ALTERNATORS AND SYNCHRONOUS MACHINES 11

1. In aircraft industry, the material used for the conductor is
a) aluminium
b) copper
c) silver
d) iron
ans. a

2. Which of the following class is mainly used in electrical machines for insulation
a) class Y
b) class A
c) class F
d) class C
ans. b

3. The resistance of insulating material is
a) proportional to its resistivity
b) inversely proportional to its resistivity
c) independent of the temperature
d) none of these
ans. a

4. when a substance is placed in a magnetic field , its ability to get magnetized depends upon its
a) permeability
b) permitivity
c) susceptibility
d) none of these
ans. c

5. The material used for manufacture of armature covers of dc motors and dynamos should have
a) small B-H loop area
b) large B-H loop area
c) very large B-H loop area
d) none of these
ans. a

6. For steady flux and good mechanical properties, the material extensively used for magnetic circuit is
a) iron
b) cast iron
c) grey cast iron
d) cast steel
ans. c

7. When 3-phase synchronous motor is switched on. there exists a rotating magnetic field. The magnitude of this field flux
a) is constant at all loads
b) varies with load
c) varies with power factor
d) none of these
ans. a

8. When the excitation of synchronous motor is varied
a) the pf and the armature current varies
b) the pf varies but the armature current does not vary
c) the armature current varies but the pf does not vary
d) neither the pf nor the armature current varies
ans. a

ALTERNATORS AND SYNCHRONOUS MACHINES 10

1. The armature mmf wave of a dc machine is
a) sinusoidal and depends on the speed
b) square and independent of speed
c) sinusoidal and independent of speed
d) triangular and independent of speed
ans. d

2. The armature mmf wave for a dc machine is
a) sinusoidal and rotates wrt armature
b) sinusoidal and rotates wrt field
c) triangular and rotates wrt armature
d) triangular and stationary wrt armature
ans. c

3. Rotational losses in electrical machines consists of
a) friction and windage losses
b) stator core , friction and windage losses
c) rotor core, friction and windage losses
d) stray load losses and friction and windage losses
ans. c

4. Efficiency of electrical machines should be calculated by measuring
a) output and input
b) losses and output
c) losses and input
d) losses
ans. d

5. Electrical machines are designed to have maximum efficiency at
a) full load
b) 50% of full load
c) near about full load
d) no load
ans. c

6. In 3-phase ac machines, phase spread of 120 degrees is preferred over a phase spread of 60 degrees
a) true
b) false
c) depends upon type of machine
d) none of these
ans. b

7. The mmf produced by a single phase winding is
a) pulsating and stationary
b) pulsating and rotating
c) constant in amplitude and stationary
d) constant in amplitude and rotating
ans. a

8. The material used for magnetic circuit where high value of flux density required is
a) cast iron
b) ferro cobalt
c) soft steel
d) gray cast iron
ans. b

9. The material suitable for the manufacture of transformer and large turbo-alternator is
a) hot rolled grain oriented steel
b) cold rolled grain oriented steel
c) cast steel
d) none of these
ans. b

10. The material used for the magnetic circuit where high value of flux density required is
a) cast iron
b) ferro cobalt
c) soft steel
d) gray cast iron
ans. b

ALTERNATORS AND SYNCHRONOUS MACHINES 9

1. Electromagnetic torque in rotating machines is present when
a) air gap is uniform
b) stator winding alone carries current
c) rotor winding alone carries current
d) both stator and rotor windings carry current
ans. d

2. The interaction torque depends on
a) stator field strength alone
b) both the stator field and rotor field strengths
c) stator field strength and torque angle
d) stator field rotor field strengths and the torque angle
ans. d

3. Armature winding is one in which working
a) flux is produced by field current
b) flux is produced by working current
c) emf is produced by the working flux
d) emf is produced by the leakage flux
ans. c

4. Large synchronous machines are constructed with armature winding on the stator because stationary armature winding
a) can be insulated satisfactorily for higher voltages
b) can be cooled more efficiently
c) have reduced slip ring losses
d) all of the these
ans. d

5. A coil consists of
a) two conductors
b) two coil-sides
c) two turns
d) four turns
ans. b

6. One turn consists of
a) two-coil sides
b) two conductors
c) four conductors
d) four coil sides
ans. b

7. The main advantages of distributing the winding in the slots is to
a) add mechanical strength to the winding
b) reduce the amount of copper required
c) reduce the harmonics in the generated emf
d) reduce the size of the machine
ans. c

8. The distribution factor is defined as the ratio of
a) arithmetic sum of coil emfs to phasor sum of coil emfs
b) phasor sum of emf per coil to the arithmetic sum of the emf per coil
c) phasor sum of coil emfs to the arithmetic sum of coil emfs
d) phasor sum of coil emfs to the per phase voltage
ans. c

9. The main advantage of using fractional-pitch winding is to reduce
a) amount of copper in the winding
b) size of the machine
c) harmonics in the generated emf
d) cost of the machine
ans. c

10. The pitch factor, in rotating electrical machines is defined as the ratio of resultant emf of a
a) full pitched coil to that of a chorded coil
b) full pitched coil to the phase emf
c) chorded coil to the phase emf
d) chorded coil to that of a full pitched coil
ans. d

ALTERNATORS AND SYNCHRONOUS MACHINES 8

1. If the armature current is in phase with generated emf, the effect of armature reaction is
a) cross magnetizing
b) magnetizing
c) demagnetizing
d) none of these
ans. a

2. The value of voltage regulation as determined by synchronous impedance method is
a) greater than that found by actual loading
b) less than that found by actual loading
c) the same as that found by actual loading
d) none of these
ans. a

3. An alternator is connected to the bus bars and is supplying load. Its prime mover is suddenly shut down. The alternator will
a) continue to work as an alternator
b) continue to run as synchronous motor but direction of rotation will reverse
c) continue to run as synchronous motor and the direction of rotation will remain the same
d) none of the above
ans. c

4. Alternators generally use short pitch coils to remove harmonics from emf wave shape. As compared to full pitch coils, the short pitch coils need
a) more conductor material
b) the same amount of conductor material
c) less conductor material
d) none of the above
ans. c

5. The biggest size of the alternators used in India is
a) 200 MW
b) 350 MW
c) 500 MW
d) 1000 MW
ans. c

6.  In order to extract energy from an electrical system , the coupling magnetic field must react with
a) electrical system
b) mechanical system
c) both a and b
d) none of these
ans. a

7. Electromagnetic force or torque developed in any physical system tends to
a) increase the magnetic stored energy at constant flux
b) decrease the magnetic stored energy at constant current
c) decrease the reluctance
d) decrease the inductance
ans. c

8.  Reluctance motors are
a) doubly -excited
b) singly- excited
c) multiply-excited
d) none of these
ans. b

9. The doubly excited magnetic systems are
a) moving iron instruments
b) electromagnetic relays
c) solenoids
d) synchronous motors
ans. d

10. Reluctance torque in rotating machines is present when
a) air-gap is not uniform
b) reluctance seen by stator mmf is constant
c) reluctance seen by stator mmf varies
d) reluctance seen by rotor mmf is constant
ans. c

ALTERNATORS AND SYNCHRONOUS MACHINES 7

1. In a synchronous motor
a) the rotor mmf and stator mmf are stationary with respect to each other
b) the speed of rotor mmf is slightly less than the speed of stator mmf
c) the speed of rotor mmf is slightly more than the speed of stator mmf
d) none of these
ans. a

2. In a 3-phase alternator the unsaturated synchronous reactance is 3 ohms per phase. The saturated synchronous reactance is
a) 3 ohms
b) more than 3 ohms
c) less than 3 ohms
d) none of these
ans. c

3. The short-circuit characteristic of an alternator is
a) always linear
b) always non-linear
c) sometimes linear and sometimes non-linear
d) none of these
ans. a

4. Under short circuit conditions, the power factor of an alternator is
a) almost zero lagging
b) unity
c) about 0.6 lagging
d) none of these
ans. a

5. High speed alternators usually have
a) salient pole rotors
b) cylindrical rotors
c) both a and b
d) none of these
ans. b

6. Cylindrical rotor alternators have
a) large length to diameter ratio
b) small length to diameter ratio
c) vertical configuration
d) none of these
ans. a

7. Salient pole machine have
a) large no of poles
b) small no of poles
c) small diameters
d) long cores
ans. a

8. To ensure proper cooling, cylindrical rotor alternators use
a) radial ducts only
b) axial ducts only
c) both radial and axial ducts only
d) none of these
ans. c

9. Modern alternators usually have short pitched winding
a) to improve the voltage wave shape
b) to improve the magnitude of the generated voltage
c) to increase the machine rating
d) none of these
ans. a

10. If the armature current is leading the generated voltage by 90 degrees the effect of armature reaction will be
a) demagnetizing
b) magnetizing
c) partly magnetizing and partly cross magnetizing
d) none of these
ans. b

ALTERNATORS AND SYNCHRONOUS MACHINES 6

1. Synchronous motors generally have
a) cylindrical rotor
b) salient pole rotor
c) both  a and b are correct
d) none of these
ans. b

2. A synchronous motor is supplying a certain load and is operating at unity power factor. If field current and terminal voltage are kept constant but the load is increased, the power factor
a) will remain the same
b) will become leading
c) will become lagging
d) none of these
ans. c

3. Synchronous motors are usually operated at
a) unity power factor
b) leading power factor
c) lagging power factor
d) none of these
ans. b

4. 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 speed slightly less than synchronous speed
d) none of these
ans. c

5. In a synchronous motor
a) the generated emf must always be less than the terminal voltage
b) the generated emf must always be more than the terminal voltage
c) the generated emf can be less than or greater than the terminal voltage
d) none of these
ans. c

6. As the load on a synchronous motor is increased, the torque angle
a) increases
b) decreases
c) remains the same
d) none of these
ans. a

7. An under excited synchronous motor operates at

a) unity power factor
b) leading power factor
c) lagging power factor
d) none of these
ans. c

8. Synchronous motors are used in
a) sizes greater than about 50 HP
b) all sizes
c) generally small sizes
d) none of these
ans. a

9. A synchronous motor is supplying its rated load. If the excitation is increased
a) the power factor becomes more leading
b) the power factor becomes more lagging
c) the power factor becomes less lagging
d) none of these
ans. a

10. The power output of a synchronous motor is
a) directly proportional to synchronous reactance
b) inversely proportional to synchronous reactance
c) independent of synchronous reactance
d) none of these
ans. b

ALTERNATORS AND SYNCHRONOUS MACHINES 5

1. An unexcited single phase synchronous motor is
a) reluctance motor
b) universal motor
c) repulsion motor
d) ac series motor
ans. a

2. The damping winding in a synchronous motor is generally used
a) to provide starting torque only
b) to reduce noise level
c) to reduce eddy currents
d) to prevent hunting and provide the starting torque
ans. d

3. A synchronous motor working on leading power factor and not driving any mechanical load is known as
a) synchronous induction motor
b) spinning motor
c) synchronous condenser
d) none of the above
ans. c

4. The amortisseur winding in a synchronous motor
a) provides starting torque only
b) provides starting torque and eliminates hunting
c) improves the power factor of the machine.
d) none of these
ans. b

5. An over excited synchronous motor operates at
a) unity power factor
b) lagging power factor
c) leading power factor
d) none of these
ans. c

6. Under full load running condition , the slip of a synchronous motor is
a) 0
b) about 0.2
c) about 0.01
d) none of these
ans. a

7. When the synchronous motor is to be started, the field system is energised
a) in the very beginning
b) after the motor has attained a speed slightly less than the synchronous speed
c) after the motor has attained synchronous speed and has been loaded
d) none of these
ans. b

8. When a synchronous motor is started, the field winding is initially
a) short circuited
b) open circuited
c) excited by a dc source
d) none of these
ans. a

9. V-curves of a synchronous motor show the relation between
a) armature current and field current
b) applied voltage and field current
c) applied voltage and armature current
d) none of these
ans. a

10. For a given load , the armature current of a synchronous motor is minimum when the power factor is
a) unity
b) slightly less than unity and lagging
c) slightly less than unity and leading
d) none of these
ans. a

ALTERNATORS AND SYNCHRONOUS MACHINES 4

1. The magnitude of the emf induced in the stator due to revolving flux will depend upon the
a) speed of the motor
b) dc excitation current
c) load on the motor
d) speed and rotor flux
ans. b

2. If the field of synchronous motor is under excited, the power factor will be
a)  more than unity
b) unity
c) lagging
d) leading
ans.  c

3. If one of the 3-phase of synchronous motor is short circuited, motor will
a) start
b) not start
c) over heated
d) fail to pull into synchronism
ans. b

4. The advantages of synchronous motor over slipring induction motor are that it's
a) power factor can be varied
b) speed can be easily varied
c) speed is independent of supply frequency
d) rotor has two slip rings
ans. a

5. The standard full load power factor of a synchronous motor is
a) unity
b) unity or 0.8 leading
c) unity or 0.8 lagging
d) zero or 0.8 leading
ans. b

6. The synchronous motor can be started by
a) coupling with dc compound motor
b) coupling with ac induction motor
c) providing damper windings
d) all of these
ans. d

7. The advantages of synchronous motor as compared to induction motor are that
a) it runs at constant speed
b) it can run over wide range of power factors both lagging and leading
c) it's torque is less sensitive to change in supply voltage
d) all of these
ans. d

8. Rotor winding of the synchronous motor is excited by
a) induction from stator current
b) ac supply
c) dc supply supplied by the exciter
d) the revolving field
ans. c

9. The speed of synchronous motor depends upon
a) number of poles
b) supply frequency
c) either a or b
d) both a and b are correct
ans. d

10. The magnitude of the induced emf in the stator of a synchronous motor
a) is equal to the supply voltage
b) is less than the supply voltage
c) can be increased than the supply voltage
d) can be increased or decreased than the supply voltage
ans. d

ALTERNATORS AND SYNCHRONOUS MACHINES 3

1. A synchronous motor can run at
a) a leading power factor
b) unity power factor
c) lagging or leading or unity power factor
d) zero power factor
ans. c

2. The power factor of the input power to a synchronous motor is adjusted by varying
a) number of poles
b) magnitude of excitation
c) magnitude of armature reaction
d) none of these
ans. b

3. At constant load , the magnitude of armature current of a synchronous motor has large values for
a) low values of field excitation only
b) high values of field excitation only
c) both a and b
d) none of these
ans. c

4. For a synchronous motor, the inverter "V" curve is the relation between
a) field current and power factor
b) field current and armature current
c) armature current and power factor
d) none of these
ans. a

5. In a synchronous motor , minimum armature current occurs at
a) zero power factor
b) unity power factor
c) lagging power factor
d) leading power factor
ans. b

6. A 3-ring synchronous converter supplying a dc 3-wire system must have the secondary of its transformer connected as
a) delta only
b) delta or star
c) zig-zag only
d) star or zig-zag
ans. c

7. The construction of synchronous motor is similar to
a) dc compound motor
b) slip ring induction motor
c) dc shunt motor
d) alternator
ans. d

8. The synchronous motor runs at
a) less than synchronous speed
b) synchronous speed
c) more than synchronous speed
d) none of these
ans. b

9. The synchronous motor runs on
a) 3-phase ac supply
b) 3-phase dc and ac supply
c) dc supply only
d) 3-phase ac and single phase ac
ans. b

10. If a synchronous motor is switched on to 3-phase supply with its rotor winding short circuited, it will
a) start
b) not start
c) start and continue to run as induction motor
d) start and continue to run as synchronous motor
ans. c

ALTERNATORS AND SYNCHRONOUS MACHINES 2

1. Salient pole type rotors are
a) smaller in axial length
b) larger in axial length
c) smaller in diameter
d) larger in diameter and smaller in axial length
ans. d

2. The advantage of using short pitched winding in an alternator is that it
a) reduces harmonics in generated emf
b) saves copper used in the winding
c) give the better shape to the waveform
d) all of these
ans. d

3. The emf generated in an alternator depends upon
a) coil span factor
b) frequency
c) flux per pole
d) all the above three
ans. d

4. The conditions for running two alternators in parallel are
a) terminal voltage should be same
b) frequency should be same
c) phase sequence should be same
d) all the above three
ans. d

5. The alternators are operated in parallel because
a) it is convenient and economical in repairing
b) it maintains better stability of supply
c) it is easy to install an additional unit as and when required
d) all the above three
ans. d

6. Different methods of synchronising the alternators generally used are
a) dark and bright lamp method
b) stroboscopic method
c) dark lamp method
d) both a and b are correct
ans. d

7. when two alternators are running in parallel , if prime-mover of one of the alternators is disconnected, that alternator will
a) stop running
b) run as synchronous motor
c) run as generator
d) none of these
ans. b

8. Two alternators are running in parallel. If the driving force of both the alternators is changed, this will result in change in
a) frequency
b) back emf
c) generated voltage
d) all the above
ans. a

9. The frequency per revolution in an alternator is equal to
a) number of poles
b) no of armature conductors
c) number of pairs of poles
d) none of these
ans. c

10. For a full pitch winding , the generated voltages in both coil sides are
a) exactly in phase
b) in quadrature
c) exactly 180 degrees out of phase
d) approx. 180 degrees out of phase
ans. a

ALTERNATORS AND SYNCHRONOUS MACHINES

1. Alternator generates
a) dc
b) ac
c) both ac and dc
d) pulsating dc
ans. b

2. Alternator works on the principle of
a) mutual induction
b) Faraday's laws of electro-magnetic induction
c) self induction
d) self and mutual induction
ans. b

3. The rotor of the alternator requires
a) dc
b) ac
c) pulsating dc
d) none of these
ans. d

4. In large size alternators, flux is kept
a) stationary
b) rotating
c) either a or b is correct
d) none of these
ans. b

5. The rotor of an alternator has
a) 4 slip rings
b) 3 slip rings
c) 2 slip rings
d) no slip rings
ans. c

6. The generator which gives dc supply to the rotor is called
a) converter
b) exciter
c) inverter
d) rectifier
ans. b

7. The different types of rotor of an alternator are
a) salient pole type
b) cylindrical pole type
c) both a and b
d) none of these
ans. c

8. Salient pole type rotors are generally used with prime movers of
a) high speed
b) low speed
c) medium speed
d) low and high speed
ans. b

9. Cylindrical pole type rotors are generally used with prime movers of

a) high speed
b) low speed
c) medium speed
d) low and high speed
ans. a

10. The frequency  of emf generated depends upon
a) speed of the alternator
b) number of poles of the alternator
c) type of alternator
d) both a and b are correct
ans. d