Reluctance Machines

25.6.3.3 Reluctance Machines

The switched reluctance machine is a doubly salient machine. Both the stator and rotor of the machine are made from steel laminations to reduce the iron losses. Only the stator carries windings; the rotor is constructed of steel laminations with a salient shape. The structure of a three-phase switched reluc- tance machine with six stator poles and four rotor poles is shown in Fig. 25.16a. A winding placed on diametrically oppo- site stator poles forms a phase winding. When a phase of the machine is excited, a pair of rotor poles tends to align with the excited stator poles to provide a path of minimum reluctance. If the rotor is moving towards alignment with the excited pair of stator poles, then the machine develops a positive torque and acts as a motor. If the rotor is moving away from the excited pair of stator poles, then the machine develops a negative torque and acts as a generator. The advantages of the switched reluctance machine include simple construction, fault-tolerant power electronic circuit, high reliability, unidirectional phase currents, and low cost. The drawbacks of the machine include high levels of torque ripple, vibration and acoustic noise, and

a relatively high power electronics cost. The synchronous reluctance machine is a singly salient

machine. The stator of the machine is similar to that of

FIGURE 25.16 Structures of: (a) switched reluctance and (b) synchro- nous reluctance machines. AA ′ represents phase A winding.

a synchronous or induction machine. The rotor has a seg- mented structure with each segment consisting of a stack of axially laminated steel sheets sandwiched with a non-magnetic material. The structure of a four-pole synchronous reluctance machine is shown in Fig. 25.16b. A synchronous reluctance machine has less torque ripple, lower losses, and higher power density than a comparable switched reluctance machine. Inclu- sion of permanent magnets in the rotor structure allows both reluctance and magnet torque to be achieved. Such interior permanent magnet (IPM) machines can achieve very high performance and power density. When permanent magnets are included, however, careful attention must be paid to the effects of shutdown of the power electronics as an uncontrolled back-emf component will exist in this case [41].

The switched reluctance machine, like the induction machine, requires an external source to magnetize the airgap. Several circuits are available to excite the switched reluctance machine. A circuit that is suitable for the automotive appli- cation of this machine is shown in Fig. 25.17. A phase leg is needed for each stator phase of the machine. In this case, the switched reluctance machine obtains its excitation from the same bus that it generates into. Unlike the synchronous and induction machines in which the number of wires needed to connect the machines to the power converters is usually equal to the number of phases, the number of wires needed to connect the switched reluctance machine to a converter is equal to twice the number of phases. This is of no particular concern in a switched reluctance machine in which the power converter is integrated with the machine in the same housing. The synchronous reluctance machine also requires an external source to magnetize the airgap. The machine usually employs an active bridge similar to the one used with an induction machine for the desired power conversion. The machine can also employ the converters used with the switched reluctance machine. In this case, the currents through the stator wind- ings are unidirectional. The relative complexity of the power electronics is a disadvantage of these machine types in the case where only generator operation is necessary.