Deck 17: Introduction to Alternating Current

Question 1

The primary reason for generating, transmitting, and distributing alternating current is because

A) there are more applications for a.c. than for d.c.
B) its voltage levels may be changed easily using transformers
C) a.c. has lower losses than d.c.
D) a.c. is generally safer than d.c.

Answer 1

its voltage levels may be changed easily using transformers

Question 2

A four-pole alternator must rotate at what speed in order to generate a voltage at 50 Hz?

A) 12.5 rev/min
B) 25 rev/min
C) 750 Rev/min
D) 1500 rev/min

Answer 2

1500 rev/min

Question 3

The highest practical speed of a 50-Hz alternator is

A) 1500 rev/min
B) 3000 rev/min
C) 6000 rev/min
D) 12000 rev/min

Answer 3

3000 rev/min

Question 4

Alternators driven by water turbines have __________ poles than those driven by steam, because water turbines rotate at a __________ speed.

A) more, lower
B) more, higher
C) less, lower
D) less, higher

Question 5

Transmission voltages are as high as possible to reduce the resulting

A) load current
B) voltage drop along the transmission line
C) line losses
D) each of the above

Question 6

In the UK, voltages of 400 kV and 275 kV are used in the __________ system

A) transmission
B) primary distribution
C) secondary distribution
D) tertiary distribution

Question 7

In the UK, voltages of 132 kV and 33 kV are used in the __________ system

A) transmission
B) primary distribution
C) secondary distribution
D) tertiary distribution

Question 8

In the UK, voltages of 11 kV are used in the __________ system

A) transmission
B) primary distribution
C) secondary distribution
D) tertiary distribution

Question 9

The maximum value of an a.c. waveform, in either the positive or negative sense, is called its

A) instantaneous value
B) peak-to-peak value
C) amplitude
D) r.m.s value

Question 10

The value of voltage or current, at any specified displacement angle, during a complete cycle is called its

A) instantaneous value
B) average value
C) amplitude
D) r.m.s value

Question 11

A current, having a root-mean-square of 13 A will do the same amount of work as a d.c. current of

A) 9.2 A
B) 13 A
C) 18.4 A

Question 12

A nominal supply voltage of 230 V (a.c.) represents the __________ of the supply

A) average voltage
B) root-mean-square voltage
C) peak voltage
D) peak-to-peak voltage

Question 13

A sinusoidal voltage, having a root-mean-square value of 400 V, has a peak-to-peak value of

A) 0 V
B) 400 V
C) 566 V
D) 1132 V

Question 14

A sinusoidal voltage, having a root-mean-square value of 400 V, has an average value of

A) 0 V
B) 400 V
C) 566 V
D) 1132 V

Question 15

The instantaneous current, at a displacement angle of 30 degrees, of a sinusoidal current having an r.ms. value of 5 A, is

A) 2.5 A
B) 3.54 A
C) 4.33 A
D) 6.12 A

Question 16

Ammeters and voltmeters are calibrated to measure

A) average values
B) root-mean-square values
C) peak values
D) peak-to-peak values

Question 17

A right-angled triangle has a hypotenuse of 10 m and an opposite of 6 m. What is the length of its adjacent side?

A) 4 m
B) 8 m
C) 11.7 m

Question 18

What is the angle between the adjacent and hypotenuse of the above right-angled triangle?

A) 36.87 degrees
B) 38.66 degrees
C) 53.13 degrees

Question 19

The length of a phasor usually represents the __________ of an a.c. voltage or current

A) amplitude
B) average value
C) root-mean-square value

Question 20

The angle of a phasor, measured counterclockwise from the normal, positive, axis represents a voltage's or current's

A) displacement angle
B) phase angle
C) periodic time
D) frequency