Lesson 3 - Transformer Connections (2023)
Wye-to-delta connected transformers, with properly marked leads, always have an angular-phase displacement of ? .
30°
What voltage reading would be expected across the winding?
480V E ᶩᶤᶯᵉ = E ʷᶤᶯᵈᶤᶯᵍ = 480V
What current reading would be expected on Phase B? (Round the FINAL answer to the nearest whole number.)
4A I ᶩᶤᶯᵉ = I ʷᶤᶯᵈᶤᶯᵍ × 1.73 = 2.31 × 1.73 = 3.99 = 4 A
In a wye-to-delta connection of three single-phase transformers, one unit may be disconnected from the circuit and service maintained with the secondary operating in an open delta at ? of normal bank capacity.
57.7%
A(n) ? is a 3-phase connection method that has the wire from the ends of each coil connected end-to-end to form a closed loop.
Delta connection
A transformer with a 10:1 turns ratio is connected as shown. The applied AC voltage is 240 V, and the reading taken between the high- and low-voltage leads is 216 V. This means that the transformer is wired with additive polarity.
False
In the secondary winding of a 1-phase transformer, the neutral wire generally carries a current equal to that of either of the two outside wires.
False
The majority of 3-phase transmission circuits in the United States are designed with only 3-phase transformers connected together to obtain 3-phase power.
False
The polarity of a transformer can be used to deduce the direction of the turns of the high- and low-voltage windings around the transformer core.
False
ANSI standardization rules state that high-voltage leads brought outside the transformer case are to be marked with a(n) ? and low-voltage leads are to be marked with a(n) ? .
H / X
Which of the following describes the transformer connections?
Primary delta, lower voltage; secondary delta, higher voltage
Which of the following describes the transformer connections?
Primary delta-secondary delta, single-voltage units
Which of the following describes the transformer connections?
Primary delta-secondary wye, single-voltage units
Which of the following describes the transformer connections?
Primary wye, higher voltage-secondary delta, lower voltage
Which of the following describes the transformer connections?
Primary wye-secondary delta, single-voltage units
Which of the following describes the transformer connections?
Primary wye-secondary wye, single-voltage units
A wye (Y) system is also known as a(n) ? system.
Star
What method is used to test for polarity and check transformer lead markings?
Transformer comparison DC use AC use Any of the above
A delta-to-delta connection can provide 3-phase power for machines and also 120 volts for receptacles and lighting.
True
A wye-to-delta connection may be made with three single-phase transformers.
True
All phases of a 3-phase transformer have the same relative polarity.
True
In transformers that have both windings wound in the same direction with respect to the core, the polarity is changed from subtractive to additive by interchanging the position of the two X leads.
True
Three-phase transformers are seldom connected in a delta-to-delta connection.
True
Today, the majority of dwellings are wired with 3-wire service.
True
A(n) ? connection may also be called a V-connection because only two sides on the delta triangle are included.
Open delta
A(n) ? winding is a third winding that is often used in power transformations to provide station power requirements or a tie with synchronous condensers.
Tertiary
? refers to the voltage-vector relationship of the transformer leads as brought outside the transformer case.
Transformer polarity
Match the type of connection to the appropriate item in the illustration.
1 - Wye-to-wye 2 - Delta with high leg to ground 3 - Delta-to-delta 4 - Delta-to-wye
In a balanced 3-phase, delta-connected system, the line current is equal to ? times the current in one of the coils.
1.73
In general, distribution transformers of 200 kVA or less having voltages of 7,500 V or less have ? polarity.
Additive
The transformer bank is wired in a(n) ? connection.
Delta-wye
What current reading would be expected on the Neutral, assuming a balanced load?
0A Note: When phases of a wye-connected transformer are balanced, the loads are equal and there is no neutral current.
Match the type of connection to the appropriate item in the illustration.
1 - Open Delta 2 - Delta 3 - Wye-to-delta 4 - Wye
Match the illustration to the appropriate secondary side winding type.
1 - Series-connected 2 - Parallel-connected 3 - Series-connected for 3-wire service
With a 3-phase bank of single phase transformers connected delta /delta, it is possible to remove one unit for repair and keep the 3-phase power available.
True
In a wye-connected system, the only current that flows in the neutral wire is the ? current.
Unbalanced
A(n) ? connection is a 3-phase connection that has one end of each coil connected together and the other end of each coil left open for external connections.
Wye
In a 3-phase circuit, the three windings are located within the generator to create three separate voltages that are out-of-phase with each other by ? electrical degrees.
120
Calculate the meter reading one would expect to see between Phase B and Neutral. Round down the answer to a whole number.
120V Vₚₕₐₛₑ = Vₗᵢₙₑ / 1.73 = 208V / 1.73 = 120V
What current reading would be expected on Phase B?
20A Iₗᵢₙₑ = Iₚₕₐₛₑ = 20 A
Calculate the meter reading one would expect to see between Phase C and Neutral. Round down the answer to a whole number.
120V Vₚₕₐₛₑ = Vₗᵢₙₑ / 1.73 = 208V / 1.73 = 120V Note: Phase-to-neutral should be equal across all phases in a correctly wired system.
