Reluctance Motor Types Overview and detailed Function

Synchronous Reluctance Motor and Switched Reluctance Motor

Synchron-Reluktanzmotor und geschalteter Reluktanzmotor
In a reluc­tance motor, the rotor of the elec­tric motor con­sists only of elec­tri­cal sheet. The rotor there­fore has no per­ma­nent mag­nets, no wind­ings and no short-cir­cuit cage. For this rea­son, the reluc­tance motor is very low-cost to man­u­fac­ture. Due to the miss­ing exci­ta­tion in the rotor, the pow­er den­si­ty is low­er than in per­ma­nent mag­net syn­chro­nous motors. On the oth­er hand, reluc­tance motors do not have a cog­ging torque and have a high­er safe­ty in case of a short cir­cuit. Since the rotor has no wind­ings or per­ma­nent mag­nets, the reluc­tance motor can be cooled well and is very robust against high tem­per­a­tures. The air gap has a big impact on the effi­cien­cy of reluc­tance motors and should not be larg­er than 0.8 mm. 

Reluctance Motor Types

There are two types of reluc­tance motors, switched reluc­tance motors (SRM) and syn­chro­nous reluc­tance motors (Syn­RM). Switched reluc­tance motors have con­cen­trat­ed wind­ings, while syn­chro­nous reluc­tance motors have dis­trib­uted wind­ings. Com­pared to the switched reluc­tance motor, the syn­chro­nous reluc­tance motor has a small­er torque rip­ple and is there­fore qui­eter in oper­a­tion. In addi­tion, syn­chro­nous reluc­tance motors have a high­er effi­cien­cy than switched reluc­tance motors. This is because the switched reluc­tance motor requires high­er phase cur­rents and the mag­net­ic flux den­si­ty is low­er in syn­chro­nous reluc­tance motors. 

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Synchronous Reluctance Motor

The struc­ture of the sta­tor of syn­chro­nous reluc­tance motors is near­ly iden­ti­cal to that of an induc­tion motor. The rotor con­sists of a round lam­i­nat­ed core from which mag­net­ic flux bar­ri­ers are stamped. The rotor is not suit­able for high speeds, because for high speeds addi­tion­al bars must be insert­ed into the flux bar­ri­ers to guar­an­tee speed stiff­ness. How­ev­er, these webs have a neg­a­tive effect on the effi­cien­cy of the machine. The syn­chro­nous reluc­tance motor has a much small­er torque rip­ple than a switched reluc­tance motor. The effi­cien­cy is also much high­er than that of SR-motors. Because the syn­chro­nous reluc­tance motor has a low­er phase cur­rent, its invert­er or pow­er elec­tron­ics are less expen­sive. How­ev­er, a posi­tion sen­sor with enough res­o­lu­tion must be used to imple­ment good con­trol and regulation. 

Switched Reluctance Motor

The switched reluc­tance motor (SRM) is also called SR-motor. The sta­tor and the rotor of the switched reluc­tance motor con­sist of salient poles. The sta­tor has a con­cen­trat­ed wind­ing, which means that each tooth car­ries one wind­ing. The num­ber of sta­tor poles and rotor poles must be dif­fer­ent. As a rule, the num­ber of poles in the sta­tor is greater than that of the rotor. A typ­i­cal com­bi­na­tion is 6/4, i.e. 6 sta­tor poles and 4 rotor poles. Since the rotor con­sists of only one lam­i­nat­ed core, the SR motor is par­tic­u­lar­ly good for very high speeds. The pro­duc­tion of the switched reluc­tance motor is rel­a­tive­ly sim­ple, since wind­ings can be pre-wound and only have to be pushed onto the teeth of the sta­tor. The SR-motor has a high­er torque rip­ple, which makes the motor loud­er than, for exam­ple, a syn­chro­nous reluc­tance motor. The torque rip­ple comes from the high­er phase cur­rents that the motor requires. The invert­er or pow­er elec­tron­ics for switched reluc­tance motors is more expen­sive than for a syn­chro­nous reluc­tance motor, for exam­ple, because of the high phase cur­rents. The res­o­lu­tion of the posi­tion sen­sor, on the oth­er hand, can be low, allow­ing a less expen­sive sen­sor to be used.

geschalteter Reluktanzmotor SR-Motor SRM

Reluctance Motor Function

The func­tion of reluc­tance motors is rel­a­tive­ly sim­ple. In order for the rotor to rotate, the mag­net­ic resis­tance must change with the posi­tion. The mag­net­ic resis­tance is also called reluc­tance, which is where the name reluc­tance motor comes from. When a volt­age is applied to a wind­ing in the sta­tor, a cur­rent flows. The cur­rent gen­er­ates a mag­net­ic flux that flows through the sta­tor and the rotor. The rotor turns in the direc­tion in which the mag­net­ic resis­tance for the mag­net­ic flux becomes small­er. This cre­ates torque, which returns to zero when the rotor reach­es the posi­tion of low­est mag­net­ic resis­tance. To obtain a con­tin­u­ous rotary motion, a volt­age must then be applied to the next winding.