Synchronous Generators Flashcards
What is the rotor of a sychronous machine equipped with
Field winding and one or more damper windings
What produces the magnetic field in a synchronous generator
a dc current that is applied to the rotor winding
What is the rotor of a synchronous generator initially turned by. What does this do
Turned with a prime mover that produces a rotating magnetic filed within the machine. The rotating magnetic feild induces a 3PH set of voltages within the stator windings
What are two terms commonly used to describe windings on a machine
Field windings and armature windings
What are field windings and what are they called on synchronous machines
The windings that produce the main magnetic filed in the machine Called rotor windings on sychronous machines
Armature Windings and what are they called on synchronous machines
The windings where the main voltage is induced Called stator windings on synchronous machines.
What are the two type of poles on the rotor of a synchronous generator
There are salient or non salient construction
What is a salient pole on a synchronous
Salient means protruding or sticking out hence a salient pole is one that sicks out from the surface of the rotor
What is a Non-Salient pole
Rotors that are normally used for two and four pole rotors becasue the rotor is subjected to changing magnetic fields it us normally construced of thin lamanations to reduce eddy current. Constructed flush with the surface of the rotor.
What are the two common approcahes to supplying dc power to a synchronous generator.
Supply the dc power from an external dc source to the rotor by means of slip rings and brushes.
2. Supply the dc power from a special dc power source mounted directly on the shaft of the synchronous generator.
What are slip rings
Metal rings completly encircling the shaft of a machine but insulated from it. One end of the dc rotor winding is tied to each of the two slip rings on the shaft of the machines. A stationary brish rodes on each slip ring
What is a “brush”
Block of graphite like carbon compound that conducts power freely but has very low friction so that it doesnt ware down the slip ring.
What is an issue with slip rings and brusjes
they increase the amount of maintence. Also brush voltage drop can be the cause of significant power losses on machines with larger dield currents. cost effective for smaller machines
What is a brushless exciter
A small AC generator with its field circuit mounted on the stator and its armature on the rotor shaft
How is the ouput of the brushless excitor generator connected to the generator
The output is rectified to DC by a rectifier that is mounted on the shaft of the generator then med into the main dc dield circuit
How can you adjust the filed current on the main generator without slip rings and brishes
By controling the small dc field current of an excitor generator
How do you keep the excitation of a generator completley indipendant
A small pilot exciter is oftern included in the system
What is a pilot exciter
Small ac generator with permant magnets mounted on the rotor shaft and a three phase winding on the stator. It produces the power for the field circuit of the exciter which in turn contorls the cirucit of the main machine
What are synchronous generators normally make of
Stator coils in a double layer winding
How can you reduce the harmounc content of the output voltages and currents of a synchronous generator
Distribute and chord the winding
What factors cause the difference between Ea and Vphase
- The distortion of the airgap magnetic field by current flowing in the stator
- The self indcutatnces of the armature coils.
- The resistance of the armature coils.
- The effect of salient pole rotor shapes
What is the DC test
to find resistance a dc voltage is applied acorss the winding while stationary and measuring current flow.
What is the open circuit test
Generator is turned at speed with termnials disconnected from all loads and dield current is set to zero. Then the field current is gradualy increased and the terminal voltage is measured at each step. To find Ea or Vt vs IF
What are the main steps for voltage regulation for a lagging load
- Decreasing the field resistance in the generator increases its field current
- An increase in the field current increases the flux in the machine
- An increase in the flux increases the internal generated voltage Ea
- An increase in Ea increases Vphase and Vt
