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# Multiple choice question for engineering

## Set 1

1. The flux addition is due to
(i) sinusoidal distribution of mmfs
(ii) zero relative velocity between two mmfs
(iii) spatial distribution of three phases by 120°
a) (i), (ii)
b) (i),(ii), (iii)
c) (iii)
d) (ii), (iii)

Answer: a [Reason:] Flux addition in ac machines is due to sinusoidal flux distribution and they must be stationary with respect to each other.

2. If the resultant flux will not be in phase with the resultant mmf Fr. This is due to
a) hysteresis effect
b) armature reaction
c) hysteresis effect and armature reaction
d) eddy currents

Answer: a [Reason:] Hysteresis effect causes the flux to be lagging in nature.

3. The internal power factor angle Ψ is between
a) Ef and Ia
b) Ia and field flux
c) Ef and Vt
d) Vt and Ia

Answer: a [Reason:] 4. The load angle δ is between
a) Ef and Ia
b) Ia and field flux
c) Ef and Vt
d) Vt and Ia

Answer: c [Reason:] It is the angle between excitation voltage and the terminal voltage.

5. If it is assumed that the iron part of magnetic circuit requires zero mmf, relation between Ef and If will be
a) b) c) d) Answer: a [Reason:] There will be linear relation in Ef and Ia as mmf required is zero.

6. Phasor diagram below is best describing the a) Open circuit characteristic of alternator
b) Short circuit characteristic of alternator
c) External circuit characteristic of alternator
d) Internal circuit characteristic of alternator

Answer: a [Reason:] Graph of Ef vs Ff is the open circuit characteristic of the machine.

7. The short circuit characteristic of alternator is a straight line due to
a) no saturation
b) no armature reaction
c) no eddy current
d) all of the mentioned

Answer: a [Reason:] At the short circuit of the alternator is having the linear relation as the armature does not enter the saturation mode due to demagnetizing nature of armature reaction.

8. From the OCC and SCC curves, what does the intercept AB represent? a) Armature reaction
b) Leakage reactance
c) Armature copper losses
d) Sum of armature reaction and leakage reactance

Answer: a [Reason:] It is armature reaction which is calculated based on Potier triangle.

9. From the OCC and SCC curves, what does the intercept BC represent? a) Armature reaction
b) Leakage reactance
c) Armature copper losses
d) Sum of armature reaction and leakage reactance

Answer: b [Reason:] It is leakage reactance which is calculated based on potier triangle.

10. The leakage reactance can be determined with lesser accuracy by using Potier triangle.
a) True
b) False

Answer: b [Reason:] The leakage reactance can be determined with better accuracy by using Potier triangle.

11. Potier method is very accurate due to
a) emfs are handled as voltage and mmf as AT
b) emf and mmf are handled as AT
c) emf and mmf are handled as voltage
d) none of the mentioned.

Answer: a [Reason:] The potier’s method is very accurate because emfs are handled as voltage and mmf as AT.

12. Choose the correct representation for the time phasor below.
a) b) c) d) all of the mentioned

Answer: d [Reason:] It is the angle between excitation voltage and the armature current.

## Set 2

1. The rotating amplifiers are widely used in
a) small amplified outputs
b) large amplified outputs
c) amplified outputs
d) large power gain outputs

Answer: b [Reason:] The rotating amplifiers are applicable in feedback systems giving large amplified outputs.

2. The quantity which make rotating amplifier applicable in large power levels is/are
a) high power gain and favourable transient conditions
b) favourable transient conditions
c) high power gain
d) favourable steady state conditions

Answer: a [Reason:] The rotating amplifiers have stable transient conditions, which makes them favourable to use in high power sectors.

3. The ordinary dc machines are not favourable as amplidynes due to
a) sluggish transient response
b) low stability
c) poor gain
d) high steady state error

Answer: a [Reason:] The sluggish transient response of the ordinary dc machines makes them inappropriate to use them in amplidynes.

4. In rotating amplifiers, armature reaction mmf of the first stage is made to act as field flux for second stage.
a) True
b) False

Answer: a [Reason:] Yes, the reaction mmf of the first stage is made to act as field flux for second stage to have faster transient response.

5. The cross field machines have
a) two fluxes electrically in quadrature
b) two fluxes mechanically in quadrature
c) one set of brushes per pole
d) all of the mentioned

Answer: a [Reason:] The cross field machines have two fluxes electrically in quadrature.

6. In cross field rotating amplifiers, there is
a) two stage of flux generators
b) three stage of flux generators
c) four stage of flux generators
d) eight stage of flux generators

Answer: a [Reason:] There are two stages for flux generators in order to have faster response in the system.

7. Metadyne is _________ feedback with respect to main ___________
a) negative, current
b) negative, voltage
c) positive, current
d) positive, voltage

Answer: a [Reason:] Metadyne is negative feedback with respect to main current flowing in the second stage set of motors.

8. An amplidyne, d-axis flux is always neutralized due to
a) compensating winding at all possible loads
c) feedback at all possible loads

Answer: a [Reason:] The d-flux is neutralized by compensating winding at all possible loads as the same flux will be produced but in the opposite direction so as to cancel the flux.

9. In amplidynes, the resultant d-axis flux is
a) field flux
b) difference of field flux and main flux
c) sum of field flux and main flux
d) cross magnetizing flux

Answer: a [Reason:] The resultant d-axis flux lies along the main field poles and are so field flux. The d- axis is completely under the control of field winding and is unaffected by direct axis current.

10. Amplidynes are best applicable in
a) feedback control systems with regulated power supply.
b) power supply
c) speed control of large motors
d) All of the mentioned

Answer: a [Reason:] Amplidynes find applications in feedback systems with large power handling requirements.

11. Interpoles provide mmf more than armature mmf in a dc machine.
a) True
b) False

Answer: a [Reason:] Interpoles nullify armature cross magnetizing flux and the reactance voltage. So it is more than the armature mmf for a good efficient machine.

12. The delay of the commutation occurs due base cause of emf induced due to
a) self inductance, mutual inductance
b) self inductance
c) mutual inductance
d) leakage inductance

Answer: a [Reason:] The commutation in dc machines is delayed because of the emf induced in the coils and the mutual interaction of the field with neighbour coils.

13. I. DC shunt motor – variable speed application
II. DC compound generator – constant supply voltage
III. Differentially compounded motor – welding purposes
Above are the applications of the dc machines , but all are not matched correctly. Choose the wrong marking
a) III
b) II
c) I,III
d) III,II

Answer: a [Reason:] Differentially compounded generators are used for welding purposes, not motors.

14. I. DC shunt motor – Lathe machine
II. DC compound generator – Drilling machine
III. Differentially compounded generator – welding purposes
Above are the applications of the dc machines, but all are not matched correctly. Choose the wrong marking
a) II
b) III
c) I,III
d) III,II

Answer: a [Reason:] DC compound generators are used for maintaining constant voltage across the terminals.

15. Mark the order for the decreasing order of the voltage regulation for dc generators
a) DC shunt generator > level compound generator > Differential compound generator
b) DC shunt generator < level compound generator < Differential compound generator
c) Level compound generator > DC shunt generator > Differential compound generator
d) Level compound generator < DC shunt generator < Differential compound generator

Answer: a [Reason:] Dc shunt motor is almost constant speed generator and thus its voltage regulation is very good than all and differential compound generator has most poor v.r.

## Set 3

1. Rotor leakage reactance for a three phase slip ring induction motor with slip ‘s’ is
a) s(x2)
b) (x2)/s
c) x2
d) (x2)(s2)

Answer: a [Reason:] It is s*x2.

2. In an induction motor, when the number of stator slots is equal to an integral multiple of rotor slots
a) machine will fail to start
b) machine will get heated
c) high starting torque will be achieved
d) discontinuity in torque slip characteristic

Answer: a [Reason:] Machine fails to start as the stator slots and rotor slots become equal.

3. Two of the supply terminals to a three phase induction motor gets interchanges while regular scheduling work. When the machine is switched on, then it will
a) rotate in opposite direction
b) rotate in same direction
c) not start
d) get heated and winding will burn

Answer: a [Reason:] As the supply phase sequence is reversed, the machine’s developed emf will also be reversed.

4. The rotor of the induction motor must never run at synchronous speed because then the relative speed between the rotating flux and rotor will be
a) zero and hence torque will be zero
b) zero, and torque will be maximum
c) maximum and hence, torque will be zero
d) maximum, and torque will also be zero

Answer: a [Reason:] At synchronous speed the slip will be zero and so the torque.

5. The voltage actually used to set up the working flux in the three phase induction motor is
a) equal to applied voltage
b) less than applied voltage
c) more than applied voltage
d) equal to rotor induced emf.

Answer: a [Reason:] The actual working flux requirement is met by the applied voltage and it is same as the applied voltage to it.

6. If the three phase supply is fed to the stator and it is running at its normal operating conditions. Then the synchronous speed can be defined as speed at which
a) stator magnetic field rotates
b) rotor magnetic field rotates
c) rotor rotates
d) flux is set up in the rotor

Answer: a [Reason:] If it is fed from stator then the synchronous speed is also w.r.t. stator.

7.If the three phase supply is fed to the rotor and it is running at its normal operating conditions. Then the synchronous speed can be defined as speed at which
a) stator magnetic field rotates
b) rotor magnetic field rotates
c) rotor rotates
d) flux is set up in the rotor

Answer: b [Reason:] If it is fed from rotor then the synchronous speed is also w.r.t. rotor.

8. The mmf produced by the current of the three phase induction motor
a) is standstill with respect to stator mmf
b) rotates at the speed of rotor in air gap
c) rotates at slip speed with respect to stator mmf
d) rotates at synchronous speed with respect to rotor

Answer: a [Reason:] The mmf produced will also be at synchronous speed and it will be stationary with respect to stator mmf which itself rotates at synchronous speed.

9. In an induction motor slip will be negative when
a) rotor rotates at speed more than synchronous speed and in the direction of the rotation of stator field
b) stator magnetic field and rotor rotate in opposite direction
c) rotor rotates at speed less than synchronous speed and in the opposite direction of the rotation of stator field
d) rotor rotates at speed more than synchronous speed and in the opposite direction of the rotation of stator field

Answer: a [Reason:] s = (Ns-Nr)/Ns; Slip is negative when rotor rotates at speed more than synchronous speed and in the direction of the rotation of stator field.

10. A 3-phase induction motor is operating at slip ‘s’. If the slip is reversed, its slip at the instant will be
a) 2-s
b) zero
c) 2+s
d) 1-s

Answer: a [Reason:] For the reversed phase sequence the slip becomes 2-s.

11. In a three phase induction motor, voltage between the slip rings at standstill is 50 V. At full load the slip is 0.04. The voltage between slip rings at full load is
a) 2V
b) 50V
c) 20V
d) 5V

Answer: a [Reason:] E2 = s*E = 0.04*50 = 2V.

12. A three phase induction motor is connected to 400V, 50 hz supply. If the stator to rotor turn ratio is 2, the standstill rotor induced voltage per phase is
a) 115.5 V
b) 231 V
c) 346 V
d) 200 V

Answer: a [Reason:] Emf at secondary per phase = (400/1.73)/2 = 115.47 V.

13. The torque developed in the three phase induction motor depends on
(i) standstill rotor phase emf
(ii) rotor power factor
a) (i) only
b) (ii) only
c) both (i) and (ii)
d) none of the mentioned

Answer: c [Reason:] The electromagnetic torque developed in the ac machine depends on the rotor emf and the power factor.

14. If a 400 V, 50 hz star connected, 3-phase squirrel cage induction motor is operated from 400V and 75 Hz supply. The torque that the motor can now provide while drawing rated current from the supply will
a) reduce
b) increase
c) remains same
d) increases or reduces based on the rotor resistance.

Answer: a [Reason:] The reactance will increase and so the torque will decrease.

15. For a constant load torque, the supply voltage of a squirrel cage induction motor is reduced by a factor of 0.5, its rotor current is modified by a factor
a) 2
b) 0.5
c) 4
d) 0.25

Answer: a [Reason:] For constant torque the product of emf and current should be constant.

## Set 4

1. Slip is defined as (Ns as the synchronous speed and Nr is the rotor speed)
a) Ns-Nr/Ns
b) Ns-Nr/Nr
c) Nr-Ns/Ns
d) Ns-Nr

Answer: a [Reason:] Slip is the ratio of the relative speed of between the stator mmf and the rotor speed to the synchronous speed of the machine.

2. For a 4 pole three phase induction motor having synchronous speed of 1500 rpm is operating at 1450rpm. The frequency of the induced emf in rotor is
a) 100 Hz
b) 50 Hz
c) 150 Hz
d) 0 Hz

Answer: a [Reason:] The induced frequency will be f = P(Ns-Nr)/2. f = 4(1500-1450)/2 = 100 Hz.

3. A three phase, 50 Hz induction motor has a full load speed of 1440 rpm. The number of poles of this motor is
a) 4
b) 6
c) 5
d) 2

Answer: a [Reason:] P = 120f/N = 120*50/1440 = 4(1/6). The number of the poles should be whole number and must be even. So number of poles must be 4.

4. A three phase, 50 Hz induction motor has a full load speed of 1440 rpm. The full load slip will be
a) 4%
b) 5%
c) 2.4%
d) 3%

Answer: a [Reason:] s=1500-1440/1500 = 4%

5. A three phase, 50 Hz induction motor has a full load speed of 1440 rpm. Rotor frequency is
a) 2 Hz
b) 50 Hz
c) 52 Hz
d) 58 Hz

Answer: a [Reason:] s=0.04, rotor frequency = s*f = 0.04*50 = 2 Hz.

6. A three phase, 50 Hz induction motor has a full load speed of 1440 rpm. Speed of the stator filed with respect to stator structure is

Answer: a [Reason:] Speed of the stator filed with respect to stator structure = Ns = 1500 rpm = 157 rad/s

7. A three phase, 50 Hz induction motor has a full load speed of 1440 rpm. Speed of the stator field revolving rotor structure is

Answer: c [Reason:] Speed of the stator field revolving rotor structure = 1550-1440 = 60 rpm = 6.28 rad/s

8. A three phase, 50 Hz induction motor has a full load speed of 1440 rpm. Speed of the rotor field w.r.t. rotor structure is

Answer: c [Reason:] Speed of the rotor field w.r.t. rotor structure = 120*rotor frequency/Poles = 120*2/4 = 60 rpm = 6.28 rad/s

9. A three phase, 50 Hz induction motor has a full load speed of 1440 rpm. Speed of the rotor field w.r.t. stator field is
a) zero

Answer: a [Reason:] As both the rotor field and stator file rotate at synchronous speed, the relative speed between them is zero.

10. A properly shunted center zero galvanometer is connected in the rotor circuit of a 6-pole, 50 hz wound rotor induction motor. If the makes 90 complete oscillations in one minute, the rotor speed is
a) 1000 rpm
b) 970 rpm
c) 950 rpm
d) 930 rpm

Answer: b [Reason:] Rotor frequency = s*f1 = 90/60 = 1.5 Hz. s = 1.5/50 = 0.03 rotor speed = 1000(1-0.03) = 970 rpm.

11. A 4-pole, 3-phase slip ring induction motor is used as a frequency changer. Its stator is excited from 3-phase, 50 Hz supply. A load requiring 3-phase, 20 Hz supply is connected to the star-connected rotor through three slip-rings of SRIM.
At what two speeds the prime mover should drive the rotor of this SRIM?
a) 900 rpm
b) 600 rpm
c) 1500 rpm
d) 0

Answer: a [Reason:] Ns = 1500 rpm. Speed of the rotor field with respect to rotor structure = 120 * f2/P = 120*20/4 = 600 rpm Nr = 1500-600 = 900 rpm.

12. For a 4-pole, 3-phase induction motor the rotor speed found is 2100 rpm at 50 Hz. Then the phase sequence of the emf generated will be
a) acb
b) abc
c) no emf will be induced
d) cba

Answer: a [Reason:] Stator field will be running at the speed of 600 rpm anticlockwise wrt to rotor. So the phase sequence generated at the slip rings is reversed.

13. Stator flux induces emf in the rotor conductors
a) magnitude depending on the load
b) rotating at synchronous speed around stator
c) constant in magnitude
d) none of the mentioned

Answer: a [Reason:] The emf will be induced based on the slip which depends on the load connected to the rotor.

14. The 3-phase induction motor with rotor circuit open will
a) not run
b) run normally
c) get over heated
d) make noise

Answer: a [Reason:] It will not run because there will not be any closed path for the flux in rotor circuit of the machine to generate emf.

15. The rotor of th 3-phase induction motor rotates in the same direction as that of stator field. This can be explained by
a) Newton’s laws of motion
b) Farady’s laws of electromagnetic induction
c) Lenz’s law
d) Fleming’s right hand rule

Answer: a [Reason:] Newtons’s law explains the over all behaviour of the inertia involved in the machine and a unidirectional torque generated.

16. In a three phase slip ring induction motor, brushes are connected to
a) external star connected resistors
b) dc supply
c) 3-phase ac supply
d) any of the mentioned

Answer: a [Reason:] The rotor external resistances must e connected in star and at the external end of the rotor.

17. The rotor winding for 3-phase slip ring induction motor having delta connected stator must be connected in
a) star
b) delta
c) tertiary
d) any of the mentioned

Answer: a [Reason:] We should use star connected resistors as the initial requirement is low resistance for high starting torque.

18. The squirrel cage induction motor of 6-pole can be used as ________ induction motor.
a) any number of poles
b) 6 pole
c) 6 or 12 pole
d) integral number of 4 poles

Answer: a [Reason:] The poles in a squirrel cage induction motor can be adjusted.

## Set 5

1. The PWM control of DC motor varies
a) linearly with speed
b) inversely with speed
c) parabolically with speed
d) exponentially with speed

Answer: a [Reason:] In PWM technique duty ratio is a linear function with respect to speed.

2. Ward-Leonard system of system of speed control is not recommended for
a) constant speed operation
b) Wide speed
c) frequent-motor reversed
d) very slow speed

Answer: a [Reason:] Ward-Leonard system of system of speed control is not recommended for constant speed operation.

3. Mark the wrong option. Which of the following cause and effect behaviour in speed control is correct when field resistance is increased.
a) Decrease in flux
b) Increase in armature current
c) Increase in EMF
d) Decrease in speed

Answer: c [Reason:] When field resistance is increased, emf actually decreases, not increases due to fall in flux.

4. The effect of field resistance speed control on a shunt motor speed torque characteristic is
a) b) c) d) none of the mentioned

Answer: a [Reason:] Field resistance speed control will make the speed above the base speed control.

5. At a very low speed, increase in field resistance will
a) decrease the speed of motor
b) increase the speed of motor
c) not have signicant effect on speed
d) no effect

Answer: a [Reason:] At low speed increase in armature current caused by decrease in emf, will not be enough to compensate for decrease in flux in induced torque equation.

6. Small DC motors have best speed control by
a) armature voltage control
b) field resistance control
c) any of the methods
d) none of the mentioned

Answer: a [Reason:] For the small dc motors it is difficult to vary the speed using their field control.

7. To implement armature voltage control, it must be ensured that
a) it is used on separately excited machine
b) it is used on shunt machine
c) it is used on series machine
d) any of the mentioned machine

Answer: a [Reason:] It must be separately excited mahcine to implement the armature voltage control.

8. The armature resistance speed control is best illustrated by which of the following
a) b) c) d) Answer: a [Reason:] As the field resistance increases the speed decreases.

9. Armature voltage control works for speeds _____ base speed and field resistance control works well for speed _____ base speed.
a) below, above
b) above, below
c) above, above
d) below, below

Answer: a [Reason:] For armature voltage control, armature current is increased so as to reduce speed below base speed. In field control flux is reduced so the speed would be above base speed.

10. How does the power varies with the variations in the speed, for a dc shunt motor?
a) b) c) d) Answer: a [Reason:] After the base speed is attained, the power consumed becomes constant as the speed remains constant in a shunt machine.

11. The torque limit of speed for a shunt motor
a) remains constant till base speed
b) remains constant after base speed
c) varies linearly after base speed
d) varies inversely till base speed

Answer: a [Reason:] The torque limit of the shunt motor remains constant till the base speed.

12. A laboratory group was working with a set of 3-hp shunt DC motor. But there was a mistake that it was fused with a 0.3A fuse instead 3A fuse. Then it was started
a) a flash occurred instantly
b) it ran for 3s and fuse was blown
c) it ran normal
d) none of the mentioned

Answer: b [Reason:] Due to the wrong usage of the fuse wire, it could not sustain the current and in few seconds it was blown off.

13. Run-away for large DC machine is avoided by using
a) a turn or two of cumulative compounding
b) compensating winding
c) stablizer winding
d0 any of the mentioned