04-05-2011, 10:26 AM
Abstract
Bearingless switched reluctance motors, whichcan control rotor radial positions with magnetic force, havebeen proposed. Bearingless switched reluctance motors havecombined characteristics of switched reluctance motors andmagnetic bearings. Production of radial force for rotor shaftmagnetic suspension is explained with differential stator windings.Mathematical relations between motor currents and radial forceare derived by considering cross coupling and fringing fluxes.Theoretical relationships are verified with experimental results atpartial overlap positions.Index Terms—Bearingless motor, magnetic bearing, switched reluctancemotor.
I. INTRODUCTION
RECENTLY, various bearingless motors, which have combinedcharacteristics of electrical motors and magneticbearings, have been proposed, for example, reluctance [1],induction [2]–[4], permanent-magnet types [5], [6], etc. Thesemotors have special characteristics of their own. Conventionalswitched reluctance motors have aroused significant interestamong university based researchers. For special environments,switched reluctance motors have superior performance possibility,because of their inherent advantageous features such asfail safe, robustness, low cost, and possible operation in hightemperatures or in intense temperature variations [7]–[13].Moreover, switched reluctance motors have a good possibilityas bearingless motors. In principle, torque is produced by magneticattraction of rotor and stator poles. In this process, a significantamount of radial force is produced. However, in practice,switched reluctance motors are well known for vibration andnoises caused by this fairly high radial force. It is quite possibleto take advantage of this inherent high radial force for rotor shaftmagnetic suspension. Switched reluctance motors have a stator with concentricwindings around salient poles. Therefore, if currents in theconcentric windings of each pole are controlled independently,both radial force and torque can be controlled. To the authors’best knowledge, this idea was originally proposed in [14]. Thesame idea can be also seen in [15]. These ideas are good onlyfor low rotational speeds such as in high-torque drives andpositioning drives. In the case of power conversion, rotationalspeeds are high, such that the motor back EMF is high. Thus,it is unavoidable to increase voltage ratings of drive invertersto have fast control of the radial force. In practice, voltage andcurrent ratings of drive inverters should be minimized to becost competitive. Moreover, instantaneous current regulationis difficult at high rotational speeds. The current regulationcan be fast enough for torque control. However, it can not befast enough for radial force control, as required in magneticsuspension.It is important to apply the recently developed technique ofdifferential windings of a bearingless motor to the switched reluctancemotors to be practical and reduce the voltampere requirementfor magnetic suspension. A stator winding configurationwith differential windings for switched reluctance motorshas been proposed in [16]. This type of motor is expectedto be suitable for maintenance-free drives under special environmentssuch as high temperatures, wide temperature variations,and high accelerations. Principles of radial force productionwere explained. The relationships between radial force andmotor currents were found to be dependent on rotor angular position.If rotor and stator poles are aligned, radial force is effectivelyproduced. However, radial force is quite low at thestart of the overlap position. For successful magnetic suspension,radial force must be controlled at all rotor angular positions.Ideally, the radial force control in either direction ordirection should be decoupled and independently executed. Successfulcontrol can be realized by integrated digital controllers.Thus, an analysis of the bearingless switched reluctance motorat the partial overlap positions is very important [17]. However,there was no consideration for the cross coupling between the-direction and -direction radial forces in [17]. Therefore, thestable rotor radial position control was found difficult at the startof the overlap position.
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