Electrical Machine Competitive Questions: Part-11

Electrical Machine Competitive Questions with Answers:

[1] A field excitation of 20A in a certain alternator results in an armature current of 400A in short circuit and a terminal voltage of 2000V on open circuit. The magnitude of the internal voltage drop within the machine at a load current of 200A is [GATE2009]

(a) 1V

(b) 10V

(c) 100V

(d) 1000V

Ans:D

[2] Figure shows the extended view of a 2 pole dc machine with 10 armature conductors.Normal brush positions are shown by A and B, placed at the interpolar axis. If the brushes are now shifted, in the direction of rotation, to A' and B' as shown, the voltage waveform V_A'B' will resemble. [GATE]

Ans: A

 
[3]A 220V,50Hz,single-phase induction motor has the following connection diagram and winding orientations shown.MM' is the axis of the main stator winding(M₁M₂) and AA' is that of the auxiliary winding (A₁A₂). Directions of the winding axes indicate direction of flux when currents in the windings are in the directions shown. Parameters of each winding are indicated. When switch S is closed, the motor [GATE]

(a) Rotates clockwise

(b) Rotates anticlockwise

(c) Does not rotate

(d) Rotates momentarily and comes to a halt

Ans:C

[4] The electromagnetic torque Te of a drive, and its connected load torque Tl are as shown below. Out of the operating points A,B, C and D, the stable ones are [GATE 2007]

Ans:C

[5] In a transformer, zero voltage regulation at full load is [GATE 2007]

(a) Not possible

(b) Possible at unity Power factor load

(c) Possible at leading Power factor load

(d) Possible at lagging Power factor load

Ans:C

[6] The DC motor, which can provide zero speed regulation at full load without any controller is [GATE 2007]

(a) Series

(b) Shunt

(c) Cumulative Compound

(d) Differential Compound

Ans:B

[7] A single phase 10kVA, 50 Hz transformer with 1kV primary winding draws 0.5A and 55W, at rated voltage and frequency, on no load. A second transformer has a core with all its linear v2 times the corresponding dimensions of the first transformer. The core material and lamination thickness are the same in both transformers. The primary windings of both the transformers have the same number of turns. If a rated voltage of 2kV at 50Hz  is applied to the primary of the second transformer, then the no load current and power, respectively, are  [GATE2012]

(a) 0.7 A,77.8W

(b) 0.7A,155.6 W

(c) 1A,110W

(d) 1A,220W

Ans:B

[8] The locked rotor current in a 3-phase, star connected 15kW, 4-pole, 230V, 50Hz induction motor at rated conditions is 50A. Neglecting losses and magnetizing current, the approximate locked rotor line current drawn when the motor is connected to a 236V, 57Hz supply is  [GATE2012]

(a) 58.5A

(b) 45.0A

(c) 45.7A

(d) 55.6A

Ans: B

[9] A 220V, 15kW,1000rpm shunt motor with armature resistance of 0.25O, has a rated line current of 68A and a rated field current of 2.2A. The change in field flux required to obtain a speed of 1600 rpm while drawing a line current of 52.8A and a field current of 1.8A is  [GATE2012]

(a) 18.18% increase

(b) 18.18% decrease

(c) 36.36% increase

(d) 36.36% decrease

Ans: D

 
[10] In 8 - pole wave connected motor armature, the number of parallel paths are 
(a) 8
(b) 4
(c) 2
(d) 1

Ans: C

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Electrical Machines: Part 6

[1] In a transformer the voltage regulation will be zero when it operates at
(a) Unity p.f.
(b) Leading p.f.
(c) Lagging p.f.
(d) Zero p.f. leading.

Answer: B
[2] In a stepper motor the angular displacement
(a) Can be precisely controlled.
(b) It cannot be readily interfaced with micro computer based controller.
(c) The angular displacement cannot be precisely controlled.
(d) It cannot be used for positioning of work tables and tools in NC machines.

Answer: A
[3] The power factor of a squirrel cage induction motor is
(a) Low at light load only.
(b) Low at heavy load only.
(c) Low at light and heavy load both.
(d) Low at rated load only.

Answer: A
[4] The generation voltage in India is usually
(a) Between 11 KV and 33 KV.
(b) Between 132 KV and 400 KV.
(c) Between 400 KV and 700 KV.
(d) None of the above.

Answer: A
[5] When a synchronous motor is running at synchronous speed, the damper winding produces
(a) Damping torque.
(b) Eddy current torque.
(c) Torque aiding the developed torque.
(d) No torque.

Answer: D
[6] If a transformer primary is energised from a square wave voltage source, its output voltage will be
(a) A square wave.
(b) A sine wave.
(c) A triangular wave.
(d) A pulse wave.

Answer: A
[7] A salient pole synchronous motor is running at no load. Its field current is switched off. The motor will
(a) Come to stop.
(b) Continue to run at synchronous speed.
(c) Continue to run at a speed slightly more than the synchronous speed.
(d) Continue to run at a speed slightly less than the synchronous speed.

Answer: B
[8] The frequency of the rotor current in a 3 phase 50 Hz, 4 pole induction motor at full load speed is about
(a) 50 Hz.
(b) 20 Hz.
(c) 2 Hz.
(d) Zero.

Answer: C
[9] The speed of a dc motor can be controlled by varying
(a) Its flux per pole
(b) Resistance of armature circuit
(c) Applied voltage
(d) All of the above

Answer: D
[10] Regarding Ward-Leonard system of speed control which statement is false?
(a) It is usually used where wide and very sensitive speed control is required
(b) It is used for motors having ratings from 750kW to 4000kW
(c) Capital outlay involved in the system is right since it uses two extra machines
(d) It gives a speed range of 10:1 but in one direction only
(e) It has low overall efficiency especially at light loads

Answer: D

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Electrical Machines: Part 5

[1] A stepper motor is
(a) A dc motor.
(b) A single-phase ac motor.
(c) A multi-phase motor.
(d) A two phase motor.

Answer: D
Stepper motor works on 1-phase-ON or 2-phase �ON modes of operation
[2] A motor which can conveniently be operated at lagging as well as leading power factors is the
(a) Squirrel cage induction motor.
(b) Wound rotor induction motor.
(c) Synchronous motor.
(d) DC shunt motor.

Answer: C
[3] The d.c. series motor should always be started with load because
(a) At no load, it will rotate at dangerously high speed.
(b) It will fail to start.
(c) It will not develop high starting torque.
(d) All are true.

Answer: A
[4] The �sheath� is used in cable to
(a) Provide strength to the cable.
(b) Provide proper insulation.
(c) Prevent the moisture from entering the cable.
(d) Avoid chances of rust on strands.

Answer: A
The sheath in underground cable is provided to give mechanical strength.
[5] The drive motor used in a mixer-grinder is a
(a) DC motor.
(b) Induction motor.
(c) Synchronous motor.
(d) Universal motor.

Answer: D
The universal motor is suitable for AC & DC both supply systems.
[6] A balanced three-phase, 50 Hz voltage is applied to a 3 phase, 4 pole, induction motor. When the motor is delivering rated output, the slip is found to be 0.05. The speed of the rotor m.m.f. relative to the rotor structure is
(a) 1500 r.p.m.
(b) 1425 r.p.m.
(c) 25 r.p.m.
(d) 75 r.p.m.

Answer: D
NS = 120f /P = 120 x 50 /4 =1500rpm
N = NS ( 1-s) = 1500 (1-0.05) = 1425
Relative speed = 1500 � 1425 = 75 rpm
[7] The primary winding of a 220/6 V, 50 Hz transformer is energised from 110 V, 60 Hz supply. The secondary output voltage will be
(a) 3.6 V.
(b) 2.5 V.
(c) 3.0 V.
(d) 6.0 V.

Answer: C
[8] The current from the stator of an alternator is taken out to the external load circuit through
(a) Slip rings.
(b) Commutator segments.
(c) Solid connections.
(d) Carbon brushes.

Answer: C
[9] In a 3 � phase induction motor the maximum torque
(a) Is proportional to rotor resistance r2 .
(b) Does not depend on r2 .
(c) Is proportional to square root of r2 .
(d) Is proportional to  square of r2 .

Answer: B
[10] In a d.c. machine, the armature mmf is
(a) Stationary w.r.t. armature.
(b) Rotating w.r.t. field.
(c) Stationary w.r.t. field.
(d) Rotating w.r.t. brushes.

Answer: C

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Electrical Machines: Part 3

[1]The size of the feeder is determined primarily by
(a) The current it is required to carry.
(b) The percent variation of voltage in the feeder.
(c) The voltage across the feeder.
(d) The distance of transmission.

Answer: A
[2] The emf induced in the primary of a transformer
(a) Is in phase with the flux.
(b) Lags behind the flux by 90 degree.
(c) Leads the flux by 90 degree.
(d) Is in phase opposition to that of flux.

Answer: C
[3] The relative speed between the magnetic fields of stator and rotor under steady state operation is zero for a
(a) DC machine.
(b) 3 phase induction machine.
(c) Synchronous machine.
(d) Single phase induction machine.

Answer: all options are correct
[4] As the voltage of transmission increases, the volume of conductor
(a) Increases.
(b) Does not change.
(c) Decreases.
(d) Increases proportionately.

Answer: C
Due to skin effect
[5] In a 3-phase synchronous motor
(a) The speed of stator MMF is always more than that of rotor MMF.
(b) The speed of stator MMF is always less than that of rotor MMF.
(c) The speed of stator MMF is synchronous speed while that of rotor MMF is zero.
(d) Rotor and stator MMF are stationary with respect to each other.

Answer: D
Motor is magnetically locked into position with stator, the rotor poles are engaged with stator poles and both run synchronously in same direction.
[6] An alternator is delivering rated current at rated voltage and 0.8 power-factor lagging case. If it is required to deliver rated current at rated voltage and 0.8 power-factor leading, the required excitation will be
(a) Less.
(b) More.
(c) More or less.
(d) The same.

Answer: B
Over excitation gives leading power factor and under excitation gives lagging p.f .
[7] Out of the following methods of heating the one which is independent of supply frequency is 
(a) Electric arc heating
(b) Induction heating
(c) Electric resistance heating
(d) Dielectric heating

Answer: C
[8] In a capacitor start single-phase induction motor, the capacitor is connected 
(a) In series with main winding.
(b) In series with auxiliary winding.
(c) In series with both the windings.
(d) In parallel with auxiliary winding.

Answer: B
To make single phase motor self start. We split the phases at 90 degree. Hence, motor behaves like a two phase motor.
[9] A synchro has
(a) A 3-phase winding on rotor and a single-phase winding on stator.
(b) A 3-phase winding on stator and a commutator winding on rotor.
(c) A 3-phase winding on stator and a single-phase winding on rotor.
(d) A single-phase winding on stator and a commutator winding on rotor.

Answer: C
The basic synchro unit called a synchro transmitter. It�s construction similar to that of a Three phase alternator.
[10] A ceiling fan uses
(a) Split-phase motor.
(b) Capacitor start and capacitor run motor.
(c) Universal motor.
(d) Capacitor start motor.

Answer: D
To give starting torque and to maintain speed.

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Electrical Machines: Part 4

[1] The torque-speed characteristics of an a.c. operated universal motor has a ______characteristic and it______ be started under no-load condition.

(a) Inverse, can

(b) Nearly inverse, can

(c) Inverse, cannot

(d) Nearly inverse, cannot

Answer: C

N direct proportional to 1/ T

[2] In the heating process of the ________type a simple method of temperature control is possible by means of a special alloy which loses its magnetic properties at a particular high temperature and regains them when cooled to a temperature below this value.

(a) Indirect induction over

(b) Core type induction furnace

(c) Coreless induction furnace

(d) High frequency eddy current

Answer:  D

Magnetic property of alloy changes with change of the temperature and Heat is produced due to eddy current = Square(i) * R and i proportional to square(f)

[3] In order to reduce the harmful effects of harmonics on the A.C. side of a high voltage D.C. transmission system ______are provided.

(a) Synchronous condensers

(b) Shunt capacitors

(c) Shunt filters

(d) Static compensators

Answer:  C

Xc= 1/?C

[4]An a.c. tachometer is just a ________with one phase excited from the carrier frequency.

(a) Two-phase A.C. servomotor

(b) Two-phase induction motor

(c) A.C. operated universal motor

(d) Hybrid stepper motor.

Answer:  D

It is a special purpose machine. It's stator coil can be energized by electronically switched current.

[5] The rotor frequency for a 3 phase 1000 RPM 6 pole induction motor with a slip of 0.04 is________Hz

(a) 8

(b) 4

(c) 6

(d) 2

Answer: D

f = N P/ 120 = 1000* 6/120 = 50 Hz

Rotor frequency fr=s *  f = 0.04 * 50= 2.0 Hz

[6] The speed-torque characteristics of a DC series motor are approximately similar to those of the _________motor.

(a) Universal

(b) Synchronous

(c) DC shunt

(d) Two-phase

Answer: A

Universal motor has same characteristics as DC series motor

It is known as an a.c series motor.

[7]In case of a universal motor, torque pulsation is minimized by _________.

(a) Load inertia

(b) Rotor inertia

(c) Both rotor and load inertia

(c) None of the above

Answer: C

[8] A hysteresis motor

(a) Is not a self-starting motor.

(b) Is a constant speed motor.

(c) Needs dc excitation.

(d) Can not be run in reverse speed.

Answer: B

[9] The most suitable servomotor for low power applications is

(a) A dc series motor.

(b) A dc shunt motor.

(c) An ac two-phase induction motor.

(d) An ac series motor.

Answer: B

[10] The size of a conductor used in power cables depends on the

(a) Operating voltage.

(b) Power factor.

(c) Current to be carried.

(d) Type of insulation used.

Answer:  C
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Electrical machines: Part 2

[1] In a DC motor, unidirectional torque is produced with the help of
a. Brushes
b. Commutator
c. End-plates
d. Both a and b

Answer: D
[2] The counter emf of dc motor
a. Often exceeds the supply voltage
b. Aids the applied voltage
c. Helps in energy conversion
d. Regulates its armature voltage

Answer: C
[3] The Eb/V ratio of a dc motor is an indication of its
a. Efficiency
b. Speed regulation
c. Starting torque
d. Running Torque

Answer: A
[4] The induced emf in the armature conductors of a dc motor is
a. Sinusoidal
b. Trapezoidal
c. Rectangular
d. Alternating

Answer: A
[5] A dc motor can be looked upon as dc generator with the power flow
a. Reduced
b. Reversed
c. Increased
d. Modified

Answer:B
[6] A series motor is best suited for driving
a. Lathes
b. Cranes and hoists
c. Shears and punches
d. Machine tools

Answer: B
[7]The Ta/Ia graph of a dc series motor is a
a. Parabola from no load to overload
b. Straight line throughout
c. Parabola throughout
d. Parabola upto full load and a straight line at overloads

Answer: D
[8] When load is removed, .......motor will run at the highest speed.
a. Shunt
b. Cumulative - compound
c. Differential compound
d. Series

Answer: D
[9] The power factor of a squirrel cage induction motor is
a. Low at light loads only
b. Low at heavy loads only
c. Low at light and heavy loads both
d. Low at rated load only

Answer: A
[10] The synchronous speed of a linear induction motor does not depend on
a. Width of pole pitch
b. Number of poles
c. Supply frequency
d. Any of the above

Answer: B. See the comment for explanation..
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Electrical Machine: Part 1

Electrical Machines Questions and Answers
[1] A salient pole synchronous motor is running at no load. Its field current is switched off. The motor will
(a) Come to stop.
(b) Continue to run at synchronous speed.
(c) Continue to run at a speed slightly more than the synchronous speed.
(d) Continue to run at a speed slightly less than the synchronous speed.

Answer: B

[2] The emf induced in the primary of a transformer
(a) Is in phase with the flux.
(b) Lags behind the flux by 90 degree.
(c) Leads the flux by 90 degree.
(d) Is in phase opposition to that of flux.

Answer: C

[3] The frequency of the rotor current in a 3 phase 50 Hz, 4 pole induction motor at full load speed is about
(a) 50 Hz.
(b) 20 Hz.
(c) 2 Hz.
(d) Zero.

Answer: C

[4] The two windings of a transformer is
(a) Conductively linked.
(b) Inductively linked.
(c) Not linked at all.
(d) Electrically linked.

Answer : B

[5]  The d.c. series motor should always be started with load because
(a) At no load, it will rotate at dangerously high speed.
(b) It will fail to start.
(c) It will not develop high starting torque.
(d) All are true.

Answer: A 

[6] Out of the following methods of heating the one which is independent of supply

frequency is

(a) Electric arc heating

(b) Induction heating

(c) Electric resistance heating

(d) Dielectric heating

Answer: C

[7] In a stepper motor the angular displacement 
(a) Can be precisely controlled.
(b) It cannot be readily interfaced with micro computer based controller.
(c) The angular displacement cannot be precisely controlled.
(d) It cannot be used for positioning of work tables and tools in NC machines.

Answer: A
[8] The generation voltage is usually

(a) Between 11 KV and 33 KV.
(b) Between 132 KV and 400 KV.
(c) Between 400 KV and 700 KV.

(d) None of the above.

Answer: A 
[9] When a synchronous motor is running at synchronous speed, the damper winding produces 
(a) Damping torque.
(b) Eddy current torque.
(c) Torque aiding the developed torque.
(d) No torque.

Answer: D
[10] A hysteresis motor
(a) Is not a self-starting motor.
(b) Is a constant speed motor.
(c) Needs dc excitation.
(d) Can not be run in reverse speed.

Answer: B
[11] If a transformer primary is energized from a square wave voltage source, its output voltage will be
(a) A square wave.
(b) A sine wave.
(c) A triangular wave.
(d) A pulse wave.

Ans: A
[12] The power factor of a squirrel cage induction motor is
(a) Low at light load only.
(b) Low at heavy load only.
(c) Low at light and heavy load both.
(d) Low at rated load only.

Answer: A
[13] In a d.c. machine, the armature mmf is
(a) Stationary w.r.t. armature.
(b) Rotating w.r.t. field.
(c) Stationary w.r.t. field.
(d) Rotating w.r.t. brushes.

Answer: C 
[14] In a transformer the voltage regulation will be zero when it operates at
(a) Unity p.f.
(b) Leading p.f.
(c) Lagging p.f.
(d) Zero p.f. leading.

Answer: B
[15] The primary winding of a 220/6 V, 50 Hz transformer is energised from 110 V, 60 Hz supply. The secondary output voltage will be
(a) 3.6 V.
(b) 2.5 V.
(c) 3.0 V.
(d) 6.0 V.

Answer: C  
[16] The relative speed between the magnetic fields of stator and rotor under steady state operation is zero for a
(a) DC machine.
(b) 3 phase induction machine.
(c) Synchronous machine.
(d) Single phase induction machine.

Answer: All are correct
[17] The current from the stator of an alternator is taken out to the external load circuit through
(a) Slip rings.
(b) Commutator segments.
(c) Solid connections.
(d) Carbon brushes.

Ans: C  
[18] A motor which can conveniently be operated at lagging as well as leading power factors is the
(a) Squirrel cage induction motor.
(b) Wound rotor induction motor.
(c) Synchronous motor.
(d) DC shunt motor.

Answer: C  
[19] The most suitable servomotor for low power applications is
(a) A dc series motor.
(b) A dc shunt motor.
(c) An ac two-phase induction motor.
(d) An ac series motor.

Answer: B 
[20] The size of a conductor used in power cables depends on the
(a) Operating voltage.
(b) Power factor.
(c) Current to be carried.
(d) Type of insulation used.

Answer: C   

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