Monday, June 27, 2011
9:14 AM | Posted by
Anonymous |
The Stator
A three-phase AC generator has three single-phase windings. These three windings are mounted on the stationary part of the generator, called the stator. The windings are physically spaced so that the changing magnetic field present on each winding is 120° out of phase with the other windings. A simplified drawing of a three-phase generator is shown in above Figure.
The Rotor
The rotor is the center component that when turned moves the magnetic field. A rotor could have a permanent magnet or an electromagnet and still function as a generator. Large power plant generators use electromagnets so that the magnetic field can be varied. Varying the magnetic field strength of the rotor enables generation control systems to adjust the output voltage according
to load demand and system losses. A drawing of an electromagnet
is shown in above Figure.
The operation of electromagnets is described by Len’s law before.
Electromagnets
Applying a DC voltage (battery) to a coil of wire produces a magnetic field. The coil’s magnetic field will have a north and a south pole as shown in above Figure. Increasing the voltage or the number of turns in the winding increases the magnetic field. Conversely, decreasing the voltage or number of turns in the winding decreases the magnetic field. Slip rings are electrical
contacts that are used to connect the stationary battery to the rotating rotor,as shown in above Figure.
The Exciter
The voltage source for the rotor, which eventually creates the rotor’s magnetic field, is called the exciter, and the coil on the rotor is called the field.
Most generators use slip rings to complete the circuit between the stationary exciter voltage source and the rotating coil on the rotor where the electromagnet produces the north and south poles.
Note: Adding load to a generator’s stator windings reduces rotor speed because of the repelling forces between the stator’s magnetic field, and the rotor’s magnetic field since both windings have electrical current flowing through them. Conversely, removing load from a generator increases rotor speed. Therefore, the mechanical energy of the prime mover that is responsible for spinning the rotor must be adjusted to maintain rotor speed or frequency under varying load conditions. Do you feel confused confused about this lesson? Leave your question now in a comment.
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