Some high-torque electric machines, such as lowspeed wind generators, may be very difficult to test on load because of the large and expensive mechanical equipment required to load them (motors) or to drive them (generators). In this paper, a full-load regenerative testing methodology is described which does not require any rotating equipment to be coupled to the machine where the rated active power flows in a closed loop between the machine and a suitably connected dual-stage converter. The successful application of this methodology to a 780-kW 14-r/min wind generator prototype is addressed as a study case. Relevant experimental results are reported in a companion paper.

The on-load factory testing of high-power electric machines may be a challenge due to the large mechanical equipment required to drive or load them. In a companion paper, a regenerative full-load testing strategy has been proposed, where the power flows in a closed loop between the electric machine and a dual-stage converter so that only the power corresponding to system losses needs to be supplied by the test facility and no mechanical equipment is needed. In this paper, the implementation of such testing strategy on a 780-kW 14-r/min permanent-magnet alternator with fractional-slot concentrated stator winding is described. Test results are presented and compared with the predictions made in Part I, showing a satisfactory accordance between theoretical and experimental results.

Rules for the selection of rotor bar numbers which minimize current and torque ripples are derived in this paper for a general symmetrical multiphase cage induction machine with prime phase number and integral slot winding. Analytically obtained expressions for optimal rotor bar number selection are validated by means of totally independent simulations, one based on a parameterized winding function (PWF) model of the induction machine and the other employing time-stepping finite-element analysis (TSFEA). As a case study, five-phase four-pole cage induction motors with forty stator slots and different number of rotor bars are comparatively analyzed. Results obtained from the PWF model are in excellent accordance with those independently obtained by TSFEA and both confirm the correctness of the proposed selection criteria. The practical motivation of the study is that an incorrect selection of rotor bar number can lead to parasitic torques of significant amplitude and, presently, there are no general rules available in the literature which may guide designers towards an optimal design choice for a general number of phases. INDEX TERMS Induction machines, multiphase stator winding, parasitic torques, rotor slot harmonics, skewing, winding function model.

Spoke-type interior permanent magnet (IPM) machines are an attractive topology for high-performance electric motors, especially designed for vehicle traction applications. In this paper, a special design for a spoke-type IPM motor is presented to enhance motor flux-weakening capability in the operation over a wide speed range. The proposed design consists of a simple and robust mechanical device that includes radially displaceable rotor yokes, connected to the shaft by means of springs. At high speed, the centrifugal force prevails over the elastic one due to springs, causing the mobile yokes to displace radially and establish a partial magnetic short circuit between PMs. This increases PM leakage flux and consequently reduces the air-gap field. As a result, a mechanical flux weakening effect is achieved at high speed, which helps significantly to reduce the demagnetizing d-axis current to be injected by the inverter, along with the related copper losses and demagnetization issues. The proposed design is investigated in this paper using an analytical model whose parameters are computed by finite-element analysis. The effectiveness of the solution being set forth is successfully proven by some testing on a laboratory prototype. Experimental results are compared with analytical predictions showing a satisfactory accordance.Index Terms-Flux weakening, interior permanent magnet (IPM) motors, spoke-type permanent magnet (PM) machines vehicle traction drives.

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