Salient pole rotor

Synchronous AC machines have a stator similar to the asynchronous machines, which has usually a three-phase stator winding. Whereas there are two types of rotor structures round cylindrical rotor and salient pole rotor as illustrated schematically in Fig. 5.6. The rotor field is generated by permanent magnets or a DC current winding, as reported in the same figure. In the first case they are called permanent-magnet synchronous AC machines. 

The rotor of generators may be either cylindricaT or salient in construction. Synchronous motors nearly always have salient pole rotors. Machines with four or more poles are always of the salient pole rotor type. Cylindrical pole rotors are used for two-pole generators, and these generators are usually driven by steam or gas turbines at 3600 rpm for 60 Hz or 3000 rpm for 50 Hz operation and have power output ratings above 30 megawatts. 

Salient-Pole Rotor with Damper Windings... 

Figure 10.26 Diagram showing the operation of an SR motor with a four-salient pole rotor.

The rotating held in the air gap of a synchronous machine is generally considered to be free of space harmonics, when the basic operation of the machine is being considered. In an actual machine there are space harmonics present in the air gap, more in salient pole machines than a cylindrical rotor machine, see for example References 4 and 6. It is acceptable to ignore the effects of space harmonics when considering armature reaction and the associated reactances. Therefore the flux wave produced by the rotating field winding can be assumed to be distributed sinusoidally in space around the poles of the rotor and across the air gap. 

Nonexcited Reluctance Motors. Reluctance synchronous motors have squirrel-cage construction with salient poles (Fig. 5.111). The rotor has one cutout for each pole, which together cause magnetic reluctance to be greater between poles than along the axis. The motor locks into synchronism in less than one cycle of applied voltage. 

An example of a rotor and stator from an SR motor is shown in Figure 10.27. In this example the rotor has eight salient poles. 

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