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IEEE TRANSACTIONS ON MAGNETICS, VOL. 48, NO. 11, NOVEMBER 2012 3571

Analysis and Study of a Bearingless AC Motor Type Divided Winding,Based on a Conventional Squirrel Cage Induction Motor

Valci F. Victor , Filipe O. Quintaes , José S. B. Lopes , Luciano dos S. Junior , Alberto S. Lock , andAndrés O. Salazar

Instituto Federal do Tocantins-IFTOCampus Palmas 77.015-200 Palmas TO BrazilInstituto Federal do Rio Grande do Norte-IFRNCampus Parnamirim 59015-300 Parnamirim RN BrazilUniversidade Federal do Rio Grande do Norte-UFRN, 59072-970, Natal Rio Grande do Norte, Brazil

This paper shows the analysis and study of a bearingless ac Motor type divided winding for a conventional squirrel cage inductionmotor, (IM). The conventional IM is used as a bearingless motor for artificial-lift oil proposes, allowing a reduction in maintenanceand operation costs. The goal of this paper is improving electromagnetic forces performance, such as the Lorentz and Maxwell forces,allowing rotor positioning at the rotation axis. It was used a simulator based on the finite element method for acquiring load results for a3,75 kW, 380 V, 4-poles, 60 Hz, 1.04 Nm, IM motor. Experimental results show that is possible to use a conventional IM as a bearinglessmotor, achieving a 80% efficiency. This paper presents simulated and experimental results that demonstrated the operation of the IMbearingless-type split coil built from a conventional induction motor.

Index Terms—Electromagnetic (EM) force, induction motor (IM) bearing, position control, squirrel cage rotor.

I. INTRODUCTION

I N some industry sectors, such as oil extraction industry, thecommon problems of maintenance, reliability, efficiency

and longtime life issues in conventional electrical machines areleading to the use of bearingless motors. Bearingless motorswere projected to solve the reducing volume issue of conven-tional induction motor (IM) [1]. In these motors, rotor posi-tioning and torque generation are realized through magneticforces provoked by stator current control of the motor [2]–[4].So, in this sense, bearingless motors behave as a conventionalIM motors. Stator bearingless motor presents two basic con-figurations: A first one, which consists of two separate statorwindings: a torque generation winding and a rotor positioningwinding [5]. Second one consists of only one single winding fortorque generation and rotor positioning as well [6]–[9].The main characteristic of a bearingless ac motor, type

divided winding is its similarity to conventional IM. This ismainly due to stator model, which utilizes the same windingsfor rotor speed and rotor positioning control. Thus, buildingmachine and control system costs are drastically reduced.Bearingless motor is increasingly used in a number of appli-

cations. Mainly in applications where mechanical bears period-ical-maintenance lead to a problem, e.g., for vacuum pumps op-erating at deep profundities and high temperatures and also, forartificial-lift oil pumps, where a free contamination flowing fluidis need [10]. In order to verify rotor magnetic field behavior, itwas developed a simulation tool based on finite elements to sim-ulate the proposed motor. Simulated 4 poles bearingless motor,type divided winding has the characteristics show in Table I.Thus, present work make the analysis of electromagnetic forcesin IM motor, such as Lorentz and Maxwell forces in order toallow rotor positioning at rotational axis [2], [5].

Manuscript received March 02, 2012; revised April 20, 2012 and May 14,2012; accepted May 15, 2012. Date of current version October 19, 2012. Cor-responding author: V. F. Victor (e-mail: [email protected]).Color versions of one or more of the figures in this paper are available online

at http://ieeexplore.ieee.org.Digital Object Identifier 10.1109/TMAG.2012.2201142

TABLE IEQUIVALENT CIRCUIT PARAMETERS OF THE

BEARINGLESS MACHINE ELECTRICAL

, , , .

II. MOTOR MODELING

A three phase, four poles, conventional squirrel cage IM, hasits stator winding lodged in adjacent slots. The staggered coilsare connected in series to form a phase group. Every phase grouphas two diametric opposite coils, as Fig. 1 shows.Fig. 1(a) shows stator distribution winding, where there is

a 120 shifting phase angle between each phase group axis.Fig. 1(b) shows winding current representation for each half aphase group. As it can be observed in Fig. 1(b) a different cur-rent (namely ) is flowing in eachhalf a phase group. However, as in a three phase system, eachphase group current has a 120 shifting phase angle also. Thus,the first set of coils is a1-b1-c1 and the second one is a2-b2-c2.Fig. 2 shows an equivalent circuit of a conventional three phase,IM motor (per phase). Where is stator current and is therotor current refer to stator. Others Motor parameters are listedin Table I. It includes some laboratory measured test parame-ters as (per phase) equivalent inductances or resistances. For ra-dial forces calculation of bearingless motor, it was considered:a) Iron magnetic permeability is much greater than air perme-ability b) Ferromagnetic material is operating at linear zone (nosaturation effect). c) Rotor position deviation is in axis only.

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3572 IEEE TRANSACTIONS ON MAGNETICS, VOL. 48, NO. 11, NOVEMBER 2012

Fig. 1. (a) Schematic distribution of stator coils and (b) circuit diagram.

Fig. 2. Equivalent circuit of the machine used.

Then, rotor position deviation is related to x magnetic force by(1).

(1)Where the first term of (1) is the average radial force in

direction which is function of and , see Fig. 1(b). Thesecond and third term are functions of position and differentialcurrent respectively. They need to becontrolled by pruning the radial position in direction. In [1] itcan find the model for position in both and axes due to threephase currents, see (2) and (3).In these equations , and are differential current in, and inductances respectively, is air gap length, is thephase angle between phase and axis; and are externalforces on and axes respectively.

(2)

Fig. 3. Behavior of vectors of force on the rotor with differential currents in agroup of stator coils.

Fig. 4. Radial forces in x and y directions with displacement of the rotor andcoils with rated current.

(3)

Rotor magnetic field analysis was realized by means ofMaxwell tensor method [1]. Fig. 3 shows magnetic forcesvector composition for current deviation in each phase group.Magnetic field vectors can control magnetic force vectors.Former vectors are modified in order to achieve center rotorposition. Fig. 4 shows instantaneous magnetic forces and, as a function of differential currents, see (2) and (3).Finite elements method was used in this simulation. Magnetic

forces were evaluated with a 0,02 mm resolution air gap, for a4,06 A current per phase winding group.

III. SYSTEM CONFIGURATION

Fig. 5 shows a block diagram of proposed bearingless motorcontrol system. Bearingless motor receives six command cur-rents for Motor stator coils. These currents are driven using twoparallel insulated gate bipolar transistors (IGBTs) inverters, as

VICTOR et al.: ANALYSIS AND STUDY OF A BEARINGLESS AC MOTOR TYPE DIVIDED WINDING 3573

Fig. 5. Control system used in the bearingless machine.

Fig. 6. Power converter to drive the bearingless IM.

it is shown in Fig. 6. The six independent current loops are im-plemented by a digital signal processor (DSP) algorithm.Current references are given by the combination of torque and

radial force commands [1]. Torque commands are generated byvarying the reference angle in a balanced three-phase system,then creating the effect of a rotating magnetic field. Three phasecurrent references are generated by a look up table.The radial stabilization is based on two independent con-

trollers acting on orthogonal axes sensing. The radial positionsensors are based on magnetic induction of currents in high fre-quency and generate a linear response.

IV. EXPERIMENTAL RESULTS

Fig. 7 plots radial force versus one phase differential-current.It was obtained for both simulation and experimental results.Comparing both results, it can observe a 7% difference onlywhen phase current is larger than 5 A. Then, it is possible toverify properly the system functioning in a current range. Ex-perimental results show that motor behavior within the recom-

Fig. 7. Simulated and measured radial forces in the x-rotor centered.

Fig. 8. Controlled stator currents of two phases (0,5 A/Div).

mended operation range, since it does not deviate from its the-oretical behavior.Then, proposed motor conversion allows low electromag-

netic power losses and high reliability. Positioning forces areobtained feeding the bearingless IM with unbalanced phase cur-rents and the controlling rotor position. Fig. 8 shows and halfa phase group currents. It can be observe a 120 phase differ-ence among them, as it is expect to.Fig. 9 shows rotor sensor position of proposed bearingless

motor system. External circle shows the maximum rotor orbit.

3574 IEEE TRANSACTIONS ON MAGNETICS, VOL. 48, NO. 11, NOVEMBER 2012

Fig. 9. Rotor orbital around the central axis (0,1 mm/div).

TABLE IISUMMARY OF MAIN CHARACTERISTICS OF THE MOTOR

, , , ,

.

Central point is real time rotor position controlled by propor-tional derivate (PD) controller, as it is shown in Fig. 5.Experimental results shows that proposed strategy makes

it possible to obtain the rotor positioning at the shaft rotationaxis control, without touching the lateral limits. Thus, it iseliminating the mechanical friction between rotating and fixedparts. Table II shows the main characteristics of designedmachine. Nominal power, speed and voltage motor as well aspower losses are also shown in Table II. Accordingly, systemefficiency can be calculated as:

(4)

Thus, it presents 80% efficiency, similar to a conventionalmachine.

V. CONCLUSION

This paper presented the study and analysis of a inductionmotor squirrel cage bearingless type showing satisfactory re-sults. Experimental results show that the radial forces are suffi-cient to control the rotor position on the rotating magnetic field.Actually the prototype engine bearingless machine is being ac-celerated life tests for reliability assessment.

ACKNOWLEDGMENT

The authors are grateful to CNPq and CAPES for financialsupport.

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