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# MCQ in DC Motors Part 4 | REE Board Exam

This is the Multiple Choice Questions Part 4 of the Series in DC 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.ย  The rotating part of a dc motor is known as

A.ย  pole

B.ย  stator

C.ย  armature

D.ย  carbon brush

Solution:

152.ย  The function of a commutator in a dc machine is

A.ย  to change alternating current to direct current

B.ย  to improve commutaion

C.ย  for easy speed control

D.ย  to change alternating voltage to direct voltage

Solution:

153.ย  Carbon brushes are used in electric motors to

A.ย  brush off carbon deposits on the commutator

B.ย  provide a path for the flow of current

C.ย  prevent overheating of armature windings

D.ย  prevent sparking during commutation

Solution:

154.ย  Voltage equation of dc motor is

A.ย  V = Eb + IaRa

B.ย  V = Eb โ IaRa

C.ย  Eb = V + IaRa

D.ย  Eb = V โ IaRa

Solution:

155.ย  Condition for maximum power output for a dc motor is

A.ย  Eb = V

B.ย  Eb = V/2

C.ย  Eb = IaRa

D.ย  Eb = IaRa/2

Solution:

156.ย  The speed of a motor is

A.ย  directly proportional to back emf and inversely proportional to flux

B.ย  inversely proportional to back emf and directly proportional to flux

C.ย  directly proportional to back emf as well as flux

D.ย  inversely proportional to back emf as well as flux

Solution:

157.ย  The highest speed attained by a shunt motor is

A.ย  equal to infinity of rated flux

B.ย  higher than no load speed at rated flux

C.ย  equal to no load speed at rated flux

D.ย  lower than no load speed at rated flux

Solution:

158.ย  In dc motor iron losses occur in

A.ย  the yoke

B.ย  the armature

C.ย  the field

D.ย  none of these

Solution:

159.ย  The dummy coil in a dc machines is used to

A.ย  eliminate reactance voltage

B.ย  eliminate armature reaction

C.ย  bring about mechanical balance of armature

D.ย  eliminate harmonics developed in the machine

Solution:

160.ย  An external resistance is added in the series with the field of a dc shunt motor. When the motor runs, the effect of resistance is

A.ย  to reduce the speed of the motor

B.ย  to increase the speed of the motor

C.ย  to reduce the armature current drawn by the motor

D.ย  to reduce the losses

Solution:

161.ย  The speed of a dc shunt motor is required to be more than F.L. speed. This is possible by

A.ย  increasing the armature current

B.ย  decreasing the armature current

C.ย  increasing the excitation current

D.ย  reducing the field current

Solution:

162.ย  If speed of a dc shunt motor increases, the back emf

A.ย  increases

B.ย  decreases

C.ย  remains constant

D.ย  decreases and then increases

Solution:

Unsolved problems

163.ย  REE Board Exam April 2002

A certain shunt motor armature has a resistance of 0.70 ohm and current of 50 amperes at 120 volts. Determine the counter emf.

A.ย  108.5 V

B.ย  116.5 V

C.ย  112.2 V

D.ย  118.3 V

Solution:

164.ย  REE Board Exam April 2001

A DC shunt motor develops 15 hp at 120 V. The armature efficiency is 95%. What is the armature effective resistance?

A.ย  0.10 ohm

B.ย  0.06 ohm

C.ย  0.08 ohm

D.ย  0.60 ohm

Solution:

165.ย  REE Board Exam April 2001

A DC shunt motor develops 15 hp at 120 V. If the effective armature resistance is 0.001 ohm and the field winding draws 2 amperes, what is the overall efficiency?

A.ย  90%

B.ย  93%

C.ย  95%

D.ย  94%

Solution:

166.ย  A 6 pole lap wound shunt motor takes 300 A when the speed is 500 rpm. The flux per pole is 0.06 Wb and the armature has a 870 conductors. Neglecting the shunt field current, calculate the output horsepower of the motor. Assume a constant loss of 4%.

A.ย  175 hp

B.ย  168 hp

C.ย  172 hp

D.ย  165 hp

Solution:

167.ย  At rated load, a shunt motor draws an armature current of 50 A from a 230 V dc mains while running at 1000 rpm. At no-load the armature current drops to 5 A. If the effect of armature reaction has weaken the flux by 2% from noload to full-load, determine speed of the motor at no-load. Assume the armature resistance to be 0.15 ohm.

A.ย  1202 rpm

B.ย  1221 rpm

C.ย  1122 rpm

D.ย  1010 rpm

Solution:

168.ย  A simplex lap wound armature has 580 conductors and carries a current of 125 ampere per armature current path. If the flux per pole is 20 mWb, calculate the electromagnetic torque developed by the armature.

A.ย  210.64 N-m

B.ย  252.72 N-m

C.ย  230.77 N-m

D.ย  207.63 N-m

Solution:

169.ย  A 100-volt shunt motor is developing 6 hp while operating at an over-all efficiency of 86%. The armature and shunt field resistances are 0.06 and 50 ohms respectively. Determine the stray power losses.

A.ย  378 W

B.ย  367 W

C.ย  392 W

D.ย  364 W

Solution:

170.ย  A long shunt compound motor takes a current of 42 A from a 230 V source. Armature, series field and shunt field resistances are 0.1 ohm, 0.2 ohm and 50 ohms respectively. If the friction and windage losses amount to 400 W, determine the over-all efficiency of the machine.

A.ย  82.24%

B.ย  80.55%

C.ย  81.22%

D.ย  79.53%

Solution:

171.ย  Interpole windings in direct current motors are used primarily

A.ย  as means for varying the speed of the motor

B.ย  to reduce the effect of armature reaction

C.ย  increase the efficiency of the motor

D.ย  to compensate for field leakage

Solution:

172.ย  In a series motor, what is the relation of the connection of motor field winding with respect to the motor armature?

A.ย  series

B.ย  parallel

C.ย  series-parallel

D.ย  delta

Solution:

173.ย  A 220 V shunt motor driving its normal load draws an armature current of 50 A from a 220 V dc source. The armature resistance of this motor including brushes is 0.25 ohm. How much armature current (minimum) will this motor draw from a 200 V dc source when driving the same load with the field adjusted to maintain the same speed?

A.ย  55.76 A

B.ย  58.24 A

C.ย  60.12 A

D.ย  63.48 A

Solution:

174.ย  A DC shunt motor runs at 600 rpm on a 240-volt supply while drawing a line current of 30 A. Its armature and field resistances are 0.5 โฆ and 120 โฆ respectively. What resistance should be placed in series with the armature circuit in order to reduce the speed to 400 rpm. Assume no changes in the armature or field current.

A.ย  2.69 โฆ

B.ย  2.41 โฆ

C.ย  2.07 โฆ

D.ย  2.83 โฆ

Solution:

175.ย  A 120-V shunt motor has an armature equivalent of 0.5 โฆ and a field resistance of 60 โฆ. At full load, the motor takes 10 A and the speed is 1000 rpm. At what speed must this motor maybe driven as a generator to deliver 10 A to an external load at 120 V?

A.ย  1158 rpm

B.ย  1086 rpm

C.ย  1262 rpm

D.ย  1045 rpm

Solution:

176.ย  A shunt motor draws current of 40 A from a 120 V source and runs at 1200 rpm at rated load. The armature and field circuit resistances are 0.1 โฆ and 60 โฆ, respectively. Determine the speed of the motor at half load. Assume a brush drop of 2 V rated load and 1 V half load.

A.ย  1180 rpm

B.ย  1238 rpm

C.ย  1230 rpm

D.ย  1026 rpm

Solution:

177.ย  A 220-V shunt motor delivers 40 hp on full-load at 950 rpm and has an efficiency of 88%. The armature and field resistances are 0.2 ohm and 110 ohms, respectively. Determine the starting resistance such that the starting armature current does not exceed 1.6 times the full-load current.

A.ย  0.704 ohm

B.ย  0.732 ohm

C.ย  0.712 ohm

D.ย  0.739 ohm

Solution:

178.ย  A 50 HP, 550 V shunt-wound motor draws a line current of 4.5 A at no-load. The shunt field resistance is 275 ohms and the armature resistance exclusive of the brushes is 0.3 ohm, the brush drop at full-load is 2 V. At full-load, the motor draws a line current of 84 A. Calculate the efficiency at full-load.

A.ย  92.0%

B.ย  88.5%

C.ย  91.2%

D.ย  89.9%

Solution:

179.ย  A 500 V shunt motor running at 720 rpm takes an armature current of 50 A. The equivalent armature resistance is 0.4 ohm. What resistance must be placed in series with the armature to reduce the speed to 540 rpm while maintaining the same torque?

A.ย  2.2 ohms

B.ย  2.6 ohms

C.ย  2.4 ohms

D.ย  2.8 ohms

Solution:

180.ย  A 400 V series motor has a field resistance of 0.2 ohm and an armature resistance of 0.1 ohm. The motor takes 30 A of current at 1000 rpm while developing full-load torque. What is the motor speed when this motor develops 60% of full load torque?

A.ย  1302.4 rpm

B.ย  1257.2 rpm

C.ย  1297.6 rpm

D.ย  1135.5 rpm

Solution:

181.ย  The field and armature resistances of a 220-V series motor are 0.2 ohm and 0.1 ohm, respectively. The motor takes 30 A of current while running at 700 rpm. If the total iron and friction losses are 350 W, determine the motor efficiency.

A.ย  90.6%

B.ย  91.5%

C.ย  89.4%

D.ย  92.2%

Solution:

182.ย  The input current to a 220 V, short shunt compound motor at no-load is 6 A. The shunt field circuit resistance is 220 ohms, the armature resistance is 0.10 ohm and the series field resistance is 0.08 ohm. What is the stray power loss?

A.ย  1153.31 W

B.ย  1066.22 W

C.ย  1232.54 W

D.ย  1073.25 W

Solution:

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