# MCQ in AC Motors Part 4 | REE Board Exam

(Last Updated On: August 8, 2020) This is the Multiple Choice Questions Part 4 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 4 of the Series

Choose the letter of the best answer in each questions.

151.  In a double squirrel cage induction motor, the outer cage winding has

A.  high inductance

B.  low resistance

C.  high resistance

D.  none of these

Solution:

152.  At starting of a double squirrel cage induction motor

A.  lower cage winding has low reactance

B.  outer cage winding has good p.f.

C.  lower winding is more effective

D.  none of these

Solution:

153.  Very large 3-phase induction motors (> 25 HP) are started

A.  direct on line

B.  star-delta starting

C.  by autotransformer starting

D.  none of these

Solution:

154.  If the rotor slots are made deeper, rotor resistance is

A.  increased at starting

B.  decreased at starting

C.  increased under running condition

D.  none of these

Solution:

155.  In a squirrel cage induction motor, the number of rotor bars is generally

A.  even number

B.  equal to stator poles

C.  prime number

D.  none of these

Solution:

156.  In a squirrel cage induction motor, the maximum torque developed under running conditions is

A.  equal to starting torque

C.  less than starting torque

D.  much higher than full-load torque

Solution:

157.  In a wound rotor induction motor, the external resistance is so adjusted in the rotor circuit that maximum torque developed is

A.  equal to its starting torque

B.  equal to its full-load torque

C.  less than its full-load torque

D.  none of these

Solution:

158.  For the same kVA rating, the leakage flux in induction motor is ____ that of transformer.

A.  more than

B.  less than

D.  none of these

Solution:

159.  A single-phase induction motor employs ____ rotor.

A.  squirrel cage

B.  wound

C.  either A or C

D.  none of these

Solution:

160.  For the same rating, the size of a single-phase induction motor is about ____ that of the corresponding 3-phase induction motor.

A.  3 times

B.  the same as

C.  1.5 times

D.  0.33 times

Solution:

161.  For the same rating, the p.f. of a single-phase induction motor is about ____ that of the corresponding 3-phase induction motor.

A.  the same as

B.  less than

C.  more than

D.  none of these

Solution:

162.  For the same rating, the efficiency of a singlephase induction motor is about ____ that of the corresponding 3-phase induction motor.

A.  less than

B.  the same as

C.  more than

D.  none of these

Solution:

163.  Three-phase induction motors are widely used for industrial applications because

A.  they are rugged in construction, requires less maintenance and are less expensive than other motors

B.  their speed can be controlled very smoothly over a wide range

C.  their operating characteristics are superior over other electrical motors

D.  they can be manufactured easily for any hp rating

Solution:

164.  In a three-phase induction motor

A.  three-phase supply is to be given to stator winding and dc supply to the rotor winding

B.  only three-phase supply is to be given to stator winding

C.  three-phase supply is to be given to both stator and rotor windings

D.  three-phase supply is to be given to rotor winding

Solution:

165.  The stator and rotor cores of an induction motor are made up of laminated sheets

A.  to reduce the hysteresis loss in the core

B.  to reduce the eddy-current loss in the core

C.  to make the rotor and stator mechanically strong

D.  to enable the stator and rotor cores dissipate heat more effectively

Solution:

166.  The stator and rotor cores of an induction motor are made up of magnetic material

A.  to keep the cost of construction low

B.  to reduce the magnetizing current

C.  to make the parts strong

D.  because the reluctance of the magnetic material is strong

Solution:

167.  In wound-rotor type induction motors the rotor terminals are brought out through slip rings

A.  to enable extra resistance to be connected across them during starting

B.  to enable closing the rotor circuit externally

C.  to enable three-phase supply to be applied across the rotor winding through rush and slip-ring arrangement

D.  to enable connecting the rotor windings either in star or in delta depending upon the need

Solution:

168.  A pulsating (alternating) magnetic field will be produced when

A.  two-phase supply is applied across a two-phase stator winding

B.  three-phase supply is applied across a three-phase stator winding

C.  a polyphase supply is applied across a polyphase stator winding

D.  a single-phase supply is applied across a single-phase stator winding

Solution:

169.  To make the simplest 6-pole stator winding, at least

A.  two coils are needed

B.  one coil is needed

C.  three coils are needed

D.  six coils are needed

Solution:

170.  The direction of the rotating magnetic field produced by the stator ampere-turns of a three-phase induction motor changes if

A.  the sequence of the supply to the stator terminals is changed

B.  a variable frequency voltage is applied across the stator terminals

C.  the supply voltage is changed

D.  supply to any one phase is disconnected

Solution:

171.  A 400-V, 50-Hz, 4-pole, three-phase induction motor cannot run at 1500 rpm because

A.  at 1500 rpm there will be no emf induced in the rotor circuit hence no torque will be produced

B.  an induction motor can run only at a speed higher than its synchronous speed

C.  at 1500 rpm, torque developed by the rotor may not be sufficient to rotate the rotor

D.  at 1500 rpm, the rotor will draw excessive current and may be harmful to the motor

Solution:

172.  A 400-V, 50-Hz three-phase induction motor rotates at 1440 rpm on full-load. The motor is wound for

A.  2-poles

B.  4-poles

C.  6-poles

D.  8-poles

Solution:

173.  The slip of a 400-V, three-phase, 4-pole induction motor when rotating at 1440 rpm is

A.  2 per cent

B.  3 per cent

C.  4 per cent

D.  5 per cent

Solution:

174. In a 3-phase induction motor, the rotor speed is ____ the synchronous speed.

A.  greater than

B.  smaller than

C.  equal to

D.  none of these

Solution:

175.  When a 400-V, 50-Hz, 6-pole induction motor is rotating at 960 rpm on no-load, its slip is

A.  1 per cent

B.  2 per cent

C.  3 per cent

D.  4 per cent

Solution:

176.  The torque-slip characteristic for a three-phase induction motor is such that

A.  for lower values of slip, torque is directly proportional to slip and for higher values of slip, torque is inversely proportional to slip

B.  for lower values of slip, torque is inversely proportional to slip and for higher values of slip, torque is directly proportional to slip

C.  for lower values of slip, torque is directly proportional to the square of the slip and for higher values of slip, torque is inversely proportional to slip

D.  for lower values of slip, torque is directly proportional to the slip and for higher values of slip, torque is inversely proportional to square of slip

Solution:

177.  Torque developed by a three-phase, 400-V induction motor is 100 N-m. If the applied voltage is reduced to 200-V, the developed torque will be

A.  50 N-m

B.  25 N-m

C.  200 N-m

D.  62.5 N-m

Solution:

178.  If the rotor circuit resistance of a three-phase induction is increased

A.  its starting torque will increase and the maximum torque developed will also increase

B.  both the starting torque and maximum torque will remain unchanged

C.  its starting torque will increase but the maximum torque developed will decrease

D.  its starting torque will increase but the maximum torque developed will remain unchanged

Solution:

179.  To achieve higher starting torque in a three-phase slip-ring type induction motor

A.  extra resistance should be connected across the slip-rings terminals

B.  the phase sequence of the supply to the motor should be reversed

C.  the supply voltage should be increased

D.  the windings should first be connected in star and then in delta

Solution:

180.  For a three-phase induction motor having rotor circuit resistance of 6 . Maximum torque occurs at a slip of 0.6. The value of standstill rotor circuit reactance is

A.  4.44

B.  0.36

C.  1 Ω

D.  10 Ω

Solution:

181.  While starting a three-phase induction motor, a star-delta starter is used to

A.  reduce the starting current to a safe value

B.  achieve higher starting torque

C.  enable the motor to start in the right direction

D.  be able to reverse the direction of rotation of the rotor as and when necessary

Solution:

182.  A delta-connected 400-V, 50-Hz, three-phase induction motor when started direct-on-line takes a starting current of 30 A. When the motor is started through a star-delta starter. The starting current will be

A.  3 A

B.  10 A

C.  15 A

D.  30 A

Solution:

183.  The magnetizing current a three-phase induction motor is much higher than the equivalent transformer because

A.  the size of an induction motor is higher than an equivalent transformer

B.  of the presence of air-gap between stator and rotor in an induction motor

C.  grain oriented magnetic material is used for the core of an induction motor

D.  inferior magnetic material is used for the core of an induction motor

Solution:

184.  Smooth speed control of a three-phase induction motor over a wide range is possible, by

A.  pole changing method

B.  frequency control method

C.  using consequent pole technique

D.  by voltage control method

Solution:

185.  The power input in blocked-rotor test performed on a three-phase induction motor is approximately equal to

A.  hysteresis loss in the core

B.  I2R loss in the windings

C.  eddy-current loss in the core

D.  iron-loss in the core

Solution:

186.  The power input in no-load test performed on a three-phase induction motor is approximately equal to

A.  hysteresis loss in the core

B.  I2R loss in the windings

C.  eddy-current loss in the core

D.  iron-loss in the core

Solution:

187.  The efficiency and p.f. of a SCIM increases in proportion to its

A.  speed

C.  voltage

D.  rotor torque

Solution:

188.  The phenomenon of squirrel-cage motors sometime showing tendency to run at a very low speed is known as

A.  cogging

B.  crawling

C.  damping

D.  skewing

Solution:

189.  The speed of revolving field for a 50-Hz, 8-pole machine will be

A.  1500 rpm

B.  1440 rpm

C.  1000 rpm

D.  750 rpm

Solution:

190.  The torque developed by an induction motor is

A.  directly proportional to the square of the rotor resistance

B.  directly proportional to the square of the supply voltage

C.  inversely proportional to the supply voltage

D.  inversely proportional to the slip

Solution:

191.  The power factor of an induction motor will be high when

C.  rotor is blocked

D.  the rotor is crawling

Solution:

192.  A 5-hp, three-phase, 400-V star-connected squirrel-cage induction motor meant to drive a milling machine, at starting takes about

A.  40 A

B.  100 A

C.  150 A

D.  200 A

Solution:

193.  The starting torque of an induction motor can be increased by

A.  increasing the rotor reactance

B.  increasing the rotor resistance

C.  increasing the supply frequency

D.  giving supply through star-delta starter

Solution:

194.  For a given three-phase induction motor, maximum torque will be developed at starting if the rotor parameters are as follows:

A.  R2 = 2 , X2 = 8

B.  R2 = 4 , X2 = 8

C.  R2 = 8 , X2 = 8

D.  R2 = 16 , X2 = 8

Solution:

195.  The speed of a three-phase induction motor will increase if the

A.  number of poles of the stator winding is increased

B.  number of poles of the stator winding is decreased

C.  frequency of the stator supply is decreased

D.  resistance of the rotor circuit is increased

Solution:

196.  Induction motors now-a-days use die-cast aluminum rotor because

A.  aluminum is lighter than copper

B.  aluminum is cheaper than copper

C.  aluminum is easy to cast because of low melting point and is easily available

D.  aluminum has less resistivity than copper

Solution:

197.  When the rotor circuit resistance of a polyphase induction motor is increased

A.  the staring torque increases

B.  the maximum value of torque decreases

C.  the slip at which maximum torque occurs remains unchanged

D.  maximum torque is developed at starting

Solution:

198.  In an induction motor the rotor resistance and reactance are 0.2 ohm and 5 ohm respectively. In order that the torque of the motor may be a maximum, the value of slip should be equal to

A.  10%

B.  8%

C.  4%

D.  1%

Solution:

199.  What will happen if the air gap in an induction motor is increased?

A.  the windage losses will increase

B.  the magnetizing current of the rotor will decrease

C.  the power factor will decrease

D.  the speed of the motor will increase

Solution:

200.  An induction motor is said to be crawling if

A.  it accelerates too fast

B.  it is started on full load

C.  it is subjected to fluctuating load

D.  it runs at 10 to 15 percent of rated speed

Solution:

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