Electricity 2 Final
The power input of winding of a transformer is called the
primary winding
the rotating part of an alternator is called the
rotor
When a squirrel-cage motor is first started, the value of the current draw is
several times the running current
In a wye connected system, the line voltage is higher than the phase voltage by a factor of
square root of three
Another name for a three phase wye connection is the
star connection
A transformer in which the secondary voltage is less than the primary voltage is called a
step down transformer
A transformer in which the secondary voltage is more than the primary voltage is called a
step up transformer
a wye delta transformer has a turns ratio of 2:1, it has a secondary line voltage of 480v, secondary has 50ohms, what is the secondary phase current
3.2 amps
In a transformer, even when there is no load connected, there will be some small current flow known as
excitation
In a transformer, the turns of primary wire compared to the turns of secondary wire is called the
turns ratio
A transformer has a 240v primary and a 120v secondary, with a 30 ohms load, what is the primary volt amps
480
A wye delta transformer has a primary line voltage of 7200v and a secondary line voltage of 480v, what is the turns ratio
8.66:1
Any transformer flux that does not follow the core and escapes into the surrounding air is called _____.
flux leakage
in order to increase the efficiency of the cooling, some alternators operate in a
hydrogen atmosphere
the alternator must supply_____for each of the loads
line current
a transformer supplying a house with 240/120v has a secondary which is center tapped, the conductor in the center is called the
neutral conductor
power is carried to the outside circuit of a revolving armature type alternator vie brushes riding against the
slip rings
In a three phase motor, the speed of rotation of the magnetic field is called the
synchronous speed
an open delta connection is made using____single phase transformers
two
for most squirrel cage induction motors the typical percent slip is about
two to five
The three phase connection in which one of the windings are connected together is called a
wye connection
most transformer cores are made of thin sheets of metal, this is called
laminated
In a three phase circuit, the true power is found by
multiplying apparent power by the power factor
what is one factor that controls the output frequency of an alternator
number of stator poles and rotor speed
the test equipment needed to test the rotation direction of the new motor is a
phase rotation meter
In a three phase connection, what is the voltage measured across a single winding
phase voltage
three lines supply power to a three phase motor, if one of these lines should open, the motor will be connected to single phase power, this is condition is known as
single phasing
Line current and phase current are the same in the _____ configuration.
wye
A tape wound core is also called a
toroid core
In calculating apparent power in a three phase circuit, the phase values of voltage and current are multiplied by
three
if three phase transformation is needed and a three phase transformer of the proper size is not available, three single phase transformer can be connected in a
three phase bank
A wye delta transformer has a primary voltage of 7200v and a secondary voltage of 360v, what is the turns ratio
11.55:1
Three-phase voltages are _____ out of phase with each other.
120 degrees
A wye connected alternator that has phase voltages of 277 V is feeding two loads, the frst is wye connected with 40 ohms Xl, the second is delta connected with 60 ohms Xc, what is phase current
14.93 amps
The horsepower rating of three phase motors and the kilovolt amphere rating of three phase transformers are _____ greater than single phase motors
150 percent
A 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
A transformer in which the primary and secondary are physically separated and not connected by a conductor is called an
isolation transformer
the revolving armature type alternator is capable of___power output and is, therefore____often used than revolving fields
less, less
In a three phase connection, what is the voltage measured between lines
line voltage
An isolation transformer has the same input and output voltages.
True
_____alternators have a fan attached to one end of the shaft
air cooled
alternators of small kva rating are generally
air cooled
to reverse the direction of rotation of a three phase motor, reverse
any two leads
A transformer that has one winding, which is used for both primary and secondary is called an
autotransformer
in an open delta with single phase loads, the high leg must be identified by
both a and b
a delta delta transformer has a primary voltage of 7200v and a secondary voltage of 360 what is the turns ratio
20:1
A transformer has a 240v primary and a 60v secondary, with a 5 ohm load, what is the primary current
3 amps
a squirrel cage rotor induction motor has a synchronous speed of 3600 rpm and a rotor speed of 3450 rpm what is the percent slip
4.17 percent
a wye delta transformer has a turns ratio of 2:1, it has a secondary line voltage of 480v, secondary load of 50 ohms, what is the current flow
5.54 amps
a three phase alternator containing six poles is turning at 1000 rpm, what is the output frequency
50 hz
A delta wye transformer has a turns ratio of 9:1, the primary line voltage is 3600v, what is the secondary
692.8v
what type of core is often found in high voltage distribution transformers
H type
If a three phase load is a pure reactive load, then the power calculated has units of
VARs
the frequency of the current in a squirrel cage rotor___as the rotor approaches synchronous speed
decreases
In a three phase power system, the delivered power
never falls to zero
an altenator has 8 poles per phase turning at 900 rpm, what is the output frequency
60 hz
A transformer has a 2:1 turns ratio, 120V is applied to the primary, what is the secondary voltage
60 volts
a tape wound core is assembled by
winding a long continuous tape into a sprial
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 current of 10A, what is the line current
17.32A
what function is performed by dielectric oil
all of the above
an alternator that has a separate small alternator mounted on the shaft is equipped with a
brushless exciter
by increasing or decreasing the strength of the magnetic field of the rotor, the_____is controlled
output voltage
A delta connected set of windings has a phase voltage of 100V, what is the line voltage
100 volts
In three phase transformer connections, the primary leads are labeled with the letter__ and the secondary with__
H, X
what machines operate on the same principle of electromagnetic induction as DC generators, but they have no commutator
alternators
The industrial use transformer that normally has 240/480v primary and 120 secondary is called a
control transformer
A three phase connection in which separate inductive loads are connected is a
delta connection
the amount of current that flows into a transformer primary when voltage is first applied is called
inrush current
A wye connected secondary has a phase voltage of 277.14v, what is the line voltage
480v
the squirrel cage rotor is constructed of____and end____embedded in steel laminations
bars, rings
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
The main power distribution system of a large industrial plant is generally shown using a
one line diagram
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
the squirrel cage rotor letters indicate the shape of rotor bars, different types of bars have a large effect on the____of a motor
starting torque
the non rotating part of a revolving field type alternator is called the
stator
In a balanced three phase system, the conductors need to be only ______the size of conductors for a single phase
75 percent
A delta connected three phase alternator is driving a wye connected load, output voltage is 480 volts, load resistors are 12 ohms each, what is the phase current
40 amps
a wye delta transformer has a secondary phase voltage pf 480v, with a 12 ohm load in each leg, what is the current in each load leg
40 amps
A delta connected set of windings has a phase voltage of 480V, what is the line voltge
480 volts
the transformer that converts a high voltage from the power company down to homes and businesses is called a
distribution transformer
the direct current that is an input to the alternators rotors is the
excitation current
an open delta transformer is assembled from two 150kva transformers. the output power of the open delta is
259.8kva
using_____is one method of preventing induced voltage from becoming excessive
field discharge resistor
when connecting three transformers for three phase using delta secondary, a voltage test should be done before closing the secondary, the correct voltage reading is
zero volts
Line voltage and phase voltage are the same in the _____ connection.
delta
the torque of a squirrel cage induction motor will me max 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 480v, secondary has 50ohms, what is the primary line current
1.6 amps
a wye delta transformer has a turns ratio of 2:1, it has a secondary line voltage of 480v, secondary has 50ohms, what is the primary phase current
1.6 amps
an open delta transformer is assembled from two 50kva transformers. the output power of the open delta is
86.6 kva
what is a magnetically operated device that can change values of voltage, current and impedance without changing frequency
a transformer
the higher power output alternators use a revolving
field
when using an open delta with a center tapped transformer, one wire is the high leg, which has the
highest voltage
for installations that will have both three phase and single phase loads on a transformer, the utility company often provides an
open delta with one transformer center tapped
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
the direction of magnetic field rotation is known as
phase rotation
a delta wye transformer has its____connected as a delta and its_____connected as a wye
primary, secondary
a wye delta transformer has its_____connected as a wye and its_____connected as a delta
primary, secondary
a______is used to measure the difference in voltge and frequency between two alternators
synchronoscope