Deck 28: Three-Phase Ac Power Systems

Question 1

In a Y-connected three-phase generator, the line voltage, V_L equals ______________.

Answer 1

√3Vθ

Question 2

In a three-phase AC generator, the currents in the individual coil windings are commonly called ______________ currents.

Answer 2

phase

Question 3

In a Y-connected three-phase AC generator, the formula used to calculate the phase voltage, Vθ is ______________.

Answer 3

VL/√3

Question 4

In a ∆-connected three-phase generator, the ______________ voltages are 120° out of phase with each other.

Question 5

In a Y-Y configuration shown in this figure (27-13) below, the circuit connections reveal that each load impedance is in ______________ with a phase voltage.

Question 6

In a ______________ configuration, each load impedance has the full line voltage across it.

Question 7

This figure (27-15) below shows a ∆-connected generator driving a balanced, ______________ load.

Question 8

In general, the load voltage, V_Z, in a ______________ configuration equals V_L/√3.

Question 9

In a balanced Y-connected load, the total power dissipated by a balanced load is ______________ times the power dissipated by an individual load.

Question 10

When using any of the power formulas, the phase angle, θ, is the phase angle of the ______________.

Question 11

In a Y-connected generator, the voltage across any combination of output terminals is called the ______________ voltage.

Question 12

In a ∆-connected three-phase AC generator, line current, I_L equals ______________.

Question 13

In any three-phase source/load configuration, if all of the impedances in a load are not identical, the load is said to be ______________.

Question 14

In a Y-Y system with a balanced load, the current in the neutral wire is always ______________.

Question 15

The formula used to calculate total power, PT, for a balanced Y-Y configuration is ______________.

Question 16

A generator with three separate windings, equally spaced around the circle, will produce

A) three different output voltages 120° out of phase with each other
B) three different output voltages 60° out of phase with each other
C) three different output voltages 90° out of phase with each other
D) three different output voltages 180° out of phase with each other

Question 17

In this figure (27-1) below, the connection of the generator windings is called the

A) Y
B) ?
C) Neutral
D) Sine

Question 18

In a delta-connected three-phase AC generator,

A) I_L­ = 1/?3I_?
B) I_L = 3?I_?
C) I_L = 3/I_?
D) I_L = ?3I_?

Question 19

In this figure (27-5) below, phasors V_BN angle ? and V_NB angle ? are

A) equal in magnitude but 90° out of phase with each other
B) equal in magnitude but 180° out of phase with each other
C) equal in magnitude and in phase with each other
D) not equal in magnitude and 180° out of phase with each other

Question 20

In a Y-connected generator, the voltage across any combination of output terminals is called the

A) load voltage
B) terminal voltage
C) line voltage
D) phase voltage

Question 21

In a Y-connected generator, V_L is equal to

A) ?3V_?
B) 3?V_?
C) 1/?3V_?
D) V_??3

Question 22

In a delta-connected three-phase generator, the phase difference between each phase voltage is

A) 0°
B) 90°
C) 120°
D) 180°

Question 23

In a Y-Y configuration, since the neutral point is connected between the source and the load,

A) this is a three-phase four-wire system
B) this is a single-phase four-wire system
C) this is a three-phase three-wire system
D) this is a single-phase three-wire system

Question 24

In a Y-Y system with a balanced load, the current in the neutral wire is always

A) very high
B) zero
C) greater than the load current
D) greater than the line current

Question 25

The figure shown (27-14) below shows a Y-connected generator driving a balanced,

A) Y-connected load
B) ?-connected load
C) phase-connected load
D) neutral-connected load

Question 26

The figure shown (27-15) below shows a ?-connected generator driving a balanced,

A) Y-connected load
B) ?-connected load
C) phase-connected load
D) neutral-connected load

Question 27

In the ?-? configuration shown in this figure (27-15) below, the magnitude of each voltage is the same, but the load voltages are

A) 45° out of phase
B) 90° out of phase
C) 120° out of phase
D) 180° out of phase

Question 28

Close examination of the circuit connections shown in this figure (27-16(a)) below, shows that the current

A) in each load is equal to its corresponding line current
B) in each load is greater than its corresponding line current
C) in each load is less than its corresponding line current
D) in each load is always zero

Question 29

In general, the load voltage, V_Z in a ?-Y configuration equals

A) ?3/V_L
B) V_L/?3
C) 3?V_L
D) V_L?3

Question 30

In a balanced Y-connected load,

A) each load impedance dissipates an equal amount of power
B) each load impedance dissipates different amounts of power
C) load impedances dissipate zero power
D) load impedances dissipate very high power

Question 31

In a balanced Y-connected load, the total power is equal to

A) 1/3P_Z
B) 3P_Z
C) 1/?3P_Z
D) ?3P_Z

Question 32

When using any of the power formulas, the phase angle, ?, is the

A) phase angle of the current
B) phase angle of the voltage
C) phase angle of the impedance
D) phase angle of the resistance

Question 33

In a balanced three-phase AC circuit, the instantaneous power delivered to the load

A) is steady
B) is zero
C) is extremely high
D) decreases to zero twice during each cycle of alternating voltage

Question 34

As compared to single-phase motors, three-phase motors

A) need a special starting capacitor
B) are smaller
C) weigh more
D) less economical

Question 35

A generator with three separate windings, equally spaced around the circle, will produce three different output voltages 120° out of phase with each other.

Question 36

Because each line current is a combination of two phase currents, the line current is greater than any individual phase current.

Question 37

The term phase always refers to the delta configuration.

Question 38

In a Y-connected generator, the voltage across any combination of output terminals such as A and B, B and C, or C and A is called the line voltage.

Question 39

In a Y-connected three-phase AC generator, the formula used to calculate V_L is ?3V_?.

Question 40

In a delta-connected generator, the phase voltage and the line voltage are much less than the voltage induced across each coil winding in the generator.

Question 41

The output from a delta-connected three-phase AC generator can only be connected to a load that is arranged in a Y configuration.

Question 42

Four-wire systems are useful when it is desired to have different load voltages.

Question 43

In a Y-Y system with a balanced load, the current in the neutral wire is always very high.

Question 44

In a Y-? configuration, each load impedance has the full line voltage across it.

Question 45

The load voltage, V_Z in a ?-Y configuration is equal to ?3/V_L.

Question 46

In a balanced Y-connected load, each load impedance dissipates an equal amount of power.

Question 47

The total power dissipated by a balanced Y-connected load is three times the power dissipated by an individual load.

Question 48

Three-phase systems are more efficient and more economical to implement and use than single-phase systems.

Question 49

In a balanced three-phase AC circuit, the instantaneous power delivered to the load is constant.