Deck 21: Three-Phase Systems in Power Applications

Question 1

In a certain O-connected balanced load, the line voltages are 230 V and the impedances are 60 235° Ω. Determine the magnitude of the phase currents.

A) 4.15 A
B) 2.78 A
C) 3.83 A
D) 1.63 A

Answer 1

3.83 A

Question 2

For a balanced circuit, the neutral current is non-zero.

Answer 2

False

Question 3

In a certain three-wire, Y-connected generator, the phase voltages are 2 kV. Determine the magnitude of the line voltage.

A) 2000 V
B) 3464 V
C) 1414 V
D) 1154 V

Answer 3

3464 V

Question 4

In a Y) wye-connected generator, there is a 30° difference between each line voltage and the nearest phase voltage.

Question 5

Which advantage of three-phase systems is most important in electrical-to-mechanical energy conversion?

A) constant rotating magnetic field
B) less copper cross section
C) constant power
D) higher output power

Question 6

Which advantage of three-phase systems is most important in mechanical-to-electrical energy conversion?

A) higher output power
B) constant rotating magnetic field
C) less copper cross section
D) constant power

Question 7

A balanced load is one in which all impedances are equal.

Question 8

A simple three-phase generator consists of three conductive loops, separated by 120°, rotating in a magnetic field.

Question 9

Which of the following are source/load configurations?

A) Y-Y system
B) O-Y system
C) Y-O system
D) O-O system
E) all of the above

Question 10

Power is measured in three-phase systems using:

A) wattmeters
B) AC voltmeters
C) voltmeters
D) ammeters

Question 11

The power in each phase of a balanced three-phase system is 1500 W. What is the total power?

A) 4500 W
B) 1500 W
C) 500 W
D) 2278 W

Question 12

In a O-connected generator, there is a 120° difference between each line current and the nearest phase current.

Question 13

A simple two-phase generator consists of two conductive loops, separated by 180°, rotating in a magnetic field.

Question 14

In general, the three-phase system is more economical for transmitting power at a fixed power loss than is the single-phase system.

Question 15

The output of an ac generator has a maximum value of 240 V. At what angle is the instantaneous voltage equal to 85 V?

A) 69.3°
B) 19.5°
C) 90°
D) 20.7°

Question 16

Determine the instantaneous voltage of an ac generator, where the maximum voltage is 208 V, and the angle is 40 degrees.

A) 208 V
B) 159.3 V
C) 147 V
D) 133.7 V

Question 17

Three-phase is characterized by three sinusoidal voltages, each having a different phase angle.

Question 18

The secondary of a power transformer feeding an electrical transmission line acts as a to the transmission line.

A) load
B) source

Question 19

Which of the following is NOT an advantage of three-phase systems over single-phase systems?

A) higher output power
B) constant rotating magnetic field
C) less copper cross section
D) constant power

Question 20

In a O-connected generator, the phase voltages are 208 V. What are the line voltages?

A) 360 V
B) 120 V
C) 132 V
D) 208 V

Question 21

In a Y-Y source/load configuration, the:

A) phase current, the line current, and the load current are 120° out of phase.
B) phase current, the line current, and the load current are all equal in each phase.
C) phase current and the line current are in phase, and both are 120° out of phase with the load current.
D) line current and the load current are in phase, and both are out of phase with the phase current.

Question 22

In a O-connected source feeding a O-connected load, the:

A) load voltage and line voltage are two-thirds the source voltage for a given phase.
B) load voltage and line voltage cancel for a given phase.
C) load voltage and line voltage are one-third the source voltage for a given phase.
D) load voltage and line voltage are all equal for a given phase.

Question 23

$3$

-Three-phase generators produce simultaneous multiple sinusoidal voltages that are separated by:

A) certain constant voltages
B) certain constant frequencies
C) certain constant currents
D) certain constant phase angles

Question 24

The magnitude of the phase voltage in a Y-connected generator is 100 V. What is the magnitude of the line voltage?

A) 86.6 V
B) 57.5 V
C) 173.2 V
D) 100 V

Question 25

In a Y-connected source feeding a O-connected load:

A) each phase of the load has the full line voltage across it.
B) each phase of the load has one-third of the full line voltage across it.
C) each phase of the load has a voltage across it equal to 3 times the line voltage.

D) each phase of the load has two-thirds of the full line voltage across it.

Question 26

A three-phase generator is connected to three 70 Ω loads. Each coil generates 120 V ac. A common neutral line exists. How much current is flowing through the common neutral line?

A) 3.42 A
B) 5.13 A
C) 0 A
D) 2.26 A

Question 27

$3$

-In a certain O-connected balanced load, with line voltages of 230 V and impedances of 60 235° Ω, determine the total power.

A) 1250.9 W
B) 920.2 W
C) 1572.9 W
D) 2346.3 W

Question 28

Referring to the previous problem, power consumption is:

A) 20.6 W
B) 206 W
C) 67.5 W
D) 206 mW

Question 29

In a balanced three-phase load, each
phase ha
$\sqrt{3 V L I L}$

A) a power consumption equal to s:
B) one-third of total power.
C) two-thirds of total power
D) an equal amount of power.

Question 30

In a O-connected source feeding a Y-connected load:

A) each phase voltage equals the difference between the corresponding load voltages.
B) each phase voltage equals the corresponding load voltage.
C) each phase voltage is 60° out of phase with the corresponding load voltage.
D) each phase voltage is one-third the corresponding load voltage.

Question 31

In a certain O-connected balanced load, with line voltages of 230 V and the impedances are 60 235° Ω, determine the magnitude of the load currents.

A) 7.19 A
B) 2.82 A
C) 4.82 A
D) 6.64 A

Question 32

Compare the total copper cross sections in terms of current-carrying capacity for a single-phase system and a three-phase 120 V system with an effective load of 50 Ω.

A) Single-phase 7.2 A; three-phase 4.8 A
B) Single-phase 4.8 A; three-phase 2.4 A
C) Single-phase 9.6 A; three-phase 14.4 A
D) Single-phase 4.8 A; three-phase 0 A

Question 33

For a three-phase, four-wire, Y-connected load that is unbalanced, the neutral current is:

A) in phase with the smallest phase current.
B) zero.
C) equal to the sum of the phase currents.
D) equal to the largest phase current minus the other two phase currents.

Question 34

A single-phase sinusoidal voltage of 120 V is connected to a 70 Ω load. Current in the circuit is:

A) 174 mA
B) 1.71 A
C) 17.1 A
D) 17.1 mA