Ch. 8

Question 1

856. Modern alternators consist of a rotor on which the AC voltage producing windings are placed and a rotating stator on which a DC excitation winding is placed.
     a. True
     b. False

Answer 1

b. False

Question 2

857. In the case of the single-phase alternator, the coil windings are connected in three different groups, one for each phase.
     a. True
     b. False

Answer 2

b. False

Question 2

858. An alternator is a ___________________.
     a. three phase AC converter
     b. DC generator
     c. wound rotor motor
     d. AC generator
     e. single phase AC converter

Answer 2

d. AC generator

Question 3

859. Alternators __________________________.
     a. have AC excitation on the rotor
     b. DC excitation voltage on the stator
     c. have a rotating armature on which a DC excitation winding is placed
     d. slip rings on the stator
     e. split ring on the rotor

Answer 3

c. have a rotating armature on which a DC excitation winding is placed

Question 4

861. What is the most common frequency of AC generators in North America?
     a. 40 hertz
     b. 120 hertz
     c. 110 hertz
     d. 55 hertz
     e. 60 hertz

Answer 4

e. 60 hertz

Question 4

865. For paralleling alternators, the incoming alternator must be the same ___________ as the system voltage.
     a. manufacturer
     b. conductance
     c. voltage
     d. amperage

Answer 4

c. voltage

Question 4

860. Salient pole rotors _________________________.
     a. are used on high speed alternators
     b. are used on alternators not exceeding 1800 rpm
     c. require no maintenance
     d. are used on only single phase generators
     e. are used on only three phase generators

Answer 4

b. are used on alternators not exceeding 1800 rpm

Question 5

863. Paralleling alternators require the following conditions:
     1) It must be in phase with the system
     2) The incoming alternator must be the same voltage as the system
     3) Its phase sequence must be the same as the system
     4) Alternator frequency and system frequency must be the same
     a. 1, 3, 4
     b. 1, 2, 3
     c. 2, 3, 4
     d. 1, 2, 4
     e. 1, 2, 3, 4

Answer 5

e. 1, 2, 3, 4

Question 5

862. The best method of synchronizing an alternator to a system is ____________________.
     a. by using the synchroscope
     b. by using the dark lamp
     c. by using one dark, and two bright rotating brushes
     d. by using the phase indicator
     e. by using all dark rotating brushes

Answer 5

a. by using the synchroscope

Question 5

867. The stator of the induction motor is identical to that of the three-phase alternator.
     a. True
     b. False

Answer 5

a. True

Question 5

864. To take an alternator off the line in a system involving two or more alternators, the driving torque of the prime mover of the alternator to be removed should be reduced until it is supplying no current to the busbars.
     a. True
     b. False

Answer 5

a. True

Question 5

866. The most common type of rotor used in induction motors is the squirrel cage rotor.
     a. True
     b. False

Answer 5

a. True

Question 5

875. Starting methods and devices used on electric motors are used to ____________________________.
     a. increase voltage during start up
     b. increase power factor
     c. increase current when starting
     d. increase starting torque
     e. reduce voltage during start up

Answer 5

e. reduce voltage during start up

Question 5

868. Wound rotor induction motors ___________________.
     a. require less maintenance than squirrel cage motors
     b. are less complicated in construction than other motors
     c. are more reliable
     d. are not very reliable
     e. are used to control speed

Answer 5

e. are used to control speed

Question 5

876. Line impedance starters __________________________.
     a. use resistors or inductors in series with each phase in order to reduce the starting current
     b. open relays for starting
     c. reverse the polarity
     d. increase the conductance for starting
     e. increase the line frequency

Answer 5

a. use resistors or inductors in series with each phase in order to reduce the starting current

Question 5

869. Synchronous motors ______________________________.
     a. have a lagging power factor
     b. depend on slip for its torque
     c. are only single phase
     d. can be run at a leading power factor
     e. can be run at a lagging power factor

Answer 5

d. can be run at a leading power factor

Question 6

870. Synchronous motors _____________________________.
     a. can be run at very high speeds
     b. can be run as an alternator or a motor
     c. are very expensive compared to other motors
     d. do not require brushes or slip rings
     e. are low speed machines

Answer 6

b. can be run as an alternator or a motor

Question 6

872. Synchronous motors ____________________________.
     a. are used in small plants to cut costs
     b. usually are very small and compact compared to three phase induction motors
     c. require higher rated insulation values than other motors
     d. can be used to help correct power factor
     e. can not be used to help correct power factor

Answer 6

d. can be used to help correct power factor

Question 6

874. The synchronous motor is _________________________.
     a. self starting
     b. not self starting
     c. used to create a lagging power factor
     d. a split phase type motor
     e. used to decrease the power factor

Answer 6

b. not self starting

Question 6

877. Star-delta starting _____________________________.
     a. is used for any type of starting
     b. is used on motors designed to run as delta connected machines
     c. is used only for synchronous motors
     d. can be used for only DC machines
     e. can be used for AC or DC generators

Answer 6

b. is used on motors designed to run as delta connected machines

Question 6

871. Synchronous motors _____________________________.
     a. require DC power supply to the rotor
     b. require DC power supply to the stator
     c. require a high slip to control speed
     d. require a starting motor
     e. are self-starting

Answer 6

a. require DC power supply to the rotor

Question 6

882. In a variable frequency driver, in general, the percentage drop in frequency is proportional to the percentage drop in motor speed. For example, an induction motor rotates at 1725 r/min when supplied with ac power at 60 Hz, will operate at 1581 r/min when supplied with ac power at 55 Hz
     a. True
     b. False

Answer 6

a. True

Question 6

873. One of the main advantages of the synchronous motor is that it can be run at lagging power factor unlike other motors, which run at a leading power factor.
     a. True
     b. False

Answer 6

b. False

Question 6

881. Induction motor starting when directly connected to the source _______________________________.
     a. require very little current starting up
     b. need slightly more current for start up
     c. need only normal current and voltage for starting
     d. require up to six times its full-load current when starting
     e. require a starting motor

Answer 6

d. require up to six times its full-load current when starting

Question 7

880. Line impedance starters; this method of starting is relatively more expensive but gives a lower starting torque than other methods. a. True b. False

Answer 7

b. False

Question 7

878. The autotransformer is used to step down voltage, the entire winding acts as the primary and part of the winding acts as the secondary. The action of the autotransformer is similar to the two winding transformer. a. True b. False

Answer 7

a. True

Question 7

879. Synchronous motors are started by ____________________. a. the amortisseur winding b. the shunt winding c. the capacitor d. the delta winding e. the waye winding

Answer 7

a. the amortisseur winding

Question 7

885. Auto-transformers _______________________________. a. can use less copper b. use more copper c. have large ratios of transformation d. use two windings for primary and secondary circuits e. have a lower ratios of transformation

Answer 7

a. can use less copper

Question 7

886. Current transformers are __________________________. a. power step-up transformers b. current step-up transformers c. rectifying transformers d. high power factor transformers e. voltage step-up transformers

Answer 7

e. voltage step-up transformers

Question 7

884. Transformers function on the principle of __________________________. a. conductance b. high resistance to DC current c. interaction of residual magnetism d. mutual inductance e. mutual conductance

Answer 7

d. mutual inductance

Question 8

883. Transformers are used for: 1) Stepping up AC voltage 2) Stepping down AC voltage 3) Transforming AC voltage to DC voltage 4) Rectifying DC voltage to AC voltage a. 1, 2 b. 1, 2, 3 c. 1, 2, 4 d. 1, 3, 4 e. 1, 2, 3, 4

Answer 8

a. 1, 2

Question 8

887. Modern transformers are very efficient devices with large industrial transformers often better than 95% efficient. a. True b. False

Answer 8

a. True

Question 8

888. Three phase systems often use banks of _______ ________ transformer to replace three-phase transformers. a. double phase b. single phase c. step down d. step up

Answer 8

b. single phase

Question 9

889. If the number of secondary turns is such that the secondary voltage is smaller than the primary voltage, then the transformer is a __________ ___________ transformer. a. small scale b. ramp up c. quick step d. step up e. step down

Answer 9

e. step down

Question 9

890. Transformers are cooled by: 1) Spraying water over the coils 2) Placing the core in an oil filled tank 3) Forced cooling by fans 4) Using pyrene as a coolant a. 1, 2, 3 b. 1, 3, 4 c. 2, 3 d. 2, 3, 4 e. 1, 2, 3, 4

Answer 9

c. 2, 3