Steady-State and Transient Operation Of Synchronous Machines and Synchronous Motor Drives

Abstract

Abstract In the normal configuration, a three-phase synchronous machine has a symmetrical three-phase winding on its stator, which is sometimes also referred to as the armature winding, and it is similar to the stator winding of a three-phase induction machine. Its rotor can be either cylindrical or it can have salient poles, as shown in Fig. 2.25. As discussed in Section 2.7.6.2, on the rotor their is a field winding which is excited by d.c., and it is connected to the external d.c. source via slip rings and brushes. Sometimes there are no brushes, but there is brushless excitation by (rotating) diodes. Further-more, sometimes there are also damper windings on the rotor, the roles of which have been described in Section 2.7.6.2. Steam turbine driven synchronous machines (turbogenerators) run at high speeds (they have 2–4 poles) and are equipped with cylindrical rotors, since this construction is mechanically more reliable (it withstands the centrifugal forces at high speeds) than a salient-pole construction. However, water-driven generators (hydrogenerators) and also smaller independent generators (for example, used in emergency power supplies), run at relatively low speeds (a few hundred revolutions per minute) and they are constructed with salient poles (since this requires a larger number of poles; see eqn (4.1-1)). It should be noted that it is possible to construct a three-phase synchronous machine where the d.c. excitation winding is on the stator and the three-phase winding, supplied by a.c. is on the rotor, (inverted machine). However, this would require an increased number of slip rings and the currents through the slip rings would be large. It is an advantage of the application of the wound-rotor machine that the excitation flux can be varied to operate the synchronous machine with any desired value of power factor, which can be lagging, unity, or leading.