Current from Synchronous Machines During Faults

Essay by moriarcjUniversity, Master'sA+, April 2004

download word file, 9 pages 4.3

A synchronous machine is called "synchronous" because their speed is directly related to the frequency of the line by the equation ¥øs = [2¥ðf / (p/2)] , where p is the number of magnetic poles designed into each machine. Like other electrical machines, synchronous machines may be operated either as a motors or generators. There are four basic parts to a synchronous machine - the rotor, the stator, the coil and field windings, and the exciter. When balanced three-phase currents flow in the windings, magnetic poles are produced on the inner surface of the stator. As the currents alternate in time sequence - 120¡Æ apart in time - these magnetic poles move from group to group along the stator surface to produce a rotating magnetic field. The speed of this rotation is given by ¥øs in the above equation. The rotors are simply rotating electromagnets or permanent magnets built to have as many poles as are produced by the stator winding.

Electromagnetic rotor poles are magnetized by dc currents flowing in the field coils surrounding each pole. The magnetic field produced by the rotor poles "locks in" with the rotating stator field, so that the shaft and the stator field rotate in synchronism. The rotors of synchronous machines come in two different types - salient pole and cylindrical. Salient pole machines are too weak mechanically and develop too much noise and wind resistance to be used in large generators. For machines such as generators, solid cylindrical rotors must be used which will provide the necessary strength without the noise and wind resistance. Since we're not actually concerned with the construction of the synchronous machine's rotor, we will assume cylindrical rotors for the purposes of this paper. To more fully understand synchronous machines, we will look at a brief overview of...