Electricity 2 Final Exam
In a three-phase connection, what is the voltage measured across a single winding?
Phase Voltage
The non-rotating part of a revolving-field-type alternator is called the .
stator
Three-phase voltages are ____ out of phase with each other.
120 degrees
A delta-connected set of windings has a phase current of 10 A. What is the line current?
17.32 A
The synchronous motor is an induction motor.
False
When single-phase loads are tied to a three-phase transformer, the phase currents of the transformer will be automatically balanced.
False
Another name for the three-phase wye connection is the _____ connection
Star
The frequency of the current in a squirrel-cage rotor ____ as the rotor approaches synchronous speed.
decreases
When connecting three transformers for three-phase use with a delta secondary, a voltage test should be done before closing the secondary. The correct voltage reading is ____.
zero volts
An isolation transformer has the same input and output voltages.
True
The industrial use transformer that normally has 240 V or 480 V primary and a 120 V secondary is called a(n) ____ transformer.
control
Some motors are constructed so that they will operate on either of two voltages. When they are connected to operate on the higher of the two voltages, the windings are connected in .
series
An alternator containing eight poles per phase is turning at 900 RPM. What is the output frequency?
60 Hz
A transformer has a 2:1 turns ratio. 120 V is applied to the primary. What is the secondary voltage?
60 V
A wye-connected alternator that has phase voltages of 277 V is feeding two loads. The first load is wye-connected inductors that have 40 Ω of XL. The second load is delta-connected capacitors that have XC of 60 Ω. How much is the alternator phase current?
14.93 A
What type of core is often found on high-voltage distribution transformers?
H-type
In order to increase the efficiency of the cooling, some alternators operate in a(n) _____ atmosphere.
hydrogen
The amount of current that flows into a transformer primary when voltage is first applied is called ____ current.
inrush
The revolving armature type alternator is capable of ____ power output and is, therefore, _____ often used than the revolving field type.
less; less
The main power distribution system of a large industrial plant is generally shown using a(n) ____ diagram.
one-line
The rotating part of a revolving-armature-type alternator is called the .
rotor
In a wye-connected system, the line voltage is higher than the phase voltage by a factor of ________.
square root of three
A transformer in which the secondary voltage is less than the primary voltage is called a(n) ____ transformer.
step-down
A transformer in which the secondary voltage is more than the primary voltage is called a(n) ____ transformer.
step-up
A(n) ____ is used to measure the difference in voltage and frequency between two alternators.
synchronoscope
In a three-phase motor, the speed of rotation of the magnetic field is called the ____speed.
synchronous
In a transformer, the turns of primary wire compared to the turns of secondary wire is called the _____ ratio.
turns
An open delta connection is made using ____ single-phase transformers.
two
For most squirrel-cage induction motors the typical percent slip is about ____ percent.
two to five
The direction of magnetic field rotation is known as _____ rotation.
phase
Line voltage and phase voltage are the same in the ______ connection.
Delta
A transformer that has one winding, which is used for both the primary and secondary, is called a(n) _____.
autotransformer
When a squirrel-cage motor is first started, the value of the current draw is .
several times the running current
What function is performed by dielectric oil?
All of the Above
A transformer in which the primary and secondary are physically separated and not connected by a conductor is called a(n) ____ transformer.
isolation
Most transformer cores are assembled from many thin sheets of metal. This type of construction is called ____.
laminated
In a three-phase connection, what is the voltage measured between the lines?
line-voltage
A tape-wound core is also called a(n) ____ core.
Toroid
In an open delta with single-phase loads, the high leg must be identified by .
both a and b
The torque of a squirrel-cage induction motor will be maximum when the phase angle between the stator and rotor fields is .
0 degrees
A wye-delta transformer has a turns ratio of 2:1. It has a secondary line voltage of 480 V. The secondary is connected to a wye-connected three-phase resistive load of 50 Ω in each phase. What is the primary line current?
1.6 A
An open delta transformer is assembled from two 150 kVA transformers. The output power of the open delta is _____.
259.8 kVA
A delta-connected three-phase alternator is driving a wye-connected resistive load. The alternator output voltage is 480 V. The load resistors are 12 Ω each. What is the phase current in the alternator?
40 A
A wye-delta transformer has a secondary phase voltage of 480 V. A delta-connected resistive load has a resistance of 12 Ω in each leg. What is the current in each load leg?
40 A
Line current and phase current are the same in the _______ configuration.
Wye
The three-phase connection in which one end of each of the windings are connected together is called a(n) ________ connection.
Wye
When using an open delta with a center tapped transformer, one wire is the high leg. The high leg is the one that _____.
has the highest voltage
If a three-phase load is a pure reactive load, then the power calculated has units of ______.
VARs
An alternator containing eight poles per phase is turning at 1500 RPM. What is the output frequency?
100 Hz
A delta connected set of windings has a phase voltage of 100 V. What is the line voltage?
100 V
A delta-delta transformer has a primary voltage of 7200 V and a secondary voltage of 360 V. What is the turns ratio?
20:1
A wye-delta transformer has a turns ratio of 2:1. It has a secondary line voltage of 480 V. The secondary is connected to a wye-connected three-phase resistive load of 50 Ω in each phase. What is the secondary phase current?
3.2 A
A wye-delta transformer has a turns ratio of 2:1. It has a secondary line voltage of 480 V. The secondary is connected to a wye-connected three-phase resistive load of 50 Ω in each phase. What is the current flow is in each load resistor?
5.54 A
A three-phase alternator containing six poles is turning at 1000 RPM. What is the output frequency?
50 Hz
In an open delta with a center-tapped transformer, label the two ends of the center-tapped transformer as L2 and L3 and label the high leg as L1. The current flow in the neutral (center tap) is equal to .
AN = A1 + (A2 - A3)
Alternators of small kilovolt-ampere rating are generally -cooled.
air
____ alternators have a fan attached to one end of the shaft.
air-cooled
To reverse the direction of rotation of a three-phase motor, reverse .
any two leads
The squirrel-cage rotor is constructed of ____ and end ____ embedded in steel laminations.
bars; rings
An alternator that has a separate small alternator mounted on the shaft of its rotor is equipped with a(n) .
brushless exciter
For installations that will have both three-phase and single phase loads on a three-phase transformer, the utility company often provides a(n) ____three-phase transformer.
open delta with one transformer center tapped
Power is carried to the outside circuit of a revolving armature type alternator via brushes riding against the .
sliprings
A wye-delta transformer has a primary voltage of 7200 V and a secondary voltage of 360 V. What is the turns ratio?
11.55:1
A delta connected set of windings has a phase voltage of 480 V. What is the line voltage?
480 V
The alternator must supply the _____ for each of the loads.
line current
A transformer supplying a house with 240/120 V has a secondary which is center tapped. The conductor connected to the center is called the ____ conductor.
neutral
Some motors are constructed so that they will operate on either of two voltages. When they are connected to operate on the lower of the two voltages, the windings are connected in .
parallel
If a piece of machinery needs a replacement motor and will be damaged if the new motor rotates the wrong way. The test equipment needed to test the rotation direction of the new motor is a .
phase rotation meter
A wye-delta transformer has its ____ connected as a wye and its ____connected as a delta.
primary; secondary
The output winding of a transformer is called the ____winding.
secondary
The horsepower rating of three-phase motors and kilovolt-ampere rating of three-phase transformers are about __________ greater than for single-phase motors or transformers with a similar frame size.
150 percent
A transformer has a 240 V primary and a 120 V secondary. With a 30 ohm load connected, what is the primary volt- amps?
480
A wye-connected secondary has a phase voltage of 277.14 V. What is the line voltage?
480 V
A delta-wye transformer has a turns ratio of 9:1. The primary line voltage is 3600 V. What is the secondary line voltage?
692.8 V
In a balanced three-phase system, the conductors need be only about ______ the size of conductors for a single-phase, two-wire system of the same kilovolt-ampere (kVA) rating.
75 percent
A wye-delta transformer has a primary line voltage of 7200 V and a secondary line voltage of 480 V. What is the turns ratio?
8.66:1
An open delta transformer is assembled from two 50 kVA transformers. The output power of the open delta is _____.
86.6 kVA
A three-phase connection in which three separate inductive loads have been connected together is a(n) ______ connection.
Delta
In three-phase transformer connections, the primary leads are labeled with the letter ____ and the secondary leads are labeled with the letter ____.
H; X
In three-phase power systems, the delivered power __________?
Never falls to zero
If three-phase transformation is needed and a three-phase transformer of the proper size and turns ratio is not available, three single-phase transformers can be connected to form a ____.
three-phase bank
What is a magnetically operated device that can change values of voltage, current, and impedance without changing frequency?
a transformer
A transformer has a 240 V primary and a 60 V secondary. With a 5 ohm load connected, what is the primary current?
3 A
A wye-delta transformer has a turns ratio of 2:1. It has a secondary line voltage of 480 V. The secondary is connected to a wye-connected three-phase resistive load of 50 Ω in each phase. What is the primary phase current?
1.6 A
The direct current that is an input to the alternator's rotor is the ____ current.
excitation
What is one factor that controls the output frequency of an alternator?
number of stator poles and rotor rotation speed
What machines operate on the same principle of electromagnetic induction as DC generators, but they have no commutator?
alternators
Any transformer flux that does not follow the core and escapes into the surrounding air is called ____.
Flux Leakage
A tape-wound core is assembled by .____
Winding a long continuous tape into a spiral
The transformer that converts a high voltage from the power company down to 240/120 V for homes and businesses is called a(n) ____ transformer.
distribution
In a transformer, even when there is no load connected, there will be some small current flow known as ____ current.
excitation
The higher power output alternators use a revolving .
field
Using a(n) ____ is one method of preventing induced voltage from becoming excessive.
field discharge resistor
When calculating the power of a pure resistive load, the voltage and current are in phase with each other and the power factor is _____.
one
In calculating apparent power (VA) in a three-phase circuit, the phase values of voltage and current are multiplied by _____.
three
If one alternator does not produce sufficient power to supply a load, another alternator can be connected to the circuit. The second alternator is connected in .
Parallel
In a three-phase circuit, the true power (watts) is found by _____.
multiplying apparent power by the power factor
By increasing or decreasing the strength of the magnetic field of the rotor, the _______________ of an alternator is controlled.
output voltage
The power input winding of a transformer is called the ____ winding.
primary
A delta-wye transformer has its ____ connected as a delta and its ____connected as a wye.
primary; secondary
