Explore topic-wise MCQs in Electrical Machines.

This section includes 132 Mcqs, each offering curated multiple-choice questions to sharpen your Electrical Machines knowledge and support exam preparation. Choose a topic below to get started.

101.

Constant losses of DC motor can be measured by:

A. Swinburne's Test
B. Retardation Test
C. Hopkinson's Test
D. Load Test
Answer» B. Retardation Test
Discussion
102.

Plugging provides

A. Zero torque braking
B. Highest torque braking
C. Indeterminate torque braking
D. Smallest torque braking
Answer» C. Indeterminate torque braking
Discussion
103.

In a DC motor which of the following can sustain the maximum temperature rise?

A. armature winding
B. field winding
C. commutator
D. slip ring
Answer» C. commutator
Discussion
104.

In which of the following method, the effect of armature reaction is more?

A. Field weakening method
B. Armature resistance control
C. Same in both methods
D. Cannot be determined
Answer» B. Armature resistance control
Discussion
105.

A 200-V Dc generator supplies 4 kW at a terminal voltage of 200 V, the armature resistance being 0.5 Ω. If the machine is operated as a motor at the same terminal voltage with the same armature current, find the ratio of the generator speed Ng, to the motor speed Nm .

A. \(\dfrac{N_g}{N_m}=1.25\)
B. \(\dfrac{N_g}{N_m}=1.105\)
C. \(\dfrac{N_g}{N_m}=0.905\)
D. \(\dfrac{N_g}{N_m}=0.833\)
Answer» C. \(\dfrac{N_g}{N_m}=0.905\)
Discussion
106.

In a dc compound motor, 4-point starter provided as _____.

A. to reduce the field current
B. to increase the field current
C. not to affect the current flowing through 'Hold on' coil even when the field current changes
D. none of these
Answer» D. none of these
Discussion
107.

A 220 V DC motor has an armature resistance of 0.8 Ω. Find the induced emf when the motor has a full load armature current of 40 A.

A. 220 V
B. 180 V
C. 188 V
D. 32 V
Answer» D. 32 V
Discussion
108.

Direction: The following item consists of two statements, one labelled as ‘Statement (I)’ and the other as ‘Statement (II)’. Examine these two statements carefully and select the answers to these items using the code given below:Statement (I): A Direct-On-Line (DOL) starter for starting dc motor is used for reasons of economyStatement (II): DOL starter limits the starting current to a safe limit

A. Both Statement I and Statement II are individually true and Statement II is the correct explanation of Statement I
B. Both Statement I and Statement II are individually true but Statement II is not the correct explanation of Statement I
C. Statement I is true but Statement II is false
D. Statement I is false but Statement II is true
Answer» D. Statement I is false but Statement II is true
Discussion
109.

For a dc motor operating under condition of maximum transfer of power, the efficiency of the motor is

A. 100 %
B. about 90%
C. 75%
D. less than 50%
Answer» E.
Discussion
110.

Neglecting saturation, if the current taken by a series motor is increased from 10 A to 12 A, the percentage increase in its torque will be:

A. 44%
B. 40%
C. 20%
D. 16%
Answer» B. 40%
Discussion
111.

A 200 V d.c machine has Ra = 0.5 Ω and its full-load la = 20 A. Determine the induced e.m.f when the machine acts as a motor.

A. 210 V
B. 190 V
C. 200 V
D. 215 V
Answer» C. 200 V
Discussion
112.

Consider the following figure for a 4-pole, 3-phase, 24-slots double layer winding. Find the pole pitch and the slot pitch angle.

A. 4 and 40 electrical degrees
B. 5 and 50 electrical degrees
C. 6 and 30 electrical degrees
D. 6 and 60 electrical degrees
Answer» D. 6 and 60 electrical degrees
Discussion
113.

In a cumulative compound DC motor with rated voltage applied with rated load, if the series winding part is shorted, then speed will be:

A. less than full speed
B. more than rated speed
C. same
D. zero
Answer» C. same
Discussion
114.

A dc motor develops an electromagnetic torque 150 N-m in a certain operating conduction. from this operating conduction a 10% reduction in field flux and 50% increase in armature current is made. What will be new value of electromagnetic torque?

A. 225 N-m
B. 202.5 N-m
C. 22.5 N-m
D. 20.25 N-m
Answer» C. 22.5 N-m
Discussion
115.

Directions: It consists of two statements, one labelled as the ‘Statement (I)’ and the other as ‘Statement (II)’. You are to examine these two statements carefully and select the answer using the codes given below:Statement (I): In large dc motors which drive reversing rolls of a steel mill and in the generators which supply them, large fluctuations of currents cause higher voltages being induced in the coils located between adjacent commentator segments. This may result in the so called ‘flash-over’ between adjacent brushes, causing a heavy short-circuit.Statement (II): To counteract the possibilities of a ‘flash-over’, a ‘compensating winding’, which is embedded in the slots on the pole faces, is connected in series with armature winding, and which supplies an mmf that magnetizes in the opposite direction to that of the armature mmf.

A. Both Statement (I) and Statement (II) are individually true and Statement (II) is the correct explanation of Statement (I)
B. Both Statement (I) and Statement (II) are individually true but Statement (II) is NOT the correct explanation of Statement (I)
C. Statement (I) is true but Statement (II) is false
D. Statement (I) is false but Statement (II) is true
Answer» C. Statement (I) is true but Statement (II) is false
Discussion
116.

A 240v dc shunt motor with an armature resistance of 0.5 has a full load current of 40A. Find out the ratio of the stalling torque to the full load torque when a resistance of 1 ohm is connected in series with the armature.

A. 4
B. 6
C. 12
D. None
Answer» B. 6
Discussion
117.

A dc shunt motor runs at 600 rpm from a 240 V supply. Its armature has a resistance of 0.5 ohm and draws 30 amp. If an external resistance of 2.5 ohm is connected in series with the armature, while there is no change in armature or filed current, the new speed is

A. 400 rpm
B. 800 rpm
C. 600 rpm
D. None of these
Answer» B. 800 rpm
Discussion
118.

A 240 V shunt motor with the armature resistance of 0.1 Ω runs at 850 r.p.m. for an armature current of 70 A. If its speed is to be reduced to 650 r.p.m., the resistance to be placed in series for an armature current of 50 A is nearly

A. 0.82 Ω
B. 1.14 Ω
C. 1.24 Ω
D. 1.34 Ω
Answer» C. 1.24 Ω
Discussion
119.

For which of the following applications is DC shunt motor preferred over other motors?

A. For high speed application
B. For constant speed application
C. For low speed application
D. For applications requiring high starting torque
Answer» C. For low speed application
Discussion
120.

THYRISTOR_CONTROLLED_STARTER_IS_PREFERRED_OVER_DOL_STARTER_DUE_TO?$

A. lesser losses
B. controlled direction
C. least resistance offered
D. all of the mentioned
Answer» B. controlled direction
Discussion
121.

The effect of fringing increases as we$

A. increase air gap
B. decrease air gap
C. increase in flux density
D. introduce more ferric core material
Answer» B. decrease air gap
Discussion
122.

For_a_7.46_kW,_200_V_dc_shunt_motor_with_full_load_efficiency_of_85%_has_armature_resistance_of_0.25_ohms._Calculate_the_value_of_starting_resistance_in_ohms_for_a_current_1.5_times_of_the_full_load_current.$

A. 2.788
B. 3.038
C. 2.688
D. 2.588
Answer» B. 3.038
Discussion
123.

For a dc machine , its commutator has a diameter of 50 c rotating at 1000 rpm. For a brush width of 1 cm, the time commutation taken by the machine will be

A. 0.382 ms
B. 0.456 ms
C. 0.573 ms
D. 0.312 ms
Answer» B. 0.456 ms
Discussion
124.

A dc machine is run at rated speed in forward direction and then in backward direction. It is observed that, speeds of the rotation are different, then

A.
B. incorrect brush placement
C. incorrect pole and core alignment
Answer» B. incorrect brush placement
Discussion
125.

If the students give a forward shift of 10° to the dc generator, then it$

A. reduces flux per pole
B. improves flux per pole
C. increases the flux density in core
D. none of the mentioned
Answer» B. improves flux per pole
Discussion
126.

The post effects of the armature reaction is

A. main field distortion
B. shift in MNA
C. reduction in main field
D. none of the mentioned
Answer» E.
Discussion
127.

The direct-on-line starter is used to start a small dc motor because it limits initial current drawn by armature circuit?

A. True
B. False
Answer» C.
Discussion
128.

The shunt motor starters that can be used is/are

A. 3-point and 4-point starter
B. 5-point starter
C. 4-point starter
D. 5-point and 3-point starter
Answer» B. 5-point starter
Discussion
129.

Following are the observations due to large starting current in dc motor.

A. Sparking at brushes
B. Mechanical shock at shaft
C. Source current fluctuations
D. Burning of the field winding
Answer» B. Mechanical shock at shaft
Discussion
130.

A 100 hp, 250 V, 350 A shunt dc motor with an armature resistance of 0.05 ohms. To limit maximum starting current to twice the rated of its value, what will be the number of stages of starting resistances?

A. 3
B. 2
C. 4
D. 5
Answer» B. 2
Discussion
131.

Considering a human handed control system for the dc motor speed control, if the resistance wire cut out too slowly, then

A. starting resistance would burn
B. field winding would burn
C. speed will rise steeply
D. any of the mentioned
Answer» B. field winding would burn
Discussion
132.

A starting resistance is inserted at the starting in an induction motor as well as dc motor.

A. Induction motor has to control starting torque whereas in dc motor, it is done to avoid large current
B. To limit starting current in both the machines
C. To limit starting speed
D. All of the mentioned
Answer» B. To limit starting current in both the machines
Discussion