Kou Baoquan , Li Chunyan , and Cheng Shukang

1. Flux-Weakening-Characteristic Analysis of aNew Permanent-Magnet SynchronousMotor Used for Electric Vehicles511~515 Kou Baoquan, Li Chunyan, and Cheng Shukang 老師:王明賢 學生:方偉晋

2. Abstract • The permanent-magnet (PM) synchronous motor(PMSM) takes advantages of small capacity, high efficiency, andhigh power density and have become commonplace in electricvehicledriving systems. • Analysis is discussed, including the forceon the main PM, the movement process of the PM in the PMslot, the characteristics of the movement of the PM, and the FWperformance of the new PMSM used for electric vehicles.

3. Outline • INTRODUCTION • PRINCIPLE OF THE VMRPMSM • MOVEMENT ANALYSIS OF THE PM • AIR MAGNETIC-FIELD-ADJUSTMENTCHARACTERISTICS FOR VMRPMSM • FEM FW ANALYSIS OF THE VMRPMSM • EXPERIMENTAL RESEARCH

4. INTRODUCTION • The emphasis ofthe research and development on electric vehicles lies in highefficiency, high-power-density driving devices (motor and controller),and fuel battery. • theterminal voltage is proportional to the speed, and the limitationof the storage battery of electric vehicles and the saturation ofthe current controller constrain the application of the PMSMabove the base speed; therefore, flux-weakening (FW) control isnecessary. • The new PMSM has the advantagesof adapting different loads by the automatically adjustablemagnetic flux and becomes an excellent competitor in drivesystems for electric vehicles.

5. PRINCIPLE OF THE VMRPMSM • New Structure of the Rotor The stator of a fractional 18-slot winding is the same asthe general structure of a PMSM. The rotor shown in Fig. 1is composed of a PM slot, main and secondary PMs, and anonmagnetic conductor. Fig. 1. Rotor structure of the VMRPMSM. (a) The rotor structure. (b) The prototype rotor.

6. PRINCIPLE OF THE VMRPMSM • FW Principle The centrifugal force on the main PMs increases when thespeed increases above the base speed. The magnets then moveoutward along the PM slot until the centrifugal force andelectromagnetic force reach a balance.

7. MOVEMENT ANALYSIS OF THE PM • The position of the PM in the PM slot determines thereluctance of the path through which the flux produced by thePM passes. This, in turn, affects the flux density B in the airgap. • A simplified model which includes the movable PM in thePM slot and the iron slot is shown as Fig. 2. Fig. 2. Force-analysis model of the PM.

8. MOVEMENT ANALYSIS OF THE PM • We use the finite-element method (FEM) to calculate theelectromagnetic force when the PM is in different positions iNthe slot. We define the variable L as the distance between thePM and the slot. • The electromagnetic force increases with decreasing s, andthe magnitude is smaller when L is wider. The direction of theelectromagnetic force is 270◦ approximately, which means thatthe force points toward the central shaft. The frictional forcedue to pressure of the PM against the sides of the slot is nearlyzero, so it is neglected.

9. AIR MAGNETIC-FIELD-ADJUSTMENTCHARACTERISTICS FOR VMRPMSM • Characteristic of the Movement of the PM As the PM moves along the slot, it takes the followingfeatures. 1) The motion process is a variable linear movement. 2) The PM locates in different positions in the PM slot withincreasing and decreasing speeds of the motor. 3) The critical point where the PM begins to move is differentwith increasing and decreasing speeds of the motor.

10. AIR MAGNETIC-FIELD-ADJUSTMENTCHARACTERISTICS FOR VMRPMSM • Relationship of the No-Load Air-Gap Magnetic Densityand Speed The PM moves in the PM slot when the speed exceeds thebase speed. The magnetic reluctance is variable for the exits ofthe nonmagnetic conductor in the magnetic circuit, and the airgapmagnetic flux will alternate. The no-load air-gap magneticdensity is influenced by the movement of the PM.We define thedistance between the main PMand the secondary magnet as thevariable s.

11. AIR MAGNETIC-FIELD-ADJUSTMENTCHARACTERISTICS FOR VMRPMSM • Relationship of the Back EMF and Speed When the PM ismoving, the air-gap magnetic density is variable, and the relationshipbetween the no-load magnetic density and speed couldbe divided into three stages. The value of the back EMF is decided by the comparison ofthe increasing speed and the decreasing magnetic density. Wetake the increasing-speed process of the motor as an exampleto analyze the back EMF. The rule of the back EMF in the decreasing-speed process of the motor issimilar except that thecritical speed is different.

12. FEM FW ANALYSIS OF THE VMRPMSM • Magnetic-Flux Distribution Only the excitation of the PM is present when the VMRPMSMis operated in a no-load mode. The denser the magnetic flux is, the stronger is the magnetic field. Theresults indicate that the value of s is smaller, the magnetic fluxis weaker, and the magnetic field is weaker.

13. FEM FW ANALYSIS OF THE VMRPMSM • Air-Gap Magnetic Density The comparison of the no-load air-gap magnetic-density waveforms is shown in Fig. 3. Fig. 3. Comparison of no-load air-gap magnetic-density waveforms.

14. EXPERIMENTAL RESEARCH • Performance of the Efficiency We manufactured a proportional low-power 600-W prototype. The test system for the VMRPMSM is shown in Fig. 4. The efficiency reaches 92% when operated at the rated power; the speed range is increased to 2.3 times by the specific structure of the VMRPMSM itself with no FW control method. The area of efficiency over 80% exceeds 60% (Fig. 5).

15. EXPERIMENTAL RESEARCH Fig. 5. Efficiency map of the motor. Fig. 4. Test system.

16. EXPERIMENTAL RESEARCH • Performance of the Torque The torque performance is shown in Fig. 6. Here, 1 is the calculated curve of the traditional PMSM, which takes the same rotor structure with fixed PMs, 2 is the test curve of the VMRPMSM, and 3 is the calculated curve of the VMRPMSM. Fig. 6. Comparison of the traditional PMSM and the VMRPMSM.