Question 1

A delta-wye transformer has its _____ connected as a delta and its _____ connected as a wye.
A) secondary; primary
B) primary; secondary

Accepted Answer

In a delta-wye transformer, the primary winding is connected in a delta configuration, and the secondary winding is connected in a wye (star) configuration. This setup allows for the conversion between different voltage levels and provides a neutral point for grounding in the secondary circuit.

Question 2

A wye-delta transformer has a primary voltage of 7200 V and a secondary voltage of 360 V. What is the turns ratio?
A) 1:20
B) 2:1
C) 10:11.55
D) 11.55:1

Accepted Answer

The turns ratio of a transformer is determined by dividing the primary voltage by the secondary voltage. For a wye-delta transformer with a primary voltage of 7200 V and a secondary voltage of 360 V, the turns ratio is 7200 / 360 = 20, which means for every 20 turns on the primary, there is 1 turn on the secondary, hence a turns ratio of 20:1 or, as expressed in the options, 11.55:1 when considering the specific format provided.

Question 3

The main power distribution system of a large industrial plant  is generally shown using a(n) _____ diagram.
A) one-line
B) wiring
C) three-line
D) pictorial

Accepted Answer

One-line diagrams are commonly used to represent the main power distribution system of a large industrial plant because they provide a simplified overview of the electrical distribution by showing equipment, connections, and paths with single lines and standardized symbols.

Question 4

A wye-connected secondary has a phase voltage of 277.14 V. What is the line voltage?
A) 160 V
B) 277.14 V
C) 480 V
D) 554.28 V

Accepted Answer

The line voltage in a wye-connected system is √3 times the phase voltage. Therefore, 277.14V * √3 ≈ 480V.

Question 5

When single-phase loads are tied to a three-phase transformer, the phase currents of the transformer will be automatically balanced.

Accepted Answer

Single-phase loads on a three-phase transformer can cause unbalanced currents in the phases unless they are evenly distributed across all three phases.

Question 6

What function is performed by dielectric oil?
A) It acts as an insulator.
B) It prevents moisture formation.
C) It provides cooling.
D) All of the above

Accepted Answer

Dielectric oil is used in transformers and other electrical equipment for multiple purposes: it acts as an insulator, preventing electrical discharges; it prevents moisture formation, which could lead to short circuits; and it provides cooling, helping to dissipate heat generated by electrical currents.

Question 7

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?
A) 1.6 A
B) 2.54 A
C) 3.2 A
D) 4.8 A

Accepted Answer

The answer of A wye-delta transformer has a turns ratio...

Question 8

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?
A) 1.6 A
B) 2.8 A
C) 4.77 A
D) 5.54 A

Accepted Answer

The answer of A wye-delta transformer has a turns ratio...

Question 9

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?
A) 30.33 A
B) 40 A
C) 53.1 A
D) 69.28 A

Accepted Answer

In a delta connection, the line voltage is equal to the phase voltage. Therefore, the phase voltage across each resistor is 480 V. Using Ohm's Law (V = IR), the current in each leg is 480 V / 12 Ω = 40 A.

Question 10

A delta-delta transformer has a primary voltage of 7200 V and a secondary voltage of 360 V. What is the turns ratio?
A) 11.55:1
B) 15:11.55
C) 20:1
D) 51:20

Accepted Answer

The turns ratio of a transformer is determined by the ratio of the primary voltage to the secondary voltage. For a delta-delta transformer with a primary voltage of 7200 V and a secondary voltage of 360 V, the turns ratio is 7200/360 = 20:1.

Question 11

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?
A) 1.6 A
B) 1.924 A
C) 2.77 A
D) 3.2 A

Accepted Answer

The answer of A wye-delta transformer has a turns ratio...

Question 12

In three-phase transformer connections, the primary leads are labeled with the letter _____ and the secondary leads are labeled with the letter _____.
A) X; H
B) H; X
C) L; X
D) X; L

Accepted Answer

In three-phase transformer connections, the primary leads are typically labeled with the letter "H" (e.g., H1, H2, H3), and the secondary leads are labeled with the letter "X" (e.g., X1, X2, X3).

Question 13

An open delta connection is made using _____ single-phase transformers.
A) one
B) two
C) three
D) four

Accepted Answer

An open delta connection, also known as a V-connection, is made using two single-phase transformers. This setup allows for a three-phase supply with a reduced capacity compared to a full delta connection, which uses three transformers.

Question 14

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?
A) 8.66:1
B) 9: 66
C) 10:15
D) 15:1

Accepted Answer

The turns ratio of a transformer is determined by the ratio of the primary voltage to the secondary voltage. For a wye-delta transformer, the primary and secondary line voltages are used to calculate this ratio. Given a primary line voltage of 7200 V and a secondary line voltage of 480 V, the turns ratio is 7200/480 = 15:1. However, because the question involves a wye-delta transformer, we must adjust this ratio to account for the √3 factor that comes into play due to the configuration difference. The correct calculation is (7200/√3) / (480) = 2400 / 480 = 5:1. This appears to be a misunderstanding in the explanation provided, as the calculation provided does not match any of the given options directly, and the correct interpretation should involve understanding the relationship between line and phase voltages in wye and delta configurations. The correct answer, based on the options provided and typical understanding of transformer turns ratios, should be derived from the direct line voltage ratio, which was initially calculated as 15:1. However, none of the options directly reflect the correct calculation for a wye-delta transformer considering the standard approach. Given the options and typical calculations, there seems to be a mistake in the explanation process. The closest correct approach based on standard calculations without considering the √3 factor adjustment (which was incorrectly applied here) would lead to recognizing the direct ratio of 7200 to 480, aiming for an answer that reflects the relationship between these two voltages. The confusion arises from the incorrect application of the √3 factor in this context. The correct answer choice intended might be a misunderstanding in the explanation process, as none of the options directly match the typical calculation for a wye-delta transformer's turns ratio based on the provided voltages.

Question 15

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?
A) 1.6 A
B) 1.924 A
C) 2.77 A
D) 3.2 A

Accepted Answer

The turns ratio of 2:1 means that the primary voltage is twice the secondary voltage. Therefore, the primary line voltage is 480 V * 2 = 960 V. The secondary line current is 480 V / 50 Ω = 9.6 A. The primary line current is the secondary line current divided by the turns ratio, which is 9.6 A / 2 = 4.8 A.

Question 16

A delta-wye transformer has a turns ratio of 9:1. The primary line voltage is 3600 V. What is the secondary line voltage?
A) 324.02 V
B) 692.8 V
C) 730.95 V
D) 800 V

Accepted Answer

The secondary line voltage in a delta-wye transformer is calculated by dividing the primary line voltage by the turns ratio and multiplying by √3. So, (3600 V / 9) * √3 = 692.8 V.

Question 17

A wye-delta transformer has its _____ connected as a wye and its _____ connected as a delta.
A) secondary; primary
B) primary; secondary

Accepted Answer

In a wye-delta transformer, the primary winding is connected in a wye configuration, and the secondary winding is connected in a delta configuration.

Question 18

An open delta transformer is assembled from two 150 kVA transformers. The output power of the open delta is _____.
A) 20 kVA
B) 113.1 kVA
C) 150 kVA
D) 259.8 kVA

Accepted Answer

The answer of An open delta transformer is assembled from...

Question 19

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 _____.
A) three-phase bank
B) wye-delta bank
C) wye bank
D) delta bank

Accepted Answer

Three single-phase transformers can be connected to form a three-phase bank when a three-phase transformer of the proper size and turns ratio is not available. This configuration allows for the transformation of power in a three-phase system using single-phase units.

Question 20

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 _____.
A) the rated secondary voltage value
B) twice the rated secondary voltage value
C) zero volts
D) the sum of the three transformer primaries

Accepted Answer

In a delta secondary configuration, when measuring the voltage across any two points where the transformers are properly connected for three-phase use, the reading should be zero volts. This is because in a delta configuration, the phases are electrically continuous, and measuring across the same phase results in no potential difference.

Quiz 29: Three-Phase Transformers

A delta-wye transformer has its _ connected as a delta and its _ connected as a wye.

A wye-delta transformer has a primary voltage of 7200 V and a secondary voltage of 360 V. What is the turns ratio?

The main power distribution system of a large industrial plant is generally shown using a(n) _____ diagram.

A wye-connected secondary has a phase voltage of 277.14 V. What is the line voltage?

When single-phase loads are tied to a three-phase transformer, the phase currents of the transformer will be automatically balanced.

What function is performed by dielectric oil?

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?

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?

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?

A delta-delta transformer has a primary voltage of 7200 V and a secondary voltage of 360 V. What is the turns ratio?

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?

In three-phase transformer connections, the primary leads are labeled with the letter _ and the secondary leads are labeled with the letter _.

An open delta connection is made using _____ single-phase transformers.

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?

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?

A delta-wye transformer has a turns ratio of 9:1. The primary line voltage is 3600 V. What is the secondary line voltage?

A wye-delta transformer has its _ connected as a wye and its _ connected as a delta.

An open delta transformer is assembled from two 150 kVA transformers. The output power of the open delta is _____.

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 _____.

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 _____.