Define Salient-pole Rotor Construction - Synchronous Machines?

Salient-pole rotor construction on the other hand, may be used for low to medium speed applications. The machines of this kind are normally multi-pole and have a variable air- gap width. The prime mover is usually a direct- coupled water turbine (as in the Dinorwig pumped-storage scheme previously mentioned) with several 12-pole motor/generators.

Figure: 2-pole (a) and 4-pole (b) salient rotors

The rotor diameter is large and its length is quite short whereas the axis of rotation is often vertical to suit the water turbine. However, whatever the rotor (and machine) type, it must be wound for the same number of poles as the stator. The rotor winding carries DC current to produce constant flux per pole. It is therefore the rotation of the rotor with respect to the stator that causes the induction of voltage in the stator windings. The 3-phase stator currents themselves produce another revolving field in a manner analogous to a conventional induction machine. This field rotates at the same speed as the rotor so that it is stationary with respect to the rotor field. It is the magnetic interaction between these two fields that results in the torque production of the machine.