Final
in line ammeters that have multiple range values cannot use a current transformer to provide the different ranges
F
motor power factor can be computed using this formula PF=VA/P
F
the primary windings of three single phase transformers have been connected to form a wye connection, in a wye connection the phase voltage is less than the line voltage by a factor of
1.732
to find line current for load 2 use the following formula IL(Load2)=IP(Load2) times what
1.732
assume that an isolation transformer has 240 turns of wire on the primary and 60 turns of wire on the secondary. this is a ration of 4:1 (240/60=4). Now assume that 120V is connected to the primary winding. the voltage of the secondary winding is
30 because 120 from the primary is divide by 4
in a balanced three phase system the conductors need to be only about___% of the size of conductors for a single phase two wire system of the same KVA rationg
75%
One of the formulas for determining current in a single phase current is
I=VA/E
the amount of core material needed for a particular transformer is determined by the what
Power
the wye or ____ connection is made by connecting one end of each of the three phase windings together
Star
Auto transformers, like isolation transformers can be used as step up or step down transformers
T
a basic law of concerning transformers is that all values of a transformer are proportional to its turns ration
T
what of a current transformer must be connected in series with the load
The Primary Winding
CT's are designed to operate with what winding shorted
the secondary
what is a problem with many clamp on ammeters
their lowest scale value is too high to accurately measure low current values
to use the clamp on type of AC ammeter, clamp the jaw of the meter around how many of the conductors supplying the power to the load
1
the amount of current flow in a secondary circuit can be computed using the formula I=Z/E
F because I=E/Z
three single phase transformers are connected to forma a wye-delta bank. once the voltage across each of the loads resistors is known, the current flow through the phase of the load can be computed using the formula
IP(load)=EP(load)/R
to calculate the current flow supplied to the primary, use the formula IP=
IS/Rations
the clam on type of AC ammeter uses a current transformer to operate
T
the turns ration of the transformer can be changed by looping the power wire through the opening in the transformer to produce a primary wining of more than one turn
T
transformer formulas can be used to determine the number of secondary turns needed to produce desired ranges
T
when the primary of a transformer is connected to power but there is no load connected to the secondary, current is limited by the inductive reactance of the primary
T
a connection involving the us of two transformers to supply three phase power is the T-connection. in this connection, one transformer is generally referred to as the main transformer and the other is called the
Teaser Transformer
to find apparent power in the circuit use the formula
VA=1.732*EL*IL
a volt meter is a low impedance device
F
a wye connected three phase alternator supplies power to a delta-connected resistive load. the alternator has a line voltage of 480V. each resistor of the delta load has 8 ohm of resistance. the load is connected directly to the alternator. therefore the line voltage supplied by the alternator is 160V.
F
a wye-delta transformer has its primary winding connected in a delta and its secondary winding connected in wye
F
current transformers ate used to change the full scale range of
AC ammeters
the polarity of transformer windings can be determined by connecting one lead of the primary to one lead of the secondary and testing for an increase or decrease in voltage. this is often referred to as what connection
Buck or Boost
a dielectric oil provides electrical insulation between the windings and the
Case
in a ___________ connection line voltage and phase voltage are the same
Delta
one of the most common three phase to six-phase connections is the
Diametrical Connection
magnetic materials contain tiny magnetic structures in their molecular material know as magnetic what
Domains
in a three phase transformer, the phase value of the primary voltage can be computed using the formula
EP(primary)=EL/1.732
primary current can be computed by using the formula
EP/ES=IS/IP
a formula used to compute the voltage in a wye connected system is
EPhase=Eline/1.732
a common type of isolation transformer found throughout industry is the control transformer, which is used to increase the line voltage to the value needed to operate control circuits
F
a good rule for any type of transformer is that power in is always slightly greater than power out
F
if a rotary switch was to be removed and replaced with a sliding tap that made contact directly with the transformer winding the turns ration could be adjusted continuously. this type of transformer is commonly referred to as a _____ depending on manufacturer
Variate or Powerstat
only phase values of _____ can be used when computing transformer values
Voltage and Current
a transformer nameplate does not list the current rating of the windings. the current rating for a winding can be determined by dividing the KVA rating by the
Winding Voltage
according to ansi rules, the high voltage leads of a transformer are to be marked H1-H2, and so forth, ansd the low voltage leads are marked by what and so forth
X1-X2
if power is applied to the transformer bank before the delta connections is closed the volt meter should indicate
Zero Volts
transformers can be divided into several classifications, which include
Isolation, Auto, Current
a very common type of isolation transformer is the distribution transformer, which is used to supply power to
Most homes and many businesses
the number of turns needed for a full scale range of amperes can be calculated using the formula
NP/NS=IS/IP
the auto transformer has one disadvantage: since the load is connected to one side of the power line, there is ___ between the incoming power and the load
No Line Isolation
a _______ Diagram which illustrates a delta-wye connection is generally used to show the main power distribution system of a large industrial plan
One-Line
the _____ transformer is basically an isolation transformer with a high-voltage rating on the primary winding, and that is intended for use in metering circuits
Potential
a three phase transformer is constructed by winding three single phase transformers on a single core
T
almost all transformer nameplates will list the primary voltage, secondary voltage, and KVA rating
T
if three phase transformation is needed and a three phase transformer of the proper size and turns ration is not available, three single phase transformers can be connected to form a
Three Phase Bank
what transformer is constructed with a hollow core, similar to a donut in that it has a hole in the middle
Toroid
the horsepower rating of three phase motors and the kilovolt amp rating of three phase transformers are about 75% greater than for single phase motors or transformers with a similar size frame
F
the primary winding of a transformer is the side of the transformer that is connected to the driven load
F
the reason that the isolation transformer can greatly increase any voltage spikes before they reach the secondary is because of the rise time of current through an inductor
F
an auto transformer is literally a self transformer
T
correcting the power factor of a three phase circuit is similar to the procedure used to correct the power factor of a single phase circuit
T
if the phase values of voltage and current in a three phase circuit are known, the apparent power can be computed using the formula
VA=3*EPhase*Iphase
an ammeter with a full scale range of 0-6 amperes would not be able to measure accurately a current flow of 0.2 amperes. the answer to this problem is
wrap multiple turns of wire around the jaw of the ammeter to change the secondary scale value
an auto transformer uses the same windings as the primary transformer but does not use the same winding as the secondary transformer
F
auto transformers rarely have multiple taps to provide different voltages
F
delta connections should only be checked for proper polarity after making the final connection and applying power
F
in a wye connection, the line voltage is lower than the phase voltage by a factor of the square root of 3
F
the amount of phase current can be determined using the equation IP(load)=Z/EP(load)
F
the clamp on ammeter does not have different range settings
F
the scott connection is used to convert three phase power into one phase power using three single phase transformers
F
there will always be some amount of current flow in the primary of any voltage transformer even if there is no load connected to the secondary. this is called the relaxation current of the transformer
F
when current transformers are used, the main power line is inserted through the opening in the transformer. the power line acts as the primary of the transformer and is considered to be two turns
F
when the secondary winding of a CT is opened when there is power applied to the primary, this will cause the transformer to produce a step down in voltage that could still be high enough to kill anyone who comes in contact with it
F
when three single phase transformers are used to make a three phase transformer bank, their primary windings are always connected in wye
F
if the number of primary turns is the sum of the turns between taps(such as 80+100+60=240 turns) and 120 volts are connected across the 240 turns this transformer will have a volt per turn ration of 2.0
F because 240/120=.5
current transformers differ from voltage transformers in that the primary winding is generally part of the power line
T
factors such as the amount and type of core material, wire size is used to construct the windings, number of turns, and the degree of magnetic coupling between the windings greatly affect the transformers impedance
T
in a delta connection the phase voltage and line voltage are the same so it would be appropriate to say that EP(secondary)=EL(secondary)
T
in a wye connected system, phase current and line current are the same
T
many electricians use the clamp on type of AC ammeter
T
the amount of inrush current in the primary of a transformer is limited by the amount of applied voltage, resistance of the wire in the primary winding, and flux change of the magnetic field in the core
T
the amount of power input and output for an autotransformer must be the same
T
the auto transformer's one disadvantage can cause problems with certain types of equipment and must be a consideration when designing a power system
T
the inductive VARs in a three phase circuit can be computed using the formula VARsL equals the swuare root of VA2-P2
T
the line current of a delta connection is higher than the phase current by a factor of the square root of 3
T
the open delta transformer connection is often used when the amount of three-phase power needed is not excessive, such as in a small business
T
the primary current can be computed by using the same formula that is used to compute primary current for an isolation type transformer
T
the reason for the difference between line current and phase current in a delta connection is that current flows through different windings at different times in a three phase circuit
T
the secondary winding of a CT should never be opened when there is power applied to the primary
T
when three phase transformers are connected as wye or delta, the primary and secondary windings themselves become the phases of a three single phase connection. this is true whether a three phase transformer is used or whether three single phase transformers are employed to form a three phase bank
T
a three phase transformer is connected in a delta configuration. the load is connected as wye, and each phase has an impedance of 7 ohm. the primary is connected to a line voltage of 4160 volts, and the secondary line voltage is 440 volts. therefore, the turns ration is 9.45:1
T 4160/440= 9.45:1
if the primary section of a transformer is connected across part of a winding and the secondary is connected across the entire winding, the secondary section contains more windings than the primary. this type of transformer is a step down transformer
F\