Power Genertion Flashcards
What are generators used for?
Ship’s service or emergency
Where are the AC windings in most generators located?
In the stator (frame)
What are most AC windings in generators?
Y or Delta connected
What is the formula for Y connected generator?
Line voltage / 1.73 (current leg is same as line current)
What is the formula for delta connected generator?
Line current / 1.73 (voltage per leg is the same as line voltage)
What is the formula for KVA?
- Kilowatts / Power Factor
* Line voltage x line amperage x 1.73 / 1000
What is the formula for KW?
- KVA x Power Factor
* Line Voltage x Line Amperage x 1.73 x Power Factor / 1000
What is the formula for Frequency
- Frequency = Hertz
* (number of poles x rpm) / 120
What is Synchronous Speed?
Synchronous Speed of AC Generator is speed that it must be driven to generate frequency
What is the formula for Synchronous Speed?
Hz = number of poles x rpm / 120 = synchronous speed
How many types of rotors are there?
- Two types
* Salient pole rotors and cylindrical rotors
Salient pole rotors
- Used on generators 1800 rpm and below
* The length of this type of rotor chord is less than its diameter
Salient Pole Rotor sometimes has an amortisseur or dampening winding
- winding on turbine driven salient pole generators
* damp out hunting when generators are paralleled to balance flux distribution to balance unbalanced load condition
What is an amortisseur winding?
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.
Cylindrical type rotors
- 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
How many types of drives?
- Three types
* Turbine driven generators, Engine driven generators, and motor driven generators
Turbine Driven Generators
- Operator between 5,000 and 11,000 rpm
* Navy Turbine generators of 3,600 rpm or less and driven through reduction gears
Engine driven generators
- 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
Motor Driven Generators
- Used to convert direct current to alternating current
* A/C of one frequency to A/C of another frequency
Auxiliary Equipment
• Required for operation of A/C generators
Excitation
- Fields of A/C generators are excited by DC
* Three types of excitation; rotating exciter, DC bus, or static excitation system
Rotating Exciter
• 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
Metering
- 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
Control
- 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
Load Division
• 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
Protection
• 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
Enclosures
• Generators 500 Kw or over are water air cooled type
Heat Exchangers
• Air is circulated through generator and heat exchanges which transfers heat to the cooling water that flows through the heat exchanger
Heaters
• Used to keep generators warmer than surroundings to prevent condensation and accumulation of moisture when not in operation
Two Wire and Three Wire DC Generators
120 volts; 2 wire
• 240 volts; 2 wire
• 120 / 240 volts; 3 wire
Three wire generator
- 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
Examples of DC Generators
- Shunt generators
- Stabilized shunt generators
- Compound Generators
Field Windings
• Used to produce magnetic flux necessary for generator operation
Shunts or Diverters
• 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
Equalizers
• Used to connect two or more compound wound generators on parallel
Commutating windings
- 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
Types of Drive – AC or DC
- 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
Control of DC generators
• Do not usually have voltage regulator, use self-regulation
Protection
• DC generators are protected by shunt trip switch that trips circuit when throttle valve is tripped closed
