【正文】 f permanent magner synchronous motor (PMSM) such as the open load permanent magnet flux, d axis reactance Xd And q axis reactance Xq are most essential to the performance analysis and optimization design of the motor. Based on the numerical analysis of 3D electromagnetic field, the three electromagnetic parameters of permanent magnet Synchronous motors with U from interior rotor structures are calculated by FEA. The rules of the leakage coefficient and reactance parameters changing with the air gap length, permanent magnet magnetism length and isolation magnetic bridge dimensions in the rotor is given. The calculated values agree well with the measured values. The FEA results are integrated with the self piled electromagnetic design program to optimize the prototype motor. The tested performances of the prototype motor prove that the method is fitted to optimize motor structure.I. INTRODUCTIONPMSM possess many merits such as high efficiency, long life duration, pact size, light weight and simple structure[1]. And it has some characteristic such as high precision, high ratio of adjust speed precision. So it is used at more factory field. The capability of output torque, power factor and efficiency is the important performance of adjustable speed PMSM. When the applied condition is confirmed, three main electromagnetic parameters[23], the open load permanent magnet flux, d axis reactance Xd and q axis reactance Xq make very important influence on the motor operation performance. The electromagnetic power formula of interior adjustable speed PMSM: (1)From (1), we can see that three main electromagnetic parameters have obvious influence on the system performance. As for adjust speed PMSM, the exact calculated reactance parameter is very important for realizing the control algorithm and is necessary to predict steady state and transient characteristics of PMSM. When the stator structure is invariable, heightening the ratio of Xaq and Xad by changing rotor structure can improve the system power factor and efficiency. Rational parameter design is the core of the successful designing of PMSM.For the medium capacity variable speed high efficiency permanent magnet synchronous motor (PMSM) such as 200kW or 300kW, to obtain the good cooling effect, there are radius channels and axis holes inside the motors. At present, the influences of radius channels and core length on noload leakage coefficient are hard to be obtained with existing calculation method. This made the blind choice of noload leakage coefficient while designing the motors. It prolonged the course of research and development (Ramp。D), increased the cost of Ramp。D, and lacked of scientific basis in design optimization and performance simulation. This influences the application of PMSM. So as for the medium capacity the exact noload leakage coefficient of PMSM is more essential to the optimal design of motor than small motor.Due to the effect of permanent magnet material, the reactance parameters calculating method is different form other AC machines. For PMSM there are no open circuit and short circuit states like the electrical excited motor. The excitation of permanent magnet material is effected at all time. At the same time, different magnetic motive force and field saturation the reactance parameters are different. The parameters calculating method must take the influence of PM material into account. Based on the above reasons, the traditional circuit calculating method is not fit for the PMSM. So the finite element method is applied to calculate the parameters , and . is the noload leakage flux coefficient. In this paper the 3D electromagnetic field is used to get the exact parameters of U form medium capacity PMSM[46]. Through analyzing the electromagnetic parameter changing with the structure, the detailed experience data and curve can be obtained. Finally selecting rules of three main electromagnetic parameters are integrated with the self piled electromagnetic design program to optimize the prototype motor and a good foundation will make for designing and applying of PMSM..II. NO LOAD LEAKAGE FLUX COEFFICIENTIn this part, Ansoft is used to calculate the noload leakage coefficient of U form interior PMSM, On the basis of the calculations of many PMSM’s, the change law and influence factor of the noload leakage coefficient with motor structure parameters are analyzed, and the curve of different structure dimension is given. is the Cross section of Multisegment U form rotor structure. is the U form magnetic circuit structure separate magnetic bridge diagram. is the solved magnetic density of PMSM.A. Motor modelingDuring the calculating process of electromagnetic parameter only based on exact analysis model of motor analysis model we can get valid simulation result. Therefore, a 3D FEA model is established as following: 1) According to the motor structure dimensions, the motor simulation model is established. 2) Confirm the material attribute。 3) Ensure the boundary condition and outside source parameter。 4) Mesh。 5) Setting the resolve parameter, calculate.B. FEM analysis results of leakage flux coefficient is the curve. is the air gap lengthen, and is the length of the permanent magnet material. is the noload leakage coefficient change with magnetic bridge length w of proposed U form motor. is the noload leakage coefficient change with magnetic bridge width b.III. INFLUENCE OF STRUCTURE DIMENSION ON REACTANCE PARAMETERSAs for the Xad, the air gap fundamental flux and the effective air gap fundamental flux were gotten when the direct axis armature current is equal to the . (2)As for the reactance , directly give the winding current , and resolve the air gap flux .(3)Where, Kdpis the waveform coefficient of flux. From the above 3D FEA results, the dq axes reactance parameters are calculated by self piled program in matlab software. A．Influence of air gap