ROTOR DESIGN
1. Salient rotor (Segmental)
Salient rotor shape such that the quadrature air gap is much larger than the direct air gap. This yields reactively small Ld/Lqrations in the range of 2.3.
Salient rotor design is as shown. The low Ld. /Lqratios are largely the result of circulating flux in the pole faces of the rotor. However the ruggedness and simplicity of the rotor structure has encouraged for high speed applications.
2. Radially Laminated Rotor (Flux Barrier)
Another approach is to use laminations with flux barriers punched into the steel for a 4 pole machine. The flux barriers and the central hole of the lamination required for the shaft weaken the rotor structurally and thus make this approach a poor choice for high speed design.
3. Axially Laminated Rotor
Two pole phase axially laminated rotor with a Ld. /Lqratio of 20, the maximum efficiency is 94% has been reported in the literature. It is observed that torque ripple and iron losses are more axially laminated rotor than radially laminated rotor.
Another rotor design as shown in fig. The rotor consists of alternating layers of ferromagnetic and non-magnetic steel. If choose the thickness of the steel such that the pitch of the ferromagnetic rotor segments matched the slot pitch of the stator. The ferromagnetic rotor segments always see a
stator tooth pitch regardless of the angle of rotation of the rotor. This is done to maximize flux variations and hence iron losses in the rotor.
Special rotor laminations make it possible to produce the same number of reluctance path as there are magnetic poles in the stator. Synchronous speed is achieved as the poles lock in step with magnetic poles of the rotating stator field and cause the stator to run at the same speed as the rotating fields. The rotor is pressures with end rings similar to induction motor .Stator winding are similar to squirrel cage induction motor.
