Fractional Number Of Slots Research Articles

Novel permanent magnet motor drives for electric vehicles

Novel permanent magnet (PM) motor drives have been successfully developed to fulfil the special requirements for electric vehicles such as high power density, high efficiency, high starting torque, and high cruising speed. These PM motors are all brushless and consist of various types, namely rectangular-fed, sinusoidal-fed, surface-magnet, buried-magnet, and hybrid. The advent of novel motor configurations lies on the unique electromagnetic topology, including the concept of multipole magnetic circuit and full slot-pitch coil span arrangements, leading to a reduction in both magnetic yoke and copper, decoupling of each phase flux path, and hence an increase in both power density and efficiency. Moreover, with the use of fractional number of slots per pole per phase, the cogging torque can be eliminated. On the other hand, by employing the claw-type rotor structure and fixing an additional field winding as the inner stator, these PM hybrid motors can further provide excellent controllability and improve efficiency map. In the PM motors, by purposely making use of the transformer EMF to prevent the current regulator from saturation, a novel control approach is developed to allow for attaining high-speed constant-power operation which is particularly essential for electric vehicles during cruising. Their design philosophy, control strategy, theoretical analysis, computer simulation, experimental tests and application to electric vehicles are described.

A novel polyphase multipole square-wave permanent magnet motor drive for electric vehicles

A novel high-power-density permanent magnet (PM) motor drive for electric vehicles (EVs) is proposed. The motor is a polyphase multipole square-wave PM motor, which can be classified as a kind of PM brushless DC motor. The distinct features of the proposed motor as compared to those of the conventional PM brushless DC motor are as follows. First, the multipole magnetic circuit arrangement enables the minimization of the magnetic yoke, resulting in the reduction of motor volume and weight. Second, the coil span is purposely designed to be equal to one slot pitch, thus saving the amount of copper used. Third, by using a fractional number of slots per pole per phase, the arrangement of the numbers of poles and slots is so unique that the magnetic force between the stator and the rotor at any rotating position is uniform, hence eliminating the cogging torque that usually occurs in PM motors. Finally, the motor can be controlled to operate at a constant torque region and a constant power region with field weakening, thus both high starting torque and high cruising speed can be achieved. Therefore, as the proposed motor drive possesses the distinct advantages of high power density, high efficiency, and superior dynamic performance, it is very suitable for EV applications. A prototype of a five-phase 22-pole 5 kW motor drive has been designed for an experimental EV.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>

Balanced Fractional-Slot Wave windings

The a-c wave windings which are used mostly in rotors of induction motors are “modified” d-c wave windings whose winding pitch is changed after each p winding elements (p = number of pole pairs). These modified wave windings are balanced, but they can be applied to integral-slot and (integral+1/2)-slot windings only. The standard d-c wave winding is not used for wound rotors of induction motors for it seldom is balanced. The aim of this article is to give a method of laying out balanced wave windings for any fractional number of slots per pole per phase for which the fractional-slot lap winding is balanced. For this purpose the usual chart method is to be abandoned and the slot-star of the winding used instead.

Alternating-current wave windings

Simple algebraical formulae are developed by means of which, with close approximation, the angular positions of the phases of wave windings can be calculated. In this way, the amount of asymmetry can be determined in fractional-slot windings, d.c. wave windings and windings with extra empty slots. Tables are given showing the numbers of extra empty slots that can be used, and the positions in which they must be placed, in order to obtain practically symmetrical windings. Methods are developed for determining the connection points of windings, and charts are given, suitable for use in practice, for phase windings, both with a whole number and a fractional number of slots per pole and phase

The theory of armature windings

In this paper an attempt is made to outline the theory of armature windings in heteropolar machines (i.e. machines with alternate north and south poles).Section 1.—Attention is drawn to the fundamental importance of the relation between the number of slots and the number of poles, and, by the aid of simple vector diagrams, the condition is established for obtaining a winding in which the sum of all the induced pressures is zero at any instant. If all the coils in such a winding are joined in series to form a closed winding, no internal current can circulate. Following this, the condition is found for obtaining a number of similar symmetrical polyphase systems in such a winding, this being essential when parallel circuits are needed, as in most commutator machines. A winding fulfilling these conditions is called symmetrical, and an extension of the argument reveals the relationship that must exist between the number of slots and the number of poles in a symmetrical 3-, 4-, or 6-phase winding. Table 1 shows the number of similar circuits it is possible to have with various numbers of poles.Section 2.—This deals solely with single-layer windings. The arrangement of the coils for various symmetrical and hemisymmetrical polyphase windings is discussed. The effect of the number of slots on the number of similar parts in each phase is shown, and the means indicated for suppressing tooth effects.Section 3.—The arrangement of open and closed double-layer windings is illustrated, and the methods of loading closed windings are compared. The connecting rules for lap and wave windings are deduced, and the restrictions examined for making these windings symmetrical. These results are given in Tables 2 and 3, along with the slottings possible for N-phase, lap and wave windings. These tables enable the designer to see at once the number of coil-sides per slot, the number of slots, and the number of phases possible in any symmetrical winding.Lastly, it is shown how to find the points where a winding must be tapped or opened to obtain phases, examples being given to illustrate the various cases.The sections of the paper are subdivided as follows :—1. Armature windings.(I) Condition for obtaining a closed winding.(II) Condition for obtaining a symmetrical winding.(III) Examples of symmetrical and unsymmetrical windings.(IV) Conditions for obtaining a symmetrical N-phase winding.(V) Phase-spread and coil-span.2. Single-layer windings.(I) Arrangement of single-layer windings.(II) Number of armature slots.(a) Whole number of slots per pole.(b) Fractional number of slots per pole.(i) Use of empty slots.(ii) Use of unequal coil groups.3. Double-layer windings.(I) Arrangement of double-layer windings.(a) Phase tappings off closed windings.(i) Polygon tappings.(ii) Diametral tappings.(b) Open double-layer windings.(II) Connecting rules for closed windings.(a) Closing rule for lap windings.(b) Closing rule for wave windings.(III) Conditions for obtaining symmetrical lap and wave windings.(a) Symmetrical lap windings.(b) Symmetrical wave windings.(IV) Conditions for obtaining symmetrical N-phase, lap and wave windings.(a) Symmetrical N-phase lap windings.(b) Symmetrical N-phase wave windings.(i) Number of poles.(ii) Number of slots.(V) Location of tappings and openings.(a) Tappings off symmetrical lap and wave windings.(b) Opened wave windings.(c) Illustrative examples.(VI) Wave windings with a whole number of slots per pole.