3A2 Chapter 08 Flashcards
Modern alternators consist of a rotor on which the AC voltage producing windings are placed and a rotating stator on which a DC exciting winding is placed
A) true
B) false
B) false
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
B) false
An alternator is a _______
A) three phase AC converter B) DC generator C) wound rotor motor D) AC generator E) single phase AC converter
D) AC generator
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
C) have a rotating armature on which a DC excitation winding is placed
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 shed on only three phase generators
B) are used on alternators not exceeding 1800 rpm
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
E) 60 hertz
The best method of synchronizing an alternator to a system is _______
A) by using a 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
A) by using a synchroscope
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 my 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
E) 1,2,3,4
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 u tilt it is supplying no current to the busbars
A) true
B) false
A) true
For paralleling alternators, the incoming alternator ,use be the same _______ as the system voltage
A) manufacturer
B) conductance
C) voltage
D) amperage
C) voltage
The most common type of rotor used in induction motors is the squirrel cage rotor
A) true
B) false
A) true
The stator of the induction motor is identical to that of the three phase alternator
A) true
B) false
A) true
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
E) are used to control speed
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
D) can be run at a leading power factor
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
B) can be run as an alternator or a motor
Synchronous motors ________
A) require DC power supply to the rotor B) require DC power supply to the stator C) require high slip to control speed D) require a starting motor E) a self starting
A) require a DC power supply to the rotor
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
D) can be used to help correct power factor
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
B) false
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
B) not self starting
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 startup
E) reduce voltage during startup
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
A) use resistors or inductors in series with each phase in order to reduce the starting current
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
B) is used on motors designed to run as delta connected machines
The auto transformer 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 auto transformer is similar to the two winding transformer
A) true
B) false
A) true
Synchronous motors are started by ________
A) the amortisseur winding B) the shunt winding C) the capacitor D) the delta winding E) the Wayne winding
A) the amortisseur winding
Line impedance starters; this method of starting is relatively more expensive but gives a lower starting torque
A) true
B) false
B) false
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
D) require up to six times its full load current when starting
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 rpm when supplied with AC power at 60 Hz, will operate at 1581 rpm
A) true
B) false
A) true
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
A) 1,2
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
D) mutual inductance
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
A) can use less copper
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
E) voltage step up transformers
Modern transformers are very efficient devices with large industrial transformers often better than 95% efficient
A) true
B) false
A) true
These phase systems often use banks of ______ ______ transformers to replace three phase transformers
A) double phase
B) single phase
C) step down
D) step up
B) single phase
If the nu,bet 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
E) step down
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
C) 2,3
In a single phase transformer, it can be assumed that all flux produced by the primary cuts every turn of both the primary and secondary winding, thereby inducing the same voltage in every turn. If the number of secondary turns is less than the number of primary turns, the the voltage induced in the secondary will be larger than that induced in the primary
A) true
B) false
B) false
