A Permanent Magnet Traction Motor for Electric Forklifts : Design and Iron Loss Analysis with Experimental Verifications
Abstract: This thesis presents the design analysis of a permanent magnet synchronous motor (PMSM) for the traction application in electric forklifts. Within the scope of the study, an existing induction traction motor for electric forklifts benchmarks the expected performances of the proposed PMSM designs, as design specifications. The possibility of using the same stator geometry as the one used in the induction motor is explored for fast prototyping. The eventual prototype design is expected to be field-weakened and to have a constant power speed range (CPSR) of 2.5 to 3.A simple analytical design approach based on the CPSR contour plot in an interior permanent magnet parameter plane is derived to obtain the possible designs that meet all the design specifications and the targeted CPSR. A prototype design with an inset permanent magnet (IPM) rotor configuration is obtained with this approach. Finite Element Method (FEM) analysis is employed to verify the expected performance. In addition, further examinations are also carried out by taking into consideration the magnetic saturation and the stator resistance. Contour plots of torque and phase voltage are applied to procure the advanced current angle required at different speeds in the field-weakening operating region.Two prototype motors have been manufactured during this thesis work, and various experimental tests are carried out to examine and validate the expected performance. The prototype can deliver a maximum output power of 9.4 kW at the rated speed of 1500 rpm, and it has an outer diameter of 180 mm, a shaft height of 112 mm, a bore diameter of 110 mm and an active length of 165 mm. Search coil windings are implemented in the main prototype to monitor and measure the flux density waveforms in the stator tooth and the yoke back. The prototypes are naturally cooled with the cooling fins and the ventilation holes in the stator housing. The thermal analysis based on the lumped-circuit approach and the numerical method are investigated and examined by the measured results. It has been shown that an accurate loss estimation is a pre-requisite to enable both approaches to accurately analyze the heat transfer phenomenon in electric machines. The strengths and disadvantages of each method are also discussed.An analytical approach to estimate the iron loss in permanent magnet (PM) electrical machines is also developed and extensively investigated. The proposed technique is based on the flux density waveforms predicted in the various parts of the stator, namely the tooth, tooth projection and the yoke back. The waveform in the respective region is derived from the air gap flux density that consists of a fixed PM excitation and the armature field due to the fundamental current in the stator winding. The model can be applied at any operating point with different load, including the field-weakening region. This simple approach gives a good indication on how iron loss varies at various speeds and operating points. The predicted loss shows a very satisfactory agreement (± 4%) with the measured results at no-load or open-circuit condition, but larger discrepancies are found under the load condition. In the constant torque operation region, estimated losses are approximately on average 15% lower than the measured values. Under the field-weakening operation, the model becomes inadequate due to the excess eddy current loss caused by the highly distorted tooth flux density waveform. A correction factor for the eddy current loss is therefore essential to account the harmonic effect. The rectified estimations are then within ± 21% of the measured values. This simple approach has proved to be capable of estimating and modelling the difficult phenomenon of iron loss in PM motors, and it can be easily embedded in the design process for routine use in loss estimations.Keywords: Constant Power Speed Range, Electric Forklift, Finite Element Analysis, Field-weakening Capability, Iron Loss, Permanent Magnet Electrical Motor, Saliency, Thermal Analysis
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