฆโฆ

D.ย  A, AB, B, BC โฆโฆ.

Solution:

338.ย  The rotational speed of a given stepper motor is determined solely by the

B.ย  step pulse frequency

C.ย  polarity of stator current

D.ย  magnitude of stator current

Solution:

339.ย  A stepper motor may be considered as a ____ converter.

A.ย  dc to ac

B.ย  ac to ac

C.ย  dc to ac

D.ย  digital-to-analogue

Solution:

340.ย  The rotor of a stepper motor has no

A.ย  windings

B.ย  commutator

C.ย  brushes

D.ย  all of these

Solution:

341.ย  Wave excitation of a stepper motor results in

A.ย  microstepping

B.ย  half โ stepping

C.ย  increased step angle

D.ย  reduced resolution

Solution:

342.ย  A stepper motor having a resolution of 300 steps/rev and running at 2400 rpm has a pulse rate of ____ pps.

A.ย  4000

B.ย  8000

C.ย  6000

D.ย  10,000

Solution:

343.ย  The torque exerted by the rotor magnetic field of a PM stepping motor with unexcited stator is called ____ torque.

A.ย  reluctance

B.ย  detent

C.ย  holding

D.ย  either B or C

Solution:

344.ย  A variable reluctance stepper motor is constructed of ____ material with salient poles.

A.ย  paramagnetic

B.ย  ferromagnetic

C.ย  diamagnetic

D.ย  nonโmagnetic

Solution:

345.ย  Though structurally similar to a control transmitter, a control receiver differs from it in the following way

A.ย  it has 3-phase stator winding

B.ย  it has a rotor of dumbbell construction

C.ย  it has a mechanical damper on its shaft

D.ย  it has a single-phase rotor excitation

Solution:

346.ย  The control ____ synchro has three-phase winding both on its stator and rotor

A.ย  differential

B.ย  transformer

D.ย  transmitter

Solution:

347.ย  Regarding voltages induced in the three stator windings of a synchro, which statement is false?

A.ย  they depend on rotor position

B.ย  they are in phase

C.ย  they differ in magnitude

D.ย  they are polyphase voltages

Solution:

348.ย  The low-torque synchros cannot be used for

A.ย  torque transmission

B.ย  error detection

C.ย  instrument servos

D.ย  robot arm positioning

Solution:

349.ย  Which of the following synchros are used for error detection in a servo control system?

A.ย  control transmitter

B.ย  control transformer

D.ย  both A and B

Solution:

350.ย  For torque transmission over a long distance with the help of electrical wires, which of the following two switches are used?

A.ย  CX and CT

B.ย  CX and CR

C.ย  CX and CD

D.ย  CT and CD

Solution:

#### Questions and Answers in AC Motors

Following is the list of practice exam test questions in this brand new series:

MCQ in AC Motors
PART 1: MCQ from Number 1 โ 50                Answer key: included
PART 2: MCQ from Number 51 โ 100            Answer key: included
PART 3: MCQ from Number 101 โ 150           Answer key: included
PART 4: MCQ from Number 151 โ 200           Answer key: included
PART 5: MCQ from Number 201 โ 250           Answer key: included
PART 6: MCQ from Number 251 โ 300           Answer key: included
PART 7: MCQ from Number 301 โ 350           Answer key: included
PART 8: MCQ from Number 351 โ 400            Answer key: included
PART 9: MCQ from Number 401 โ 450            Answer key: included
PART 10: MCQ from Number 451 โ 500          Answer key: included

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