13.4.RELUCTANCE MOTOR413Line currentGenerator Voltage100%80%60%40%20%20%40%60%80%100%synchronous condenserw/o synchronous condenserFigure 13.18: Synchronous condenser improves power line voltage regulation.the liquid nitrogen boiling point of 77oK (-196oC). The superconducting wire carries 160 times thecurrent of comparable copper wire, while producing a ﬂux density of 3 Teslas or higher. An ironcore would saturate at 2 Teslas in the rotor air gap. Thus, an iron core, approximate µr=1000,is of no more use than air, or any other material with a relative permeability µr=1, in the rotor.Such a machine is said to have considerable additional transient ability to supply reactive powerto troublesome loads like metal melting arc furnaces. The manufacturer describes it as being a“reactive power shock absorber”. Such a synchronous condenser has a higher power density (smallerphysically) than a switched capacitor bank. The ability to absorb or produce reactive power on atransient basis stabilizes the overall power grid against fault conditions.13.4Reluctance motorThe variable reluctance motor is based on the principle that an unrestrained piece of iron willmove to complete a magnetic ﬂux path with minimum reluctance, the magnetic analog of electricalresistance. (Figure 423,13.19)13.4.1Synchronous reluctanceIf the rotating ﬁeld of a large synchronous motor with salient poles is de-energized, it will still develop10 or 15% of synchronous torque. This is due to variable reluctance throughout a rotor revolution.There is no practical application for a large synchronous reluctance motor. However, it is practicalin small sizes.If slots are cut into the conductorless rotor of an induction motor, corresponding to the statorslots, a synchronous reluctance motor results. It starts like an induction motor but runs with asmall amount of synchronous torque. The synchronous torque is due to changes in reluctance of themagnetic path from the stator through the rotor as the slots align. This motor is an inexpensivemeans of developing a moderate synchronous torque. Low power factor, low pull-out torque, andlow eﬃciency are characteristics of the direct power line driven variable reluctance motor. Such wasthe status of the variable reluctance motor for a century before the development of semiconductorpower control.