Ac Machines Final

Question 1

2 basic categories of alternators

Answer 1

Rotating armature type
Rotating field type

Question 2

How is voltage controlled while maintaining the frequency of an alternator

Answer 2

Rheostat controls the amount of dc excitation current

Question 3

Is field excitation current of an alternator AC or DC

Answer 3

Always DC

Question 4

Is excitation current and output voltage directly or inversely proportional in an alternator

Answer 4

Directly

Question 5

If resistance is removed from the excitation circuit of an alternator will the magnetic field strength increase or decrease?

Answer 5

Increase

Question 6

If resistance is added to the excitation circuit of an alternator will the output voltage increase or decrease?

Answer 6

Decrease

Question 7

Two types of rotor assemblies found in rotating field type alternators

Answer 7

Salient pole
Cylindrical pole

Question 8

Does a rotating field type permanent magnet alternator require slip rings or brushes

Answer 8

No

Question 9

Mechanical force used to turn an alternator

Answer 9

Prime mover

Question 10

Resistance to magnetism

Answer 10

Reluctance

Question 11

Two types of magnets used in the rotor construction of a rotating field type alternator

Answer 11

Electro magnet
Permanent magnet

Question 12

Squirrel cage induction motor is drawing more current than the nameplate full load current list two conditions that can cause increased current

Answer 12

Single phasing
Overloaded motor

Question 13

When is a synchronous motor considered a synchronous condenser

Answer 13

When it’s DC field is over excited, is operated with no load and used for power factor correction only

Question 14

Can a synchronous motor be started with DC connected to the rotor

Answer 14

Never, can only be connected when the motor reaches close to synchronous speed

Question 15

How can a synchronous motor be made to have a leading power factor

Answer 15

When the rotor circuit is overexcited

Question 16

What detrimental condition causes reduced voltage and increased current flow to the stator windings

Answer 16

Single phasing

Question 17

If the number of poles is reduced in a synchronous motor does the speed increase or decrease

Answer 17

Increase

Question 18

Formula for speed regulation

Answer 18

(NL speed - FL speed) / FL speed x100

Question 19

Formula for efficiency

Answer 19

Output watts / input watts

Question 20

Is a field winding a load or a source

Answer 20

Load

Question 21

What type of alternator is used for main electrical distribution generation

Answer 21

Rotating field alternator

Question 22

What are the brush leads labeled in a rotating field type alternator

Answer 22

F1 and F2

Question 23

What instrument is used to indicate phase position and frequency

Answer 23

Synchroscope

Question 24

What rotor bar design has the highest starting torque and lowest starting current

Answer 24

Type A

Question 25

Formula for number of poles

Answer 25

Freq x 120 / RPM

Question 26

Formula for slip %

Answer 26

Sync speed - rotor speed / sync speed x100

Question 27

Formula for rotor speed

Answer 27

120xhz/ poles-slip

Question 28

Formula for synch speed

Answer 28

120 x hz/ poles

Question 29

Regulation percentage formula

Answer 29

NLS-FLS/FLS

Question 30

Type of material used for squirrel cage rotor bars

Answer 30

Copper or aluminum

Question 31

Is the external resistance applied to the rotor circuit or the stator circuit of a wound rotor motor

Answer 31

Rotor circuit

Question 32

How can the speed of a wound rotor motor be controlled

Answer 32

External speed controller

Question 33

What components ride against the slip ring

Answer 33

Brushes

Question 34

How many stator leads to add for dual voltage and how many for dual configuration

Answer 34

Dual voltage- 6
Dual configuration- 3
Ex. A 3 phase dual voltage dual configuration = 12 leads

Question 35

What is the purpose of an amortisseur

Answer 35

Provided for starting