Akeshi Takahashi

Transient-torque analysis for line-starting permanent-magnet synchronous motors

Using the analysis method for resolving the starting torque of line-starting permanent magnet (PM) synchronous machines with starting cage, the electromagnetic torque can be separated into two components, the driving ldquocage torquerdquo and the braking ldquomagnet torquerdquo. The correct saturation of the iron must be considered, when calculating the separate torque components. The sum of the two torque components exhibits a good agreement with the originally calculated starting torque with the Finite Element time stepping technique. This novel method also helps to have a better knowledge of the synchronization mechanism. Results are given for a two-pole prototype motor with PN=5 kW, nN=3000 min-1, VN=200 V, Y-connection.

Reluctance Torque Utility for Line-Starting Permanent Magnet Motors

This paper presents two types of prototype line-starting permanent magnet motors for compressor drives; one is a nonsalient-pole machine, and the other is a salient-pole machine. It is proved through both the analyses and experiments that the utility of reluctance torque not only boosts steady-state characteristics but also line-starting capabilities. It was also noteworthy that the salient-pole machine in the steady-state operation could achieve not only better efficiency but also a higher power factor. Finally, the designed motors achieved a 4% to 5% increase in efficiency over all operational points, which prevented a rise in temperature by 30 K, compared with a conventional two-pole induction motor. The results are given for a two-pole prototype motor with PN = 5 kW, nN = 3000 min -1, VN = 200 V, and Y-connection.

Asynchronous Torque of Line-Starting Permanent-Magnet Synchronous Motors

A modified theory and a proposed analytical approach clarified asynchronous torque characteristics of line-starting permanent-magnet (PM) motors, taking into consideration the mutual effect among the fields due to armature current, cage-bar current, and PMs. Theoretical and analyzed results revealed that cage torque had an oscillating component with slip frequency due to magnetic saturation. This component cannot be seen in the conventional theory. It was also found that the coupling effect between PM flux and cage-bar flux caused on magnet torque larger oscillation and more significant negative dc component, compared with the case in the conventional theory. The theoretical and analyzed results were validated on the basis of experimental approaches. Results are given for a two-pole prototype motor with PN = 5 kW, nN = 3000 min-1, VN = 200 V, and Y-connection.

Fast calculation of load conditions of permanent magnet synchronous machines using finite elements

This paper proposes a method of characteristics calculation for permanent magnet synchronous machines (PMSMs) using static finite element analysis (FEA) and steady-state dq-axis equations. The method calculates characteristics such as torque, voltage, current, inductances, etc. under any balanced steady-state load conditions. Various types of input condition can be selected to identify the load condition. For example, the characteristics are calculated at the load condition that has the smallest current amplitude and the inputted torque and frequency. The computing time of the method is small because the FEA is static. The calculated results of a 750-watt PMSM under minimum-current control and field-weakening control have agreed well with the measurements. The method is useful in PMSM development and design.

Circulating current in parallel connected stator windings due to rotor eccentricity in permanent magnet motors

Rotor eccentricity in permanent magnet (PM) motors produces a voltage imbalance between parallel circuits that then creates a circulating current. This circulating current causes conduction loss, which is one of the stray losses in motors. The conduction losses of circulating currents need to be reduced in order to make PM more highly efficient. This paper describes the effect of neutral point connections on circulating current caused by static rotor eccentricity in a PM motor, as the circulating current depends on the use of neutral point connections in stator windings. We developed a theoretical expression for a circulating current with a closed circuit and compared its validity with that of measured results. The results of how well the connections of neutral points reduced the circulating current loss are provided.

Evaluation of Torque Characteristics in Saturated Magnetic Field for Permanent-Magnet Synchronous Motors

The evaluation of torque characteristics in saturated magnetic field for permanent-magnet (PM) synchronous motors is presented. Using the modified two-axis machine model that considers the cross-coupling effect, two important phenomena resulting from magnetic saturation are indicated: the decrease in magnet torque because of an anomalous flux of PMs and the existence of cross-magnetizing torque. These two kinds of torque characteristics cannot be considered by the conventional two-axis machine model, which assumes a constant value of induced electromotive force due to PMs. Measurement results indicate the validity of the modified method while they demonstrate the conventional method has difficulty in deducing torque components.

Dynamic and steady-state performance of line-starting permanent magnet motors

This paper presents two types of prototype motors for line-starting permanent magnet motors; one is a non-salient- pole machine, and the other is a salient-pole machine. Through both the analyses and the experiments, it is proved that the reluctance torque utility boosts up not only the steady-state characteristics but also the line-starting capability. It is also noteworthy that in the steady-state operation the salient-pole machine can realize not only the better efficiency but also the higher power factor. Results are given for a two-pole prototype motor with PN = 5 kW, nN = 3000 min−1, VN = 200 V, Y-connection.

Explicit criteria for reluctance torque utility of permanent magnet motors

This paper presents an explicit criteria for reluctance torque utility of permanent magnet (PM) motors. The criteria can be quantified by PM flux linkage Ψ_p of stator phase windings, and current I, as well as direct- and quadrature-axis inductance L_d and L_q. Through both theoretical formula and experiments, it is proved that the motor with large Ψ_p can utilize little reluctance torque even if the salient-pole ratio L_q/L_d is over 2.0. In such a case, the large salient-pole ratio only leads to an increase in iron loss and hence a decrease in motor efficiency. The invented criteria helps one to understand the property inherent in each motor, and also indicates that in the above case selecting the rotor geometry with less saliency can improve the efficiency. Results are given for two types of 4-pole prototype motors with P_N = 0.6 kW, n_N = 3600 min^-1, Y-connection.

Development of self-starting permanentmagnet synchronous motors for compressor drives

A lot of saving energy policies are being globally instituted from the viewpoints of global warming and environmental preservation. Although a lot of effort has been put into improving industrial air conditioners, there are still efficiency problems with multisystem air conditioners in buildings where constant speed scroll compressors are used. We present the measurement results of a self-starting permanent magnet synchronous motor applied to a compressor drive. Compared with an isometric induction motor, efficiency improvements of 9% and 7% were confirmed for rated operation under 50 Hz and 60 Hz, respectively. The power factors also increased by 17% and 10% for the same rated operations, respectively. Moreover, the efficiency of the compressor equipped with the proposed motor was 9% and 7% higher than that with the conventional induction motor, under 50 Hz and 60 Hz operations, respectively. This result is consistent with the efficiency improvement of the proposed motor itself.