GB2225173A - Stator windings for homopolar electrical machines - Google Patents

Abstract

A Homopolar electrical machine has a lobed rotor (2) which rotates inside a cylindrical stator (12) which is provided with axially extending slots. The slots contain armature windings (14) of coils which are preferably arranged with their axes radially of the machine but which axes may be tangential (Fig. 3 not shown) to the machine. Fan blades (16) between pairs of lobes on the rotor (2) increase cooling air flow through the machine. The stator is divided into three sections (13). <IMAGE>

Description

ELECTRICAL MACHINES The present invention relates to homopolar rotating electrical machines, and, in particular, to such machines that are specifically adapted to act as generators although motor operation is possible.

A homopolar rotating electrical machine is described in my earlier UK Patent Application Serial No.

2175147A. In that specification there is described a machine in which a lobed rotor is used with fixed excitation windings or permanent magnets to produce a unidirectional or homopolar fluctuating magnetic field which is directed radially outwards towards a surrounding cylindrical stator which supports the armature windings.

The flux is linked with the stator windings which are coils with their axes aligned tangentially to the stator cylinder. The stator windings are arranged to give three phase alternating current.

Such a homopolar machine is also described in UK Patent Specification No. 1374481 (Plessey). This specification, however, provides no teaching as to the construction of the armature winding. It has also been suggested to provide independent pole pieces which each carry a winding with a radial axis and are fixed to the interior of a stator cylinder (UK Patent Specification No. 1174784).

The present invention in one aspect is directed at improving the EMF generated by a machine of the type shown in Figure 7 of the UK Patent Application Serial No.

2175147A, and also at simplifying the construction.

This improvement is achieved in accordance with the invention by providing the armature windings in axially extending slots defined in the inner cylindrical surface of a substantially cylindrical laminated stator surrounding the rotor. Preferably the armature coils are formed by windings passing through these slots such that the resulting coils have their axes directed radially.

It is believed that such an armature winding particularly when it is characterised by successively overlapping coils, each short-corded to about 60%, spread over one phase sector of a stator core, with the coils of each phase sector physically isolated from those of the adjoining phase sectors is especially novel when used in a homopolar rotating electrical machine of the type with which the present invention is concerned. In an alternative embodiment the slots support coils with their axes tangential to the stator.

As in all generators, dissipation of heat represents a significant problem. In a preferred embodiment of the machine of the present invention, fan blades are fitted to the rotor between the lobes to increase the flow of cooling air within the machine.

Two embodiments of homopolar rotating electrical machines embodying the invention will now be described, by way of example only, with reference to the accompanying diagrammatic drawings, in which: Figure 1 is in axial section through a first embodiment of a machine in accordance with the invention; Figure 2 is a transverse cross section through the machine of Figure 1 taken along a line just outside one end of the rotor; Figure 3 is an axial section through a second embodiment of the machine; Figure 4 is a composite transverse section through the machine of Figure 3, the left hand half being through the excitation windings and the right hand half being through the rotor; Figure 5 is a schematic drawing showing a portion of the internal surface of the stator showing how the armature windings are arranged for a single phase in three different configurations; and Figure 6 is a schematic drawing showing the manner in which the three separate windings are connected to provide a three phase output.

The electrical machines to be described are primarily designed to act as generators to produce a 50Hz output when the rotor is rotating at 1500 r.p.m. A first embodiment of such a machine will now be described with reference to Figures 1, 2 and 5.

The machine comprises a lobed rotor 2 mounted on a shaft 1 which extends through a machine housing defined by end plates 3 and a cylindrical member 11.

The stator core 12 is mounted to the cylindrical member 11 and defines axial slots in its internal surface which receive the armature windings 14. The homopolar magnetic field is generated by fixed windings 4 surrounding the shaft at each side of the rotor 2.

The drive shaft 1 is divided into two parts, which, between them, support the rotor 2. One end of each part of the drive shaft is spigotted into one end face of the rotor. The rotor 2 is built up from laminations of sheet steel. The rotor has two diametrically opposed lobes though any number of pairs of lobes may be formed on the rotor depending on the required frequency and speed of rotation of the machine. The edges of the lobes are inclined relative to the shaft axis. Such slanting lobe edges considerably improve the starting of the machine when it is employed as a motor. The optimum inclination of the edges may be determined by experiment for any particular configuration of armature winding.

The inclined edges also facilitate air flow through the machine. To further improve cooling, fan blades 16 are mounted between each pair of lobes as shown in Figures 1 and 2. The rotor 2 has slots (not shown), which are also slanted relative to the shaft axes. These slots contain embedded wires. The wires are connected at their ends to a common wire to form a conventional insulated squirrel cage winding for induction motor operation. This provides for damping when the machine is used as an alternating current generator.

Each shaft part 1 is fitted into its respective end plate 3. The end of the part of the shaft 1 which is shown at the left hand side in Figure 1 is supported in a journal and thrust bearing 5. A housing 7 of this bearing is fixedly mounted to the exterior of the end plate 3. The other part of the shaft 1 is supported in a journal bearing 6. A housing 7 for this bearing is fixedly mounted to the other end plate 3. This right hand part of the shaft extends beyond the bearing housing 7 and carries a drive pulley 8 by means of which the shaft, and therefore the rotor 2, can be driven in rotation when the machine is used as a generator.

The end plates 3 are provided with openings 17 which allow through flow of cooling air within the machine housing. The excitation coils 4 are mounted around a cylindrical, inwardly extending portion cf each end plate 3. The respective shaft parts extend throuah these portions and are rotatable relative thereto. A narrow air gap 9 is defined between the shaft and these portions. This gap is sufficiently small to be traversed without undue loss by the magnetic flux.

Instead of the excitation coils 4, which are supplied with a DC current, permanent magnets may be used to produce the required magnetic field. The magnets or windings 4 are each arranged to produce a magnetic field having the polarities indicated on the respective shaft and other parts. The lines of flux produced by each excitation winding 4, as in a conventional homopolar machine, extend from the end plate 3 to the cylindrical member 11 and then into the rotor core 12, across an air gap 10 between the rotor and the stator, into the rotor 2, and then through the shaft part 1, and across the air gap 9 back to the end plate 3. When the flux passes into the rotor it is concentrated by the lobes so as to produce a fluctuating, radial, magnetic field between the periphery of the rotor and the cylindrical member 11.

The design of the components is such that as much soft steel as possible is used for the components forming this flux path.

Inside the cylindrical member 11, three stator cores 12 are fitted. Each stator core is constructed from laminated sheets and defines a 1200 sector. The cores are supported within the cylindrical member 11 by means of radially inwardly extending walls 13 to which the sectors 12 are fixed. Each core is provided with axially extending slots in its inner surface. These slots receive the armature windings 14. The winding in each core sector 12 is intended to produce a single phase of output current. The number of slots in each core sector is even and the EMF generated is improved as the number of slots is increased. Three possible arrangements of the winding within the slots of a single sector are shown in Figure 5.Although only a single wire is shown as passing through each slot in the various alternative configurations shown in Figure 5, it will be appreciated that a single wire is passed around the path indicated many times to produce the required winding.

The successively overlapping coils defined in each of the core sectors, as shown in the two lower arrangements of Figure 5, are preferably short-corded to about 60%, with each core side in its own individual slot. The overlapping coils forming a phase winding of one core sector are physically isolated from those of the other two sectors. It will be noted that the axes of the resulting windings is radial and this facilitates maximum flux linkage with the rotating radial magnetic field.

Reference may be made to "The Performance and Design of AC Machines" by Say & Pink published by Isaac Pitman for a fuller discussion of the effects of the coil span and short-cording. Since lap winding of the type proposed is more generally used in DC machines, reference can also be made to the section on "The Coil Span" in "The Performance and Design of DC Machines" by Clayton also published by Isaac Pitman.

Figure 6 shows the manner in which the terminals of each of the three different phase windings are connected in a conventional star formation to provide three phase output.

When the described machine is used as a motor, it will be noted that it operates in two distinct modes.

Each rotor lobe, when moving towards the central position of any of the three core sectors 12, experiences a strong attraction when the current in that section is at a maximum. The motor also operates in the manner of an induction motor because of the interaction of the magnetic fluxes from the stator and rotor (squirrel cage) currents. The combination of these two effects results in synchronous operation of the motor at low torques due to the first mode of operation. Increased torque is produced at low speeds from a combination of the two modes of operation.

The machine illustrated in Figures 3 and 4 is essentially similar to that already described except for the construction of the stator windings. These windings are arranged with their axes tangential to the cylindrical member 11. In this embodiment the cylindrical member is divided into two parts which support between them the core sectors 12. The windings fit, as previously, in axial slots in the sectors 12 of the core. Each winding 14 is supported on the exterior of the core sector by means of a coil support 15 to ensure firm location. In this machine it is desirable for manufacturing efficiency that the stator core be formed in more than one sector in order to allow the coils 14 to be fitted around it. Operation of this machine is on a similar principle to that previously described.

Claims (6)

1. A homopolar electrical machine comprising a lobed rotor mounted within a cylindrical casing on a shaft so as to be rotatable about a common axis of the cylinder and shaft, a substantially cylindrical laminated stator core divided into three sectors each of which is an integral laminated body defining an even number of axially extending slots on its inner surface, armature windings arranged in said slots and excitation means mounted in the casing surrounding the rotor shaft in order to produce a fluctuating magnetic field between the periphery of the rotor and the stator during rotation of the rotor.

2. A machine as claimed in claim 1, wherein the winding in each sector is arranged so that the axis of a single defined coil is radially extending.

3. A machine as claimed in claim 1, wherein coils with axes extending tangentially to the cylindrical casing are supported in the stator slots.

4. A machine as claimed in any one of the preceding claims, wherein the edges of the lobes of the rotor are inclined relative to the shaft axis.

5. A machine according to any one of the preceding claims, further comprising a fan blade fitted between each adjacent pair of lobes of the rotor.

6. A rotary electrical machine substantially as herein described with reference to Figures 1, 2, 5 and 6 or Figures 3 and 4 of the accompanying drawings.