Assessment of Losses in a Brushless Doubly-Fed Reluctance Machine

Abstract

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.

Robust vector controllers for brushless doubly-fed wind turbine generators

“…6 can be partly attributed to the MTPIA conditions when most of the reactive power for the machine magnetization comes from the primary side [22]. Nevertheless, another contributing factor is certainly the relatively weak magnetic coupling between the windings of the machine with an axially-laminated rotor [1], [5], [28]. However, recent studies have reported that advanced 'ducted' reluctance rotor designs [4] can provide competitive performance to large-scale DFIGs in terms of torque density and efficiency [2], [29].…”

Electromagnetic Considerations in the Design of Doubly-Fed Reluctance Generators for use in Wind Turbines

“…Schulz et al [9] suggested that the axially-laminated rotor experiences very high eddy current loss in the laminations and that radially-laminated rotor are more suitable. This was investigated by Scian et al [10] which found that indeed the losses in the rotor are higher; however they are of an order of the copper losses in a cage induction machine. The alternative to an axially-laminated rotor is a radiallylaminated rotor which is similar to the rotor in a switched reluctance machine.…”

“…In [10] only a minus sign was used however, in general terms, ± should be used; this indicates generality of the direction of rotation. Fig.…”

“…This simplifies the method considerably. However it is possible to model the machine with individual laminations [10]. Unfortunately for the test machine used in this study, it was converted from a synchronous reluctance machine, and the stacking factor was 0.5.…”

“…The control for this machine has been studied in several papers [2] - [6] which verified that the machine could be operated successfully while more recently papers have begun to assess the machine in terms of its electromagnetic design. [7] - [10]. The machine is a hybrid machine.…”