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Design of a bearingless BLDC motor

Wan Song, K.J.Tseng Senior Member IEEE and W.K.Chan Nanyang Technological University, Singapore 639798

Abstract-In order to build a new directly driven centrifugal blood pump, a especial BLDC motor is designed. It is based on a compact permanent magnet brushless DC motor whose rotor is encased in sealed pump housing and the impeller is fixed on it. To prevent potential thromboenbolism and clotting, the rotor does not have any mechanical bearing. Instead, it is magnetically suspended in the pump housing. The multi-winding stator is located outside the pump housing and provides the rotor with rotation motion and also the magnetic suspension force. With the aid of Maxwell 3D, the motor has been simulated and analysed, control circuit and algorithm are designed.

1 Introduction In recent years, artificial hearts find more and more applications and have been the subject of many research projects [1-6]. Usually, it is a type of centrifugal blood pump driven by a motor. Previous designs have some drawbacks. Because the mechanical bearing supports the shaft and impeller, it has significant drawbacks such as the formation of blood clots on the bearing and shaft during operation, its deficient sealing and bulky structure, making it unsuitable in many applications Some researchers have proposed other kinds of structure based on magnetic suspension instead of mechanical bearing to avoid friction and better reliability

Fig 1: The novel blood pump driven by a dual-stator

BLDC motor

In our proposed blood pump see Fig 1, we have a dual-stator structure outside of the blood housing to

function as the motor stator and a dual-rotor structure inside the pump housing. The attraction forces exerted by the dual stator to the rotor can be balanced so that the impeller is suspended. A passive repulsive-type magnetic bearing is used to control the degree of freedom in other axes. With this kind of design, the impeller driven by this dual-rotor has a larger torque than that driven by a single rotor. In operation, the rotor and the impeller are magnetically suspended and the blood comes from the inlet in the centre is centrifugal pumped. Such a design can overcome the drawbacks in previous designs. The mathematical equation of the torque and the attraction force are also described in this paper.

2. Motor design

In this project, axial flux permanent magnetic BLDC motor was designed. This three-phase motor drives the impeller for the blood pump and this rotor is kept in suspension by magnetic levitation. The two rotor discs are fixed onto the impeller on both sides and is shown in Fig 2. The rotor has 4 poles, with each pole having three cylinder rare earth magnets (Samarine Cobolt), covering 90 degrees of the rotor periphery. For this motor, the electric angle is twice the mechanical angle. Fig 5 shows the stator structure and its six concentrated windings with their axes displaced from each other by 60 mechanical degrees or 120 degrees electric angles.

Fig 2: Rotor configuration

Fig 3 shows the stator winding configuration. Compared with slotless concentrated windings, the slotted ones are preferred. Fig 6 shows the cross-section diagram the proposed blood pump. Its components are listed in the Table1. Items 1and 7 are the pump housing and impeller, respectively. The impeller is driven by an axial field BLDC. Item 2 is the stator of the motor and is assembled outside the pump housing. Items 3-6 comprise the motor rotor, which is fixed onto the impeller. Both the motor rotor

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and pump impeller are sealed inside the pump housing and levitated by the electromagnetic force between the stator and the rotor. Items 12-13 are the hall-effect sensors and the eddy current displacement sensors which are used to detect the reversal of the air-gap flux field and air gap displacement respectively. .

Fig3: Motor stator with six slotted windings

Fig4: Cross-section diagram of novel artifical heart

Table 1: Components of the motor 1 Pump housing 2 Motor stator core 3 Windings 4 Motor rotor cover 5 Aluminium cover 6 Rotor PM 7 Rotor yoke 8 Motor rotor container 9 Impeller blade 10 Rotor of magnetic bearing 11 Stator of magnetic bearing 12 Hall effect sensor 13 Displacement sensor

3 Finite element analysis Maxwell 3D field simulator [9] is a user friendly software which has been used to analyse the electromagnetic field in the proposed motor and compute the related parameters in addition to conventional analytic design procedures. The simulation procedures were carried out in five steps [10]: Step 1----Creation the geometry of motor model.

Firstly, we have an idea of the initial design configuration of the proposed motor. So in the drawing panel, we can creat the motor model which includes the stator core with 6 slots, yoke, air-gap, rotor cover, yoke, base and the cylindrical permanent magnets.

Step 2-----Selection the material The different material were selected and assigned to the different parts of the motor as shown in Table 2. The material characteristics were then assigned.

Table 2: Material assigned Name Material Region Vacuum Coil1_Coil12,S_tooth1_12, S_y1,S_y2,r_y1,r_y2

Steel_1008

R_c1,R_c2 Aluminium PM1-PM24 SmCo

Step 3------Setting the current sources

In this project, two kinds of simulation were carried out. The first is the no-load magnetic field simulation, the second is magnetic field simulation under load. Different current values were set up for all phases .

Step 4------Generating the solution After the pre-procession of the first three steps, setting up the executive parameters and then defining the solution criteria, it can automatically generate a finite element mesh of the defined model and then it was refined adaptively according to error energy analysis. Fig 5 shows the 3D finite element mesh generated in the project. Through executing repeatedly, the simulation will continue until the desired energy error has been achieved.

Step 5------Post-processing

The simulation results and data can then be visualised and analysed by means of graphics and curves.

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Fig5 Mesh

Fig6: Flux density distribution of no load

Fig 7: Flux density distribution of full load The back-emf is given by :

en =NatDfD

Fig 8: Back EMF for each phase In the simulation, the rotor position was varied with Na=200 and w=2000rpm. The curves of en-q are obtained.

where Dt=wqD

Fig 9: Torque vs. different load current

Back-EMF for each phase

-4

-3

-2

-1

0

1

2

3

4

0 50 100 150 200 250 300 350 400

degree

En

(V)

-0.010.010.030.050.070.090.110.130.150.170.190.210.230.25

0 1 2 3 4 5 6 7 8 9

Ia(A)

Te(

Nm

)

4

4 Mathematical model of BLDC motor Using the coupled winding approach, the voltage equation of the three phase take the form [13].

c

b

a

V

V

V

=

R000R000R

c

b

a

iii

+

+

c

b

a

c

b

a

eee

iii

pLMMMLMMML

Where R and L are the resistance and self-inductance of each phase and M is the mutual inductance between phases. The self and mutual inductances are the same for each phase because of the 3-Phase winding symmetry and the constant air-gap length. In 3-phase winding, the current ia +ib +ic =0 Therefore, M(ia +ib)= -M ic The state-space form of the dynamic speed equation is: pwr=(Te-Bwr-Tc-Tl)/J where wr is the angle speed of the motor, w=pq and q is the angle defining the rotor position. Te and Tl are the electromagnetic and load torque respectively. Tc is the cogging toque. p is the short-hand notation for the operator d/dt The constant B is a damping coefficent associated with the mechanical rotation system of the machine. J is the moment of inertia of the motor and connected load. The electromagnetic torque can be expressed as follows: Te=(iaea+ibeb+icec)/wr The emfs ea eb ec are trapezoidal in shape and displaced 120 electrical apart. The input current is a square waveform to achieve a constant electromagnetic torque. ea ia 180 0

t

Fig10: Idealised phase current and its emf 5. Magnetic suspension system In a conventional motor, there are mechanical bearings to support the rotor. In this project, a magnetic system has been designed to support it. The function of the magnetic bearing is to support the impeller in a desired position so that it can be rotated freely. To simplify the design, passive repulsive type of magnetic bearing is used to control 4 degrees of freedom. An eddy current displacement sensor is used to detect the rotor disturbance along z-axis so that it is possible to control the axial displacement along z-axis . This is a kind of passive repulsive type magnetic bearing[11-14]. Numerous advantages such as long life, extreme reliability, frictionless nature, lubrication-free operation, low losses are offered by magnetic bearing system consisting of permanent magnets and controlled electromagnets. The satisfactory operation of this type of magnetic bearing is strongly dependent on the characteristics of the permanent magnet and its configuration in the bearing system. The configuration of the permanent magnets was analysed by the finite-element approach to help give better estimation of the function of this bearing and to optimise the structure. Fig 11 shows the permanent magnet configuration used in this system. A circular permanent magnet with either axial or radial magnetisation, is chosen for this type of system to make it radially stable. It uses permanent magnet NdFeB35 (Br=1.20Tsela, Hc=875-915KA/m, (BH) max=260-285HJ/m3. T=80-100).

Fig 11 Configuration of the magnetic bearing

After using FEM software for simulation, we got the repulsive force with different air-gap along x and y-axis .

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Fig 12: Repulsive force along x, y axes and its

stiffness The drawback of this kind of bearing is that it cannot be controlled automatically by the system. The axial flux gap BLDC motor has an advantage that it can generate an attractive force between rotor disc and the stator. Fig 13 shows the schematic of a bi-directional axial gap combined motor bearing [15].

Fig13: Schematic of bi-d irectional axial gap

combined motor bearing The radial motions x, y,qx, qy of the rotor are constrained by radial magnetic bearings such as the repulsion bearing shown. The axial gap combined motor bearing provides the motoring torque around the z-axis, and controls the position of the rotor along this axis. The stator has three-phase windings so as to generate the rotation magnetic flux in the air gap that produces the motoring torque T to the rotor. The generation of the attractive force is controlled by changing the magnitude and phase of the currents in stator1 and 2 with respect to the rotor. Since the attractive magnetic force is unstable, position feedback control is required to stabilise the axial direction. 6.Control block diagram

Fig 14: Control block diagram The controller is built on dSPACE DS1102 DSP board [16-18], that is specifically designed for development of high-speed multivariable digital controller and real-time simulations. This board is based on Texas Instruments TMS320C31 floating-point DSP. The DSP board reads the hall-effect sensor signal and displacement signal via ADC converter. Then it calculates each coil current and does commutation controller to remove steady-state error. The position control algorithm used is a standard PID controller. The DSP outputs the signal ia1,ib1,ic1, ia2,ib2,ic2 to the power amplifier block through DAC converter. The power amplifier uses current feedback. Hence, each coil current is proportional to the input command from the DSP.

7 Conclusion In this project, a novel configuration of centrifugal blood pump was designed and simulated. The adoption of brushless DC (BLDC) motor with axial flux makes the axial magnetic suspension feasible and easily to be implemented. The double stator structure produces higher torque compared to conventional BLDC motor. With the aid of Maxwell 3D simulation software, the magnet-static characteristics of this motor is analysed. In order to control the degrees of freedom easily and make the structure compact, a form of passive repulsive type magnetic bearing was designed and simulated. By repeating the simulation for different rotor positions in space, a complete electrical cycle of the variations of magnetic fluxes in various parts of the motor are obtained, so that the back-emf of each phase is obtained. Under various load conditions, the

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magnet-static torque was simulated. In addition, conventional analytic design method was also used.

Reference [1] P.E.Allaire, H.C.Kim, E.H. Maslen, Prototype Continuous Flow Ventricular Assist Device Supported on Magnetic Bearings Artficial Organs. Vol 20(6).P582-590. 1996 [2] Robert M. Hart, Victor G. Filipenco A Magnetically Suspended and Hydrostatically Stabilized Centrifugal Blood Pump Artficial Organs. Vol 20(6).P591-596. 1996 [3] Takashi Yamane, Masahiro Nishida, Toshihiko Kijima New Mechanism to Reduce the Size of the Monopivot Magnetic Suspension Blood Pump:Direct Drive Mechanism Artficial Organs. Vol 21(7).P620-624. 1997 [4] I.Sakuma, T.Sasaki, M.shiono,S.Takatani, Development of A Novel Direct Motor Driven Seal-less Centrifugal Blood Pump(Baylor Gyrp Pump)Annual Intl conference of IEEE in Medicine and Biology society. Vol. 13. No.5 1991 [5] Pratap Khanwilar, Don Olsen, Gill Bearnson, Paul Allaire, Using Hybrid Magnetic Bearings to Completely Suspend the Impeller of a Ventricular Assist Device Artficial Organs. Vol 20(6).P597-604. 1996 [6] Robert T.V.Kung, Robert.M.hart, Design Considerations for Bearingless Rotary Pumps Artficial Organs. Vol 21(7).P645-650. 1997 [7] J.X. Shen, K.J.Tseng. D.M.Vilathgamuwa, W.K.chan. A Novel Compact PMSM with Magnetic Bearing for Artificial Heart Application IEEE Transaction on Industry Applications, Vol. 36, No.4 July 2000 [8] Ho Kok Leong Wong Nee Wei K.J.Tseng, W.K.Chan. Implementation of Controller for Centrifugal Blood Pump with Magnetic Bearing NTU, FYP, 2000 [9] Maunal of Maxwell 3D Field Solver Getting Started: A Magnetic Force Problem Ansoft Corp. Sept.1999. [10] G.H.Chen and K.J.Tseng. Design of Wheel Motor Using Maxwell 2D Simulation IEEE Catalogue, No. 95TH8130 1995. [11] S.C.Mukhopadhyay,T.Ohji.M.Iwahara,S.Yamad and F.Matsunmura.A New Repulsive Type Magnetic Bearing-Modelling and Control. IEEE. 1997. [12] S.C.Mukhopadhyay,T.Ohji.M.Iwahara,S.Yamad, Design, analysis and control of a new repulsive-type magnetic bearing system IEEE. Proc,Electr. Power appl. Vol. 146. No1. January 1999. [13] M.M.Elmissiry,S.chari.Dynamic Performance of a Permanent Magnet, Axial Flux, Toroidal Stator, Brushless D.C.Motor.1992.

[14] Carl R.Knospe, Steohen J. Fedigan, R.Winston Hope.and Ronald D. Williams, A Multitasking DSP Implementation of Adaptive Magnetic Bearing ControlIEEE.1997. [15] Satoshi Ueno and Yohji Okada Characteristics and Control of a Bidirectional Axial Gap Combined Motor-Bearing IEEE/ASME Transactions on Mechatronics Vol.5.No3. September 2000 [16] DS1102 DSP Controller board Installation and Configuration Guide May 1999. DSPACE [17] dSPACE Experiment Guide May 1999. DSPA CE [18]Real-Time Interface(RTI and RTI-MP) Implementation Guide May 1999. DSPACE