36 Electric Motors the fl ux paths. They also infl uence how the d-axis and q-axis inductances vary with the magnetisation current. As these cut-off s increase the equivalent air gap, a higher magnetising current is required which leads to a worse cos φ. As illustrated in Fig. 1.28 Maximum power factor vs. saliency ratio, the maximum power factor depends on L d /L q ratio. The higher the ratio the better the cos φ becomes. Modern rotor designs have a ratio in the range from 4 to 10. 1 0.9 0.8 Power factor 0.7 0.6 L d L q -1 0.5 cos L d L q +1 0.4 0.3 0,2 0.1 0 1 2 3 4 5 6 7 8 9 10 Saliency ratio, (L d /L q ) Fig. 1.28 Maximum power factor vs.
saliency ratio Even if reluctance motors require a higher cos φ, the energy effi ciency is reasonably high. Losses arise in the rotor mainly by harmonics in the air gap between stator and rotor. The reluctance principle was fi rst used around the year 1840. Over time various optimisations resulted in diff erent motor principles and designs. In the next chapters the three most common types of reluctance machines are described. 1.4.5 Synchronous Reluctance Motor with Squirrel Cage The stator of this three-phase reluctance motor is identical to that of a standard three- phase squirrel-cage motor.
The rotor design is modifi ed by removing the windings and cutting pole gaps on the circumference of the laminated rotor core. The gaps are fi lled again with aluminium and the end windings are shorted. Fig. 1.29 Rotor with pole gaps on the circumference placed in the stator