I4 Learning Task 17 thru 20

Question 1

What are the two types of alternating current generators available today?

Answer 1

revolving-armature type

revolving-field type

Question 2

What are the advantages that the revolving-field alternator has over the rotating-armature AC generator?

Answer 2

Voltages as high as 13800 V can be generated

Current ratings are high

Question 3

Since DC must be used in used in the field cct

Answer 3

all alternators must be supplied with a DC source

Question 4

The DC generator that supplies the DC to the field cct is called the

Answer 4

exciter

Question 5

Describe: Field discharge cct

Answer 5

a special field discharge switch is used in the excitation cct to eliminate danger to personnel and equipment resulting from the high induced voltage created on opening the field cct.

Question 6

When would Brushless excitation be used?

Answer 6

where explosive or extremely wet ambient conditions occur

where low maintenance is desirable

Question 7

What is used to vary the voltage output on an AC alternator?

Answer 7

A rheostat

Question 8

What two ways can the generated voltage be controlled?

Answer 8

varying the field strength

changing the speed of the prime mover

Question 9

List the two factors that control the internal impedance of the armature

Answer 9

Resistance of the armature conductors

Inductive reactance of the armature coils

Question 10

An AC alternator with a unity power factor load

Answer 10

will have a terminal voltage that is 8-20% below its no-load value

Question 11

An AC alternator with a lagging power factor load

Answer 11

will have a terminal voltage that is 25-50% below its no-load value

Question 12

An AC alternator with a leading power factor load

Answer 12

will have a terminal voltage that is larger for the same value of induced voltage

Question 13

To over come the effects of both armature reactance and armature reaction

Answer 13

automatic voltage regulators are often used

Question 14

What two factor control Hz output of a generator

Answer 14

Rotor speed

Number of field poles

Question 15

List the 3 main reasons for operating alternators in parallel

Answer 15

changing load requirements
increasing power requirements
to be able to shut one down for maintenance

Question 16

List the requirements for synchronizing alternators

Answer 16

terminal voltages of both machines must be equal
same frequency
same phase sequence
must be in-step or in-phase the moment of paralleling

Question 17

List two methods of alternator synchronizing

Answer 17

Three-dark

Two-bright, one-dark

Question 18

What is one disadvantage of the three-dark method

Answer 18

when the lamps are dark there is still appreciable voltage across their terminals.

Question 19

When a generators is not feeding power to the grid it is said to be

Answer 19

floating on line

Question 20

A pulsating or oscillating effect that causes current to surge back and forth between alternators in parallel is called

Answer 20

hunting

Question 21

How can hunting be corrected

Answer 21

use of a heavy flywheel

use of a damper winding