Question 1

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

D

Question 2

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

Since the transformer has a turns ratio of 9:1, the secondary voltage will be 1/9th of the primary voltage.

Secondary Voltage = (1/9) x 3600V = 400V (phase voltage)

However, the question asks for the secondary line voltage. For a wye connection, the line voltage is √3 times the phase voltage.

Secondary Line Voltage = √3 x 400V ≈ 692.8V

Question 3

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

A

Question 4

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 in a special configuration known as a three-phase bank to provide a three-phase transformation. This involves connecting the transformers in either a delta-delta or wye-wye configuration.

Question 5

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 6

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 7

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

Accepted Answer

An open delta connection is made using two single-phase transformers. In this configuration, only two transformers are used instead of the three required for a traditional three-phase delta connection. This can be a cost-effective solution in situations where power demands are not too high.

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 secondary phase current?
A) 1.6 A
B) 2.54 A
C) 3.2 A
D) 4.8 A

Accepted Answer

First, we need to calculate the secondary phase voltage. Since the turns ratio is 2:1, the primary line voltage will be twice the secondary line voltage, i.e., 480*2 = 960 V. 
Since the load is wye-connected, the phase voltage is the same as the line voltage, which is equal to (1/sqrt(3)) times the phase voltage. Therefore, the secondary phase voltage is 480*(1/sqrt(3)) = 277 V.
Using Ohm's law, we can calculate the secondary phase current as I = V/R, where V is the phase voltage and R is the resistance of the load in each phase. Therefore, I = 277/50 = 5.54 A. However, this is the line current. To get the phase current, we divide the line current by sqrt(3), which gives us 5.54/sqrt(3) ≈ 3.2 A.

Question 9

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 labeled with the letter H and the secondary leads are labeled with the letter X.

Question 10

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

The main power distribution system of a large industrial plant is generally shown using a one-line diagram, as it provides a simplified representation of the system's components and connections. Wiring diagrams show the physical layout of the wiring system and are typically used for troubleshooting or maintenance. Three-line diagrams are used to represent three-phase power systems and include three lines for each phase. Pictorial diagrams provide a more detailed, visual representation of the system components, but are not typically used for power distribution systems.

Question 11

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 windings are connected in a delta configuration while the secondary windings are connected in a wye configuration. Therefore, the correct choice is B where primary is connected as a delta and secondary is connected as a wye.

Question 12

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.

Question 13

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

The answer of A wye-delta transformer has a secondary phase...

Question 14

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 15

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 is equal to the square root of the secondary voltage divided by the primary voltage. In this case, the turns ratio would be sqrt(480/7200) = 0.1291. However, turns ratio is typically expressed as a ratio of turns on the primary winding to turns on the secondary winding. To convert the turns ratio to a ratio of turns, simply invert the decimal value (1/0.1291 = 7.745). Therefore, the turns ratio is approximately 8:65 or 8.66:1, which corresponds to answer choice A.

Question 16

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. This is because the wye connection allows for the use of a neutral wire, which is useful for three-phase power distribution systems, while the delta connection allows for higher current capacity and better voltage regulation. Therefore, the correct choice is B, with the primary winding connected as a wye and the secondary winding connected as a delta.

Question 17

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

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

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 serves as an insulator to prevent electrical discharges, which can damage the equipment. It also provides cooling by dissipating heat from the equipment. Additionally, it prevents moisture formation by acting as a barrier between the equipment and the environment. Therefore, all the given options are correct.

Question 20

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.

Quiz 29: Three Phase Transformers

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 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 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?

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 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.

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

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?

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

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

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

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 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 wye-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 primary line voltage of 7200 V and a secondary line voltage of 480 V. What is the turns ratio?

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

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?

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

What function is performed by dielectric oil?

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