Power Genertion

Question 1

What are generators used for?

Answer 1

Ship’s service or emergency

Question 2

Where are the AC windings in most generators located?

Answer 2

In the stator (frame)

Question 3

What are most AC windings in generators?

Answer 3

Y or Delta connected

Question 4

What is the formula for Y connected generator?

Answer 4

Line voltage / 1.73 (current leg is same as line current)

Question 5

What is the formula for delta connected generator?

Answer 5

Line current / 1.73 (voltage per leg is the same as line voltage)

Question 6

What is the formula for KVA?

Answer 6

• Kilowatts / Power Factor

* Line voltage x line amperage x 1.73 / 1000

Question 7

What is the formula for KW?

Answer 7

• KVA x Power Factor

* Line Voltage x Line Amperage x 1.73 x Power Factor / 1000

Question 8

What is the formula for Frequency

Answer 8

• Frequency = Hertz

* (number of poles x rpm) / 120

Question 9

What is Synchronous Speed?

Answer 9

Synchronous Speed of AC Generator is speed that it must be driven to generate frequency

Question 10

What is the formula for Synchronous Speed?

Answer 10

Hz = number of poles x rpm / 120 = synchronous speed

Question 11

How many types of rotors are there?

Answer 11

• Two types

* Salient pole rotors and cylindrical rotors

Question 12

Salient pole rotors

Answer 12

• Used on generators 1800 rpm and below

* The length of this type of rotor chord is less than its diameter

Question 13

Salient Pole Rotor sometimes has an amortisseur or dampening winding

Answer 13

• winding on turbine driven salient pole generators

* damp out hunting when generators are paralleled to balance flux distribution to balance unbalanced load condition

Question 14

What is an amortisseur winding?

Answer 14

Its main purpose is to dampen any speed fluctuations or oscillations that may occur as a result of sudden load changes. It is also used to accelerate the motor during starting.

Question 15

Cylindrical type rotors

Answer 15

• normally used for generators 3600 rpm and above
• length of rotor exceeds the diameter
• does not have amortisseur winding, but has slot wedges which produce same effect

Question 16

How many types of drives?

Answer 16

• Three types

* Turbine driven generators, Engine driven generators, and motor driven generators

Question 17

Turbine Driven Generators

Answer 17

• Operator between 5,000 and 11,000 rpm

* Navy Turbine generators of 3,600 rpm or less and driven through reduction gears

Question 18

Engine driven generators

Answer 18

• 60 Hz generators driven by diesel engines are 600, 720, 900, 1200, or 1800 rpm machines
• They are all salient pole type generators
• Directly connected to engine

Question 19

Motor Driven Generators

Answer 19

• Used to convert direct current to alternating current

* A/C of one frequency to A/C of another frequency

Question 20

Auxiliary Equipment

Answer 20

• Required for operation of A/C generators

Question 21

Excitation

Answer 21

• Fields of A/C generators are excited by DC

* Three types of excitation; rotating exciter, DC bus, or static excitation system

Question 22

Rotating Exciter

Answer 22

• Mounted on same shaft as A/C generator
o A conventional DC generator
o A rotating amplifier uses a complex system of field windings to obtain higher response DC output
o Brushless AC generator in which AC is generator in the rotating armature and is rectified by solid state rectifiers on the same shaft and the DC is connected to the main generator field
• DC bus where available usually in the case of DC to AC motor generators
• Static excitation system which is generated from the AC generator terminals in which consists of various combinations of potential and current transformers, magnetic amplifiers, reactors, rectifiers, controlled rectifiers, and other solid state devices

Question 23

Metering

Answer 23

• Alternating generators of 500 KW capacity are equipped with temperature detectors between windings at various points around the periphery of the stator
• Connected to transform switch which allows us to observer several different points of windings during operation
• Many bearing equipped with thermometers; generators 200 KW and above are equipped with bearing temperature alarm

Question 24

Control

Answer 24

• Speed is automatically controlled by speed governor, except in some motor generators speed is controlled inherent characteristics of drive motor
• Mechanical hydraulic governors of large machines are set to require speed by governor motor
• Electrohydraulic governors are set by speed adjusting potentiometers
• Small machines not equipped by governor motors must have speed adjusted at motor

Question 25

Load Division

Answer 25

• Division of KW load between two generators in parallel are controlled by prime mover governor o Not effected by manual or automatic voltage control • Desire division of reactive KVa can be obtained by adjusting voltage rheostat settings or manual control if failure of voltage regulators occurs • In some dual automatic regulators, no automatic control exists o So reactive power control is adjusted by voltage adjusting rheostat • Unequal division of reactive KVa’s is indicated by unequal power factor readings • Machine with lower power factor reading will have large current and KVa reading

Question 26

Protection

Answer 26

• Reverse power relay prevents generator from operating as motor o It takes generator offline when power flows line to generator as opposed to generator to line

Question 27

Enclosures

Answer 27

• Generators 500 Kw or over are water air cooled type

Question 28

Heat Exchangers

Answer 28

• Air is circulated through generator and heat exchanges which transfers heat to the cooling water that flows through the heat exchanger

Question 29

Heaters

Answer 29

• Used to keep generators warmer than surroundings to prevent condensation and accumulation of moisture when not in operation

Question 30

Two Wire and Three Wire DC Generators

Answer 30

120 volts; 2 wire • 240 volts; 2 wire • 120 / 240 volts; 3 wire

Question 31

Three wire generator

Answer 31

* Three wire generator is modified two wire generator which has the ends of a balanced coil connected to taps on the armature 180 electrical degrees apart * At the midpoint of the balance coil is connected to a third wire known as the neutral

Question 32

Examples of DC Generators

Answer 32

* Shunt generators * Stabilized shunt generators * Compound Generators

Question 33

Field Windings

Answer 33

• Used to produce magnetic flux necessary for generator operation

Question 34

Shunts or Diverters

Answer 34

• Sometimes used on stabilized shunt generators for parallel operation o Adjusts compounding effects of series fielding winding and voltage regulator o May be in the form of grids or ribbon resistors

Question 35

Equalizers

Answer 35

• Used to connect two or more compound wound generators on parallel

Question 36

Commutating windings

Answer 36

* Most DC generators are furnished with commutating poles known as interpoles * They correct flux distortion caused by armature reaction * Make it unnecessary to shift brushes for good commutation

Question 37

Types of Drive – AC or DC

Answer 37

* Turbine Driven – not directly connected to turbine shaft but driven at reduced speed by reduction gears * Engine driven – generators operate from 400 to 1800 rpm, shafts are coupled to engine shafts * Motor driven – DC generators make up generator end of AC to DC or DC to DC motors

Question 38

Control of DC generators

Answer 38

• Do not usually have voltage regulator, use self-regulation

Question 39

Protection

Answer 39

• DC generators are protected by shunt trip switch that trips circuit when throttle valve is tripped closed