transformers
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 ____.
primary voltage Correct! winding voltage secondary voltage kilowatt rating
The volts-per-turn for any transformer is determined by the ____.
primary winding secondary winding number of turns loads
Many electricians use the clamp-on type of AC ammeter.
t
The wye, or ____, connection is made by connecting one end of each of the three-phase windings together.
triangle square Correct! star rectangle
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.
true
Current transformers are used to change the full-scale range of ____.
wattmeters voltmeters Correct! AC ammeters DC ammeters
A(n) ____ transformer is a transformer constructed with a hollow core, similar to a donut in that it has a hole in the middle.
solenoid eloid Correct! toroid winoid
A three-phase transformer is connected in a delta-delta configuration. The load is connected as a 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 ratio is 9.45:1.
t
The clamp-on type of AC ammeter uses a current transformer to operate
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
A basic law concerning transformers is that all values of a transformer are proportional to its turns ratio.
true
The amount of inrush current in the primary of a transformer is limited by amount of applied voltage, resistance of the wire in the primary winding, and flux change of the magnetic field in the core.
true
A voltmeter is a low-impedance device.
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
The primary winding of a transformer is the side of the transformer that is connected to the driven load.
false
The amount of core material needed for a particular transformer is determined by the ____ rating of the transformer. saturation
saturation efficiency primary Correct! power
CTs are designed to operate with ____ winding shorted.
the secondary the primary both the primary and secondary neither
In a(n) ____ connection, line voltage and phase voltage are the same.
alpha beta Correct! delta gamma
The phase windings of an alternator are connected in wye. The alternator produces a line voltage of 440V and supplies power to two resistive loads. One load contains resistors with a value of 4 ohm each, connected in wye. The second load contains resistors with a value of 6 ohm each, connected in delta. Both loads are connected directly to the output of the alternator. To determine the amount of line current needed for load 2, use the following formula: IL(Load 2) = IP(Load 2) x ____.
1.732 3 You Answered 73.33 440
The primary current can be computed by using the formula ____.
EP / ES = IS / IP You Answered EP x ES = IS / IP EP / ES = IS x IP ES / EP = IS / IP
Current transformers differ from voltage transformers in that the primary winding is generally part of the power line.
t
One of the formulas for determining current in a single-phase circuit is ____.
VA = I / E I = VA / (E x 3) Correct! I = VA / E E = I x VA
A delta-connected alternator is connected to a wye-connected resistive load. The alternator produces a line voltage of 240V, and the resistors have a value of 6 ohm each. If EL(Load) = 240V, then EP(Load) =
80 Correct! 138.52 160 415.68
If the amount of voltage applied to the load is to be computed, the formula ____ can be used.
EP x ES = NP / NS Correct! EP / ES = NP / NS EP / ES = NP x NS ES / Ep = NP / NS
A formula used to compute the voltage in a wye-connected system is ____.
EPhase = ELine / 1.732 EPhase = ELine x 1.732 EPhase = 1.732 / ELine ELine = EPhase / 1.732
A good rule for any type of transformer is that power in is always slightly greater than power out.
false
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.
false
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.
false
An autotransformer is literally a self-transformer.
t
The line current of a delta connection is higher than the phase current by a factor of the square root of 3 (1.732).
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 secondary winding of a CT should never be opened when there is power applied to the primary.
t
Autotransformers, like isolation transformers, can be used as step-up or step-down transformers.
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
The amount of power input and output for an autotransformer must be the same.
t
A three-phase motor is connected to a 480V, 60-Hz line. A clamp-on ammeter indicates a running current of 68A at full load, and a three-phase wattmeter indicates a true power of 40,277W. To solve this problem, first find the amount of apparent power in the circuit using the formula ____.
VA = 1.732 / (EL x IL) VA = (EL x IL) / 1.732 VA = 1.732(EL + IL) Correct! VA = 1.732 x EL x IL
The autotransformer's one disadvantage can cause problems with certain types of equipment and must be a consideration when designing a power system.
t
A delta-connected alternator is connected to a wye-connected resistive load. The alternator produces a line voltage of 240V, and the resistors have a value of 6 ohm each. Using Ohm's law, we determine the amount of phase current, IP(Load) = ____A.
13.3 Correct! 23.1 26.6 69.28
Transformers can be divided into several classifications, which include ____.
isolation, auto, and current isolation, auto, and manual isolation, public, and current commercial, auto, and current
The autotransformer has one disadvantage: Since the load is connected to one side of the power line, there is ____ between the incoming power and the load.
line isolation You Answered no line isolation line continuation no line continuation
One reason why three-phase power is superior to single-phase power is that in a balanced three-phase system, the conductors need be only about ____% of the size of conductors for a single-phase two-wire system of the same kVA (kilovolt-amp) rating.
33.3 60 Correct! 75 85
Assume that an ammeter provides range values of 5 A, 2.5 A, 1 A, and 0.5 A. Also, assume that the meter movement requires a current flow of 100 mA (0.100) to deflect the meter full scale and that the primary of the current transformer contains 5 turns of wire. The number of turns needed for a full-scale range of 5 amperes is ____ turns.
5 25 125 Correct! 250
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 to ____.
wrap just one turn of wire around the jaw of the ammeter to change the secondary-scale value simply change the secondary-scale value setting Correct! wrap multiple turns of wire around the jaw of the ammeter to change the secondary-scale value none of the above
The phase windings of an alternator are connected in wye. The alternator produces a line voltage of 440V and supplies power to two resistive loads. One load contains resistors with a value of 4 ohm each, connected in wye. The second load contains resistors with a value of 6 ohm each, connected in delta. Both loads are connected directly to the output of the alternator. The line voltage for both loads 1 and 2 will be the same, ____V.
17.33 219.09 Correct! 220 440
Assume that an isolation transformer has 240 turns of wire on the primary and 60 turns of wire on the secondary. This is a ratio of 4:1 (240/60 = 4). Now assume that 120 V is connected to the primary winding. The voltage of the secondary winding is ____V.
25 Correct! 30 240 480
The primary winding of a transformer is between points B and N, and has 120 volts applied to it. There are 120 turns of wire between points B and N. The selector switch is set to point D, and the load is now connected between points D and N. The secondary of this transformer contains 40 turns of wire. The amount of voltage applied to the load is ____ V.
30 You Answered 40 90 None of the above
When working with any transformer, the primary is the winding that is connected to a voltage source, and the secondary is the winding that is connected to an electrical load.
t
Autotransformers have high efficiencies, most in the range of ____.
80% to 83% 84% to 86% 90% to 93% Correct! 95% to 98%
A problem with many clamp-on ammeters is that ____.
changing the turns ratio is limited to wrapping two turns of wire around the jaw of the ammeter their mid-range scale values are inaccurate they can have different range settings by changing the turns ratio of the secondary of the transformer Correct! their lowest scale value is too high to accurately measure low-current values
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.
control Correct Answer potential distribution latent
Magnetic materials contain tiny magnetic structures in their molecular material known as magnetic
domains You Answered realms points plains
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 a wye.
f
According to the NEC, fuses must be plainly marked with the interrupting current or rating if it is other than 8,000 amperes.
f
An autotransformer uses the same winding as the primary transformer, but does not use the same winding as the secondary transformer.
f
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
In a wye-connected system, the line voltage is lower than the phase voltage by a factor of the square root of 3 (1.732).
f
An autotransformer has a 2:1 turns ratio. A voltage of 480 volts is connected across the entire winding. Since the transformer has a turns ratio of 2:1, a voltage of 240 volts will be supplied to the load. Ammeters connected in series with each winding indicate the current flow in the circuit. It is assumed that the load produces a current flow of 4 amperes on the secondary. A current flow of ____ amperes is supplied to the primary.
2 4 6 12
If a rotary switch were to be removed and replaced with a sliding tap that made contact directly with the transformer winding, the turns ratio could be adjusted continuously. This type of transformer is commonly referred to as a ____ depending on the manufacturer.
Variate Rotarystat You Answered Variac or Powerstat Variate or Transtron
The amount of phase current can be determined using the Ohm's law equation, IP(Load) = Z / EP(Load).
f
The clamp-on ammeter does not have different range settings.
f
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 frame size.
f
There are several steps that should be followed when selecting a conductor. The first step is to determine the temperature rating of the devices and terminals.
f
To determine the impedance of a transformer, first compute the full load current rating of the secondary winding using the formula I = E / VA.
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 a wye connection.
f
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.
false
The inductive VARs in a three-phase circuit can be computed using the formula VARsL equals the square root of VA2 - P2.
t
Almost all transformer nameplates will list the primary voltage, secondary voltage, and kVA rating.
true
To use the clamp-on type of AC ammeter, clamp the jaw of the meter around ____ of the conductors supplying power to the load.
only one one or two two all
The primary winding of a transformer is between points B and N, and has 120 volts applied to it. There are 120 turns of wire between points B and N. The selector switch is set to point D, and the load is now connected between points D and N. The secondary of this transformer contains 40 turns of wire. Assume that the load connected to the secondary has an impedance of 10 ohm. The amount of current flow in the secondary circuit is ____ amps.
0.25 3 You Answered 4 9
A current transformer has a ratio of 600:5. If the primary power wire is inserted through the opening, it will require a current of 600 amps to deflect the meter full scale. If the primary power conductor is looped around and inserted through the window a second time, the primary now contains two turns of wire instead of one. It requires ____ amps of current flow in the primary to deflect the meter full scale.
125 200 Correct! 300 600
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 x EPhase x IPhase (EPhase x IPhase) / 3 3 / (EPhase x IPhase) EPhase x IPhase
To calculate the current flow supplied to the primary, use the formula IP = ____.
IS / Ratio IS x Ratio 2(IS / Ratio) Ratio / IS
According to ANSI rules, the high-voltage leads of a transformer are to be marked H1 - H2, and so forth, and the low-voltage leads are marked ____, and so forth.
L1 - L2 S1 - S2 V1 - V2 Correct! X1 - X2
The number of turns needed for a full-scale range of amperes can be calculated using the formula ____.
NS / NP = IS / IP NP x NS = IS / IP Correct! NP / NS = IS / IP NP / NS = IP / IS
____ of a current transformer must be connected in series with the load.
The primary winding
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 a
buck or linked patched or boost patched or tied Correct Answer buck or boost
Autotransformers 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
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 volts-per-turn ratio of 2.0.
f
If the primary current is less than 2 amperes, the short circuit protective device can be set at not more than 250% of that value.
f
In general, fuses are rarely selected for protection of high voltage circuits because they are more expensive than other types of protection.
f
In-line ammeters (ammeters that must be connected directly into the line) 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 Scott connection is used to convert three-phase power into one-phase power using three single-phase transformers.
f
The amount of current flow in a secondary circuit can be computed using the formula I = Z / E.
f
A very common type of isolation transformer is the distribution transformer, which is used to supply power to ____.
hospitals and nursing homes warehouses and factories Correct! most homes and many businesses power plants
A three-phase transformer is constructed by winding three single-phase transformers on a single core.
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
t
Occasionally, the temperature rating of devices such as circuit breakers, fuses, and terminals may be found on the piece of equipment, but this is the exception and not the rule.
t
Percentages must be converted to a decimal form before they can be used in a mathematical formula. When this conversion has been made to represent the percent impedance in decimal form, this is called the decimal impedance.
t
The NEC states that an autotransformer rated at 1000 volts or less must be protected by an overcurrent device in each of the input ungrounded conductors.
t
The actual impedance of a transformer is determined by its physical construction, such as the gage of the wire in the winding, the number of turns, the type of core material, and the magnetic efficiency of the core construction.
t
The turns ratio of the transformer can be changed by looping the power wire through the opening in the transformer to produce a primary winding 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 three-phase transformers are connected as a wye or delta, the primary and secondary windings themselves become the phases of a three-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
Factors such as the amount and type of core material, wire size used to construct the windings, number of turns, and the degree of magnetic coupling between the windings greatly affect the transformer's impedance.
true
