IntroductionThe brushless doubly-fed reluctance machine is attracting attention because it has no brushes and a reduced converter drive requirement [1] [2]. It has a reluctance type of rotor of p pole pairs. The stator will have two sets of 3-phase windings with p ± 1 pole pairs. Test machines that have been investigated have had 4 rotor poles with 2-and 6-pole stator windings although there is no reason why higher pole numbers cannot be used. While the application for these machines could be in the area of power drives, they could also be used in wind turbines as an alternative to the wound field induction generator [3]; their operation can be considered as being similar with two windings and the rotor speed being independent of the rotating field velocity. In the induction machine, the windings are couple directly by a common pole number whereas in the reluctance type of machine the MMFs are modulated by air-gap permeance to produce flux waves of p ± 1 producing cross coupling. It is necessary to use a rotor with a high d-q reluctance ratio as is vector control for correct operation [4]. One winding is fixed in frequency and volts (grid connection or power winding) and the rotor speed varies according to the load; there is a synchronizing requirement for the nongrid connected winding (control). The equation governing this is ω_c = pω_r -ω_p, where the ω is a rotational velocity (rad/sec) and c, r and p represent the control frequency, rotor velocity and grid frequency. Previous studies have concentrated on the control of the machine. The machines used usually have axial rotor laminations; this is a modification from the synchronous reluctance machine. However, in the doubly-fed application the rotor experiences rotating flux waves which can produce lamination eddy currents which could cause excess losses. This paper seeks to investigate this and compare to a radial laminated rotor using a 2/6 pole arrangement, (with 2 pole power and 6-pole control windings). Modelling The machine is modelled using Cedrat FLUX 2D. The rotor laminations are modelled individually which produces a very dense mesh (Fig 1). The laminations are stacked and form a "U" shape along the axial length. There is spacing between the laminations to produce high q axis reluctance. In this machine the stacking factor is 0.5. The other modelling alternative is to use anisotropic material [5]. The machine was simulated using a series of static solutions then a series of transient solutions. Finally the machine was compared to an equivalent radially-laminated machine. Experiments A 2/6 pole test machine was set up; it was rewound from an axially-laminated synchronous reluctance motor. It was found that the slots were small so that the loading and flux levels were not quite high enough to maximize operation. However this study was to assess the rotor losses and it served its purpose.
The paper initially reviews some previous work with respect to the effect of inter-bar currents on the performance of a cage induction machine with a skewed rotor. A multi-sliced nonlinear 2D finite element model is then put forward to account for this current. This model uses different circuits to assess how the inter-bar current should be incorporated into the simulation. It uses different arrangements in both the axial and radial representations. The starting characteristics are focused upon and simulation results are compared to measured results and it is verified that the inter-bar currents do have an effect that is not normally accounted for in standard modeling techniques.
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