# MCQ in AC Motors Part 6 | REE Board Exam

(Last Updated On: August 8, 2020) This is the Multiple Choice Questions Part 6 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 6 of the Series

Choose the letter of the best answer in each questions.

251.  If the centrifugal switch of a two-value capacitor motor using two capacitors fails to open, then

A.  electrolytic capacitor will, in all probability, suffer breakdown

B.  motor will not carry the load

C.  motor will draw excessively high current

D.  motor will not come up to the rated speed

Solution:

252.  Each of the following statements regarding a shaded-pole motor is true except

A.  its direction of rotation is from un-shaded to shaded portion of the poles

B.  it has very poor efficiency

C.  it has very poor p.f.

D.  it has high starting torque

Solution:

253.  Compensating winding is employed in an ac series motor in order to

A.  compensate for decrease in field flux

B.  increase the total torque

C.  reduce the sparking at brushes

D.  reduce effects of armature reaction

Solution:

254.  A universal motor is one which

A.  is available universally

B.  can be marked internationally

C.  can be operated either on de or ac supply

D.  runs at dangerously high speed on no-load

Solution:

255.  In a single-phase series motor the main purpose of inductively-wound compensating windings is to reduce the

A.  reactance emf of commutation

B.  rotational emf of commutation

C.  transformer emf of commutation

D.  none of these

Solution:

256.  A repulsion motor is equipped with

A.  a commutator

B.  slip-rings

C.  a repeller

D.  neither A nor B

Solution:

257.  A repulsion-start induction-run single-phase motor runs as an induction motor only when

A.  brushes are shifted to neutral plane

B.  short-circuiter is disconnected

C.  commutator segments are short-circuited

D.  stator winding is reversed

Solution:

258.  If a dc series motor is operated on ac supply, it will

A.  have poor efficiency

B.  have poor power factor

C.  spark excessively

D.  all of these

Solution:

259.  An outstanding feature of a universal motor is its

A.  best performance at 50 Hz supply

B.  slow speed at all loads

C.  excellent performance on d.c. supply

D.  highest output kW/kg ratio

Solution:

260.  The direction of rotation of a hysteresis motor is determined by the

A.  retentivity of the rotor material

B.  amount of hysteresis loss

C.  permeability of rotor material

D.  position of shaded pole with respect to the main pole

Solution:

261.  Speed of the universal motor is

A.  dependent on frequency of supply

B.  proportional to frequency of supply

C.  independent of frequency of supply

D.  none of these

Solution:

262.  In the shaded pole squirrel cage-induction motor, the flux in the unshaded part always

B.  is in phase with the flux in the unshaded pole segment

C.  lags the flux in the unshaded pole segment

D.  none of these

Solution:

263.  Which of the following motor is an interesting example of beneficially utilizing a phenomenon that is often considered undesirable?

A.  hysteresis motor

B.  reluctance motor

C.  stepper motor

Solution:

264.  Usually, large motors are more efficient than small ones. The efficiency of the tiny motor is used in a wrist watch is approximately ____ per cent.

A.  1

B.  10

C.  50

D.  80

Solution:

265.  For production of a rotating magnetic field

A.  a single-phase supply is to be connected across a single-phase winding

B.  a two-phase supply should be connected across a two-phase winding

C.  a dc supply is to be connected across a single-phase winding

D.  the polarities of a dc supply connected across a single-phase winding should be continuously reversed through a suitable switching-device

Solution:

266.  When a single-phase supply is connected across a single-phase winding, the nature of magnetic field produced is

A.  pulsating in nature

B.  rotating in nature

C.  constant in magnitude but rotating at synchronous speed

D.  constant in magnitude and direction

Solution:

267.  In a resistance split-phase type single-phase induction motor, a time-phase difference between the currents in the main and auxiliary winding is achieved by

A.  placing the two-windings at an angle of 90 degrees electrical in the stator slots

B.  applying two-phase supply across the two windings

C.  having different ratio of resistance to inductive reactance for the two windings supplied from a single-phase supply system

D.  connecting the two windings in series across a single phase supply

Solution:

268.  In a split-phase capacitor-start induction motor, a time-phase difference between the currents in the main and auxiliary winding is achieved by

A.  placing the two-windings at an angle of 90 degrees electrical in the stator slots

B.  applying two-phase supply across the two windings

C.   introducing capacitive reactance in the auxiliary winding circuit

D.  connecting the two windings in series across a single phase supply

Solution:

269.  The direction of rotation of an ordinary shaded pole single-phase induction motor

A.  can be reversed by reversing the supply terminal connections to the stator winding

B.  cannot be reversed

C.  can be reversed by open-circuiting the shading rings

D.  can be reversed by short-circuiting the shading rings

Solution:

270.  Direction of rotation of split-phase type singlephase induction motor can be reversed by

A.  reversing the supply terminals connections

B.  reversing the connection of only the auxiliary winding across the supply

C.  reversing the connections of either the main winding or the auxiliary winding terminals

D.  reversing the connections of only the main winding across the supply terminals

Solution:

271.  In a single-phase repulsion motor, torque is developed on the rotor when the brush axis is fixed

A.  at 90 degrees electrical with the stator field axis

B.  in alignment with the stator field axis

C.  at an acute angle with the stator field axis

D.  at 90 degrees mechanical with the stator field axis

Solution:

272.  If the centrifugal switch of a resistance split-phase induction motor fails to close when the motor is de-energized, then

A.  no starting torque will be developed when supply is connected again across the motor terminals

B.  a dangerously high current will flow through the main winding when supply is connected again across the motor terminals

C.  starting torque developed may not be sufficient to enable the motor to restart

D.  the motor will develop high starting torque when an attempt is made to restart

Solution:

273.  A dc series motor when connected across an ac supply will

A.  develop torque in the same direction

B.  not develop any torque

C.  draw dangerously high current

D.  develop a pulsating torque

Solution:

274.  To enable a dc series motor work satisfactorily with an ac supply, the following modifications should be done

A.  The yoke and the poles should be completely laminated

B.  Only the poles should be made of laminated steel

C.  The air-gap between the stator and the rotor be reduced

D.  Compensating poles should be introduced

Solution:

SYNCHRONOUS MOTORS

275.  EE Board Exam April 1993

A single phase synchronous motor gives an output of 9.46 kW at 0.9 lagging p.f. at 600-volts. If the stray power loss is 600 watts and the field loss is 900 watts, while the effective armature resistance is 0.6-ohm, calculate the efficiency of the synchronous motor.

A.  82.35%

B.  81.24%

C.  85.25%

D.  84.64%

Solution:

276.  EE Board Exam April 1991

A three-phase synchronous motor is measured by two wattmeters A and B. Wattmeter A reads 28.6 kW and wattmeter B reads 46.5 kW. The input power of the motor is under excited and assumed constant. What are the readings of wattmeters A and B if the power factor is 85% and the motor is over-excited?

A.  48.6 kW, 26.5 kW

B.  42.5 kW, 32.6 kW

C.  52.6 kW, 22.5 kW

D.  51 kW, 24.1 kW

Solution:

277.  EE Board Exam October 1991

A 3,600 volts three-phase star connected turbo synchronous generator of synchronous reactance of 0.5 ohm per phase and negligible resistance is supplying 30,000 kVA at 0.8 pf lagging to a large power system. If the steam supply is cut-off, calculate the amount of current that the generator will then carry assuming negligible losses.

A.  3901 A

B.  3823 A

C.  3094 A

D.  3870 A

Solution:

278.  REE Board Exam October 1994

A wye-connected turbo alternator having a synchronous reactance of 0.5 ohm and negligible resistance is connected to a large power system having a busbar voltage of 13.8 kV supplying a load of 15,000 kVA at 0.80 lagging power factor. If the steam supply is cutoff, the armature current will the machine carry assuming negligible losses?

A.  256.55 A

B.  525.62 A

C.  384.26 A

D.  627.55 A

Solution:

279.  REE Board Exam October 1994

A 1,000 kVA, 6,600 V wye-connected threephase alternator having a reactance of 8.8 ohms and a negligible resistance is supplying power to a constant frequency bus bar. The open-circuit emf at this instance is 4,311 V per phase. If steam supply is suddenly cut-off, the armature current in amperes is nearest to

A.  57

B.  60

C.  59

D.  56

Solution:

280.  EE Board Exam October 1985

An inductive load consumes 10 kW at 0.75 pf lagging. A synchronous motor with a pf of 0.9 leading is connected in parallel with the inductive load. What is the minimum required kW size of the synchronous motor so that the combined load will have a pf of 0.8 lagging?

A.  1.068 kW

B.  1.203 kW

C.  1.075 kW

D.  1.109 kW

Solution:

281.  EE Board Exam April 1980, April 1992

An industrial plant draws 500 kW at 0.6 power factor from a 3-phase system. In order to raise the power factor to 0.866 lagging and to supply needed additional power, a synchronous motor is added. This motor draws 300 kW, bringing the new total plant load to 800 kW. Neglecting the losses of the synchronous motor, calculate its exact required kVA rating.

A.  363.24 kVA

B.  345.23 kVA

C.  393.34 kVA

D.  307.35 kVA

Solution:

282.  EE Board Exam April 1988

An existing industrial plant has an average load of 900 kW at 0.6 pg lagging. To raise the overall power factor to 0.92, a synchronous motor driving a DC generator is to be installed. If the input of the synchronous motor will be 250 kW, calculate its kVA input rating.

A.  753 kVA

B.  788 kVA

C.  723 kVA

D.  715 kVA

Solution:

283.  EE Board Exam October 1994

Power factor improvement may be achieved by the use of

A.  synchronous motor

B.  synchronous converter

C.  long transmission line

D.  induction motor

Solution:

284.  EE Board Exam October 1993

A synchronous motor

A.  will have a low power factor when run below rated speed

B.  may have its speed varied by inserting a rheostat in the DC field current

C.  will slow down with an increase in load within the limits of its pull out torque

D.  will take minimum line current at any load when operating at unity power factor

Solution:

285.  EE Board Exam October 1990

In starting a 500 HP, 2,300 volts, 3-phase synchronous motor the field winding is initially short circuited so as to

A.  provide better flux distribution in the air gap

B.  increase induced voltage in fild winding

C.  produce much larger starting torque

D.  lower voltage produced between layers of the field

Solution:

286.  EE Board Exam April 1990

A synchronous motor running without load is

A.  synchronous converter

B.  synchronous condenser

C.  capacitor motor

D.  induction motor

Solution:

287.  In a synchronous motor, damper winding is provided in order to

A.  stabilized rotor motion

B.  suppressed rotor oscillations

C.  develop necessary starting torque

D.  both B and C

Solution:

288.  In a synchronous motor, the magnitude of stator back e.m.f. Eb depends on

A.  speed of the motor

C.  both the speed and rotor flux

D.  d.c. excitation only

Solution:

289.  An electric motor in which both the rotor and stator fields rotate with the same speed is called a/an ____ motor.

A.  d.c.

B.  Schrage

C.  synchronous

D.  universal

Solution:

290.  While running, a synchronous motor is compelled to run at synchronous speed because of

A.  damping winding in its pole faces

B.  magnetic locking between stator and rotor poles

C.  included e.m.f. in rotor field winding by stator flux

D.  compulsion due to Lenz’s law

Solution:

291.  The direction of rotation of a synchronous motor can be reserved by reversing

A.  current to the field winding

B.  supply phase sequence

C.  polarity of rotor poles

D.  none of these

Solution:

292.  When running under no-load condition and with normal excitation, armature current Ia drawn by a synchronous motor

A.  leads the back e.m.f. Eb by a small angle

B.  is large

C.  lags the applied voltage V by a small angle

D.  lags the resultant voltage ER by 90°

Solution:

293.  The angle between the synchronously rotating stator flux and rotor poles of a synchronous motor is called ____ angle.

A.  synchronizing

B.  torque

C.  power factor

D.  slip

Solution:

294.  If load angle of a 4 pole synchronous motor is 8° (elect.), its value in mechanical degree is ____.

A.  4

B.  2

C.  0.5

D.  0.25

Solution:

295.  The maximum value of torque angle in a synchronous motor is ____ degrees electrical.

A.  45

B.  90

C.  between 45 and 90

D.  below 60

Solution:

296.  A synchronous motor running with normal excitation adjusts to load essentially by increases in its

A.  power factor

B.  torque angle

C.  back e.m.f.

D.  armature current

Solution:

297.  When load on a synchronous motor running with normal excitation is increased, armature current drawn by it increases because

A.  back e.m.f. Eb becomes less than applied voltage V

B.  power factor is decreased

C.  net resultant voltage ER in armature is increased

D.  motor speed is reduced

Solution:

298.  When load on a normally-excited synchronous motor is increased, its power factor tends to

A.  approach unity

B.  become increasingly lagging

D.  remain unchanged

Solution:

299.  The effect of increasing load on a synchronous motor running with normal excitation is to

A.  increase both its Ia and p.f.

B.  decrease Ia but increase p.f.

C.  increase Ia but decrease p.f.

D.  decrease both Ia and p.f.

Solution:

300.  Ignoring the effects of armature reaction if excitation of a synchronous motor running with constant load is increased, its torque angle must necessarily

A.  decrease

B.  increase

C.  remain constant

D.  become twice the no-load value

Solution:

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