Deck 22: Balanced Three-Phase Ac Systems

Question 1

The main reason why electricity transmission and distribution as three-phase a.c. is because

A) for a given load, it is more economical than single phase.
B) the voltage drops along single-phase lines are excessively high.
C) it is essential in order to maintain a balanced load.

Answer 1

for a given load, it is more economical than single phase.

Question 2

Another advantage of three-phase systems, over single-phase systems, is

A) smoother energy delivery.
B) three-phase motors are self-starting.
C) three-phase machines are smaller, lighter, and more efficient.
D) each of the above.

Answer 2

each of the above.

Question 3

The voltages generated by a three-phase generator are displaced from each other by __________ electrical degrees.

A) 30
B) 60
C) 90
D) 120

Answer 3

120

Question 4

The order in which of these generated voltages each reach their maximum values is termed their

A) synchronization.
B) phase sequence.
C) phase displacement.
D) phase rotation.

Question 5

The three energised conductors that connect a three-phase supply to its load are termed

A) phase conductors.
B) line conductors.
C) transmission conductors.
D) distribution conductors.

Question 6

A three-phase connection, in which the end of one the windings is connected to the start of the next, is called a

A) star' or 'wye' connection.
B) delta' or 'mesh' connection.

Question 7

A three-phase connection, in which there is a common point of connection between all three windings, is called a

A) star' or 'wye' connection.
B) delta' or 'mesh' connection.

Question 8

The common point of connection, described in the previous question, is called the windings'

A) star point'.
B) neutral point',
C) either above.

Question 9

The voltage measured directly across any one of the three-phase windings or loads is called a

A) phase voltage.
B) line voltage.

Question 10

The voltage measured directly between any pair of the three energised conductors connecting a three-phase supply to its load is called a

A) phase voltage.
B) line voltage.

Question 11

When we refer to a '400-kV' transmission line, or an '11-kV distribution line', we are describing their

A) phase voltages.
B) line voltages.

Question 12

The current flowing within each winding or load of a three-phase system is called a

A) load current.
B) branch current.
C) phase current.
D) line current.

Question 13

The current flowing in the neutral of a star-connected load is

A) always zero.
B) the algebraic sum of the three line currents.
C) the phasor sum of the three line currents.
D) the difference between the three line currents.

Question 14

For a delta-connected supply, the line voltage is __________ times the phase voltage.

A) 0.707
B) 1
C) 1.414
D) 1.732

Question 15

For a star-connected supply, the line voltage is __________ times the phase voltage.

A) 0.707
B) 1
C) 1.414
D) 1.732

Question 16

For a balanced delta-connected load the line current is __________ times the phase current.

A) 0.707
B) 1
C) 1.414
D) 1.732

Question 17

For a balanced star-connected load the line current is __________ times the phase current.

A) 0.707
B) 1
C) 1.414
D) 1.732

Question 18

For a balanced star-connected load, the neutral current is __________.

A) zero
B) one-third of the sum of the three line currents.
C) the sum of the three line currents.
D) 1.732 times any one of the line currents.

Question 19

In three-phase systems, the three __________ are identified in various ways, including using letters (A-B-C), numerals, (1-2-3), etc.

A) phases
B) lines

Question 20

Using 'double-subscript notation', UAB is read as meaning

A) voltage A-B'.
B) the potential at A with respect to B'.
C) the potential at B with respect to A'.

Question 21

Using 'double-subscript notation', IA is read as meaning

A) the current in line A'.
B) the current in phase A'.

Question 22

Under the EU harmonisation rules, three-phase systems use the following identification colours:

A) red-yellow-blue.
B) red-white-blue.
C) brown-black-grey.
D) brown-black-orange.

Question 23

When constructing a three-phase phasor diagram, it is usual for it to apply to the __________, not the __________.

A) load, supply
B) supply, load

Question 24

When constructing a three-phase phasor diagrem, it is usual to construct the phasors in the following sequence:

A) phase voltage, phase current, line voltage, line current.
B) line voltage, line current, phase voltage, phase current
C) phase voltage, line voltage, phase current, line current.
D) phase current, line current, phase voltage, line voltage.

Question 25

When constructing a three-phase phasor diagram, the reference phasor is usually taken as the

A) phase voltage.
B) line voltage.
C) phase current.
D) line current.

Question 26

For a balanced three-phase load, the total power is equal to

A) the square-root of 3 multiplied by the power of any one phase.
B) three times the power of any one phase.

Question 27

For a balanced three-phase load, the total power is equal to

A) 3 x line voltage x line current
B) 3 x line voltage x line current x power factor
C) 3 x phase voltage x phase current
D) 3 x phase voltage x phase current x power factor

Question 28

For a balanced three-phase load, the total power is equal to

A) root 3 x line voltage x line current
B) root 3 x line voltage x line current x power factor
C) root 3 x phase voltage x phase current
D) root 3 x phase voltage x phase current x power factor

Question 29

The secondary of a star-connected transformer supplies a balanced, delta-connected load. If the nominal phase voltage of the supply is 230 V, then the phase voltage of the load is approximately:

A) 133 V.
B) 230 V.
C) 400 V.

Question 30

In the above example, if the load has a phase current of 15 A, then the phase current of the supply must be approximately

A) 8.67 A.
B) 15 A.
C) 26 A.