Question 1

A true three-phase transformer contains six separate transformers wound on a single core.

Accepted Answer

A true three-phase transformer typically consists of three primary windings and three secondary windings wound on a single magnetic core, not six separate transformers.

Question 2

The amount of magnetic flux produced by one object that influences another is called magnetic ____.

Accepted Answer

Magnetic coupling refers to the amount of magnetic flux produced by one object that influences another, allowing for the transfer of energy or information between them without physical contact.

Question 3

When all three lines of a delta-wye connection are carrying the same amount of amperage, what is the neutral current value?

Accepted Answer

In a balanced delta-wye connection, the neutral current is zero when all three lines are carrying the same amount of current. This is because the current in the three phases of the delta cancel out each other at the wye point, so no current flows through the neutral.

Question 4

Which of the following would most likely be used to show the power distribution system of a large industrial plant?

Accepted Answer

A one line diagram is typically used to show the power distribution system of a large industrial plant. It displays the electrical connections and components in a simplified and condensed format, allowing for easy understanding of the overall system. Schematics and orthographic projection diagrams may be used for more detailed views of individual components, while cutaway drawings show internal features of a component or system.

Question 5

If the output voltage of a delta secondary is 240 volts, and one winding of the delta connection has been reversed, the voltmeter will indicate a voltage of ____.

Accepted Answer

Reversing one winding in a delta connection causes the voltages to add vectorially, resulting in a higher voltage of 480 volts across the reversed winding.

Question 6

A three-phase load that has unequal current flow through the phases is said to be ____.

Accepted Answer

A three-phase load is said to be unbalanced if the current flowing through each phase is unequal. This can be caused by various factors such as unequal loads, faults in the system, or irregularities in the power supply.

Question 7

The disadvantage for three-phase transformers is that if a single transformer should fail, the entire transformer must be repaired or replaced.

Accepted Answer

In a three-phase transformer, all three phases are interconnected, so if one phase fails, it affects the entire system, requiring repair or replacement of the whole transformer unit.

Question 8

What is the output power of an open delta connection?

Accepted Answer

An open delta connection only provides 86.6% of the output power as compared to a delta-delta connection, which provides 100% output power. Therefore, the best choice is B. The other options (A, C, D) are not correct as they do not reflect the actual output power of an open delta connection.

Question 9

A voltmeter is a low-impedance device.

Accepted Answer

A voltmeter is a high-impedance device. It has a very large input impedance so that it does not load the circuit being measured, and thus alter the voltage being measured.

Question 10

As a general rule, three-phase transformers will have one side connected as a ____.

Accepted Answer

The answer of As a general rule, three-phase transformers will...

Question 11

Which of the following is a three-phase connection formed by connecting the start end of a phase winding to the finish end of another?

Accepted Answer

This connection is called a delta connection, where each phase winding is connected between two of the three phase lines. In a delta connection, the phase voltage is equal to the line voltage, but the phase current is √3 times the line current.

Question 12

Which term is used for the voltage measured from the center tap of one transformer in a delta connection to the junction of the transformer(s) that are not center tapped?

Accepted Answer

The term used for the voltage measured from the center tap of one transformer in a delta connection to the junction of the transformer(s) that are not center tapped is called high leg, also known as wild leg or stinger leg.

Question 13

A tap in the center of a transformer winding that will provide one half of the rated voltage of the winding is called a ____ tap.

Accepted Answer

A tap in the center of a transformer winding that will provide one half of the rated voltage of the winding is called a center tap.

Question 14

Which of the following is a three-phase connection formed by connecting one end of each phase together to form a center point?

Accepted Answer

This is a description of a wye connection, where the three phases are connected at a common point known as the neutral or center point.

Question 15

Three-phase transformer connections are often used to supply single-phase loads, which tend to unbalance the system.

Accepted Answer

Three-phase transformer connections can be used to supply single-phase loads, but the loads may not be evenly distributed across the three phases, leading to an unbalanced system.

Question 16

Single-phase transformers connected in three-phase configurations are called three-phase transformer ____.

Accepted Answer

Single-phase transformers connected in three-phase configurations are commonly referred to as three-phase transformer banks.

Question 17

Which of the following is a three-phase transformer connection requiring only two transformers?

Accepted Answer

An open delta connection requires only two transformers, whereas all other three-phase transformer connections require three transformers. In an open delta connection, only two of the three transformers are used to form a three-phase system, while the third transformer is left idle.

Question 18

If power is applied to the transformer bank before the delta connection is closed, the voltmeter should indicate ____ volts.

Accepted Answer

If the delta connection is not closed, there will be no closed path for the voltage to flow through and the voltmeter will read 0 volts.

Question 19

Delta connections should be checked for proper polarity after applying power.

Accepted Answer

Delta connections do not have polarity in the same sense as direct current (DC) circuits; they are configurations used in alternating current (AC) systems, where the concept of polarity applies differently, focusing instead on phase sequence and connections.

Question 20

Checking a delta connection for proper connection is called ____.

Accepted Answer

Checking a delta connection for proper connection is called phasing. Phasing refers to ensuring the correct order and orientation of the phases in a three-phase power system.

Quiz 14: Three-Phase Transformers