Technical Field.

Electro-mechanical Rotary Power Converters (ERPCs) are used for converting incoming electrical energy into outgoing electrical energy. The energy transformation may be either an incoming Direct Current (DC) to outgoing single or polyphase Alternating Current (AC); or an incoming single or polyphase AC to outgoing DC; or an incoming single or polyphase AC to outgoing polyphase or single phase AC. The ERPC rotor is driven by a motor to facilitate the electromagnetic phenomena, however in an ideal converter, the energy supplied by the motor, apart from accelerating the inertial mass to its steady state speed, only feeds mechanical losses within the converter.

Background Art.

Edison in the late 1870s first established the electric power business. Power was generated and distributed as DC, because then only DC motors existed. Very soon however, Westinghouse employed Stanley, who in 1886 developed an effective transformer. Later, in 1888 Westinghouse purchased the patents for Tesla's AC induction motor. By 1890 the Westinghouse Company was installing substantial high voltage AC transmission systems feeding lower voltage distribution systems. Until 1920, the AC and DC systems were largely competitive due to the AC to DC rotary converter, which could supply power to the commonplace DC motors. Demands for higher power levels and longer circuits however required higher voltages. Eventually DC power systems were constrained by the voltage factor. The AC systems due to the transformer, could apply the higher voltages with ease. This led to the dominance of AC over DC. DC however is now emerging as an effective and sometimes superior power transmission alternative. The resurgence of HVDC started in the 1930s and 40s when during World War II the Germans developed plans to implement an HVDC link using electronic mercury arc valves. These were to be configured as AC to DC and DC to AC power converters. The war however prevented its implementation. After the war in 1950, the Russians adopted the plans and built a successful 120kV, 65 mile long line from Moscow. From that initial scheme, HVDC power transmission has developed via power electronics through mercury arc valves, to semiconductor thyristors and latterly transistors. Now many of the world's largest power transmission schemes apply HVDC.

As power and distance limits have increased over the years, it is the turn of AC transmission schemes to be constrained. Excessive currents due to line charging and line voltage instability set power transfer limits, especially in cable systems. Power companies have consequently been turning to HVDC transmission.

To operate, a modern HVDC Power Electronic Converter (PEC), in the rectifier mode, breaks up the AC waveform and reconstructs it into DC. It does the same in the inverter mode from DC to AC. The disarrangement-rearrangement however is not a straight process reversal between the rectifier and inverter. The resulting pieced together waveforms are imperfect and contain harmonics. Energy losses occur due to conduction and switching in the semiconductor devices. Reduction of the harmonics is possible by filtering, however energy losses are then also incurred in the filters. The overall amount of

equipment required is substantial and costly. Because of their complexity and size, HVDC PECs are generally air insulated. The converter chambers contain exposed live parts and staff must follow special procedures when working within their vicinity.

In the aircraft industry multi-polar ERPCs (Moffit, G. R. "Variable reluctance device". US patent, 3041486, 6/1962) have been used for DC to polyphase AC conversion at 400Hz. A particular feature of the ERPC is its ability to maintain a very stable output voltage level when its input DC is controlled to a constant level. The number of poles and winding arrangement that these devices employ however restrict their suitability for'utility'sized machines.

In the field of energy production, some devices such as fuel cells inherently produce DC at a relatively low voltage. Most power systems however utilise AC at a substantially higher voltage. To be useful for the bulk supply of electrical energy, the output of a fuel cell would typically be converted to AC using a PEC, and then the PEC output voltage boosted to utility levels by a conventional transformer. An ERPC that combines both of these operations could replace both the PEC and transformer.

Disclosure of Invention.

A new form of ERPC, namely the (Homopolar) HERPC is disclosed. It has many advantages over the multi-polar ERPC, the PEC, and the PEC-transformer combination.

The HERPC would be substantially safer by not having exposed live parts; be relatively small and compact consisting only of a small number of assembled parts; be easy to manufacture due to its rotating homopolar machine construction; be extremely reliable, similar to that of an AC motor; have a high efficiency; retain the very stable output voltage level characteristic of the ERPC; and as with the transformer, the terminal output voltage would be determined in part by the number of turns on the respective windings. As an inverter it would develop near perfect AC waveforms.

Brief Description of Drawings.

The following figures illustrate an example of the device construction and the principle of operation. Other alternative arrangements are possible.

Fig. 1 shows the basic arrangement for the Rotary Power Converter. The upper part represents a cross section along axis of rotation. Three other cross sections, perpendicular to the axis of rotation, at defining locations, are also depicted.

Fig. 2 shows the electrical interconnections between the coils, the source of DC excitation and load burdens connected to the three phase AC output coils.

Fig. 3 shows the mathematical function used for a particular shape of the stator AC winding limb face.

Fig. 4 shows the progression of the magnetically conductive portion of the rotor (rectangular section) across the stator limb faces at intervals of 30° degrees.

In Fig. 1 the following components and features are identified: 1. Driving motor.

2. Principal winding coil.

3. Stator body.

4. Magnetically conductive rotor parts.

5. Non-magnetically conductive rotor parts.

6. AC phase winding coil.

7. Rotor and stator surfaces meet.

8. Non-magnetically conducting spacer in stator body.

9. Non-magnetically conducting locating pin to assist assembly of stator parts.

10. Cross section through rotor and stator, perpendicular to axis of rotation.

11. Pole bearing stator limbs.

In Fig. 2 the following components and features are identified: 2. Principal winding coils.

6. AC phase winding coils.

12. Typical source of DC for the device.

13. Typical load burden for the device.

Best Mode for Carrying Out the Invention.

The description is written to describe a three-phase device, however the general principles extend to a polyphase machine.

Each AC phase winding has coils 6, that encircle each stator limb that bears a pole 11, in the respective phase set. A principal winding 2, has coils that encircle all the stator limbs that bear poles 11, on all the phase sets. The principal winding is wound such that with a constant DC source, the flux always flows either radially inwards or radially outwards in all pole bearing stator limbs 11.

Energy conversion is performed by varying the reluctance of the three-phase sections of the magnetic circuit through turning the rotor.

The rotor 4 & 5, has no windings. Its function is to divert the magnetic flux on a time varying basis through each of the three-phase windings 6, on the stator pole limbs 11. It consists of some magnetically conducting portions 4, and some non-magnetically conducting portions 5. The non-magnetic portions 5, are required to channel the flux and prevent magnetic short circuits.

The output AC waveforms are determined by the shape of the AC winding limb faces. The induced AC voltage is proportional to the rate of change of flux passing through the winding. The flux however is distributed in proportion to the reluctance of the magnetic circuits-which is in turn proportional to the overlapping areas between the passing rotor and stator faces.

The mathematical function used for a particular shape of the stator AC winding limb face is shown in Fig. 3. It shows an area that changes in the form of a sinusoid. It is created from a function that is itself the combination of a positive and negative sinusoid.

At any one time, the sum of all the three-phase AC fluxes is equal to the flux flowing through the stator body 3. The sum of all the three-phase AC fluxes also equates to the total flux generated by the principal winding 2. The reluctance of the stator body, and all magnetic circuit paths in parallel with it remain constant.

In Fig. 4 the overlapping area between all facing rotor and stator parts 14, is shown at intervals of 30 degrees. In particular it shows how the area corresponding to each phase, changes in a sinusoidal pattern 15, as a result of the shape of the AC winding limb faces.

An auxiliary motor 1 drives the rotor 4 & 5, at the appropriate speed to generate the desired output AC frequency. Unlike an alternator, no electromagnetic restraining force acts. The driving torque is required for inertial accelerations and to counter friction and windage losses.

The magnetic circuits are arranged such that flux links through windings rather than cutting across conductors. The Lorentz force law (F=B. I. L) that applies to generators is therefore minimal. Restraining forces on the rotor are those due to friction and windage.

A non-magnetically conductive spacer 8, is incorporated in the stator body 3. The spacer is secured by means of a non-magnetically conductive locating pin 9. By adjusting the spacer thickness, the relationships between the input and output currents and voltages may be altered.

The cross sections 10, in Fig. 1 show two AC winding coils 6, per phase. As the rotor turns through one complete rotation, one frequency cycle of flux passes through each of the coils. Fig. 2 therefore shows two series connected coils per phase, resulting in two cycles of output current per revolution of the rotor.

Frequency conversion also occurs between the input and output currents. The frequency transformation is determined by the speed of the driven rotor.

Industrial Applicability.

In offshore oil and gas production facilities, the HERPC may be used to power installations from land based Combined Cycle Gas Turbine onshore power plants. By interconnecting using HVDC cables, and replacing offshore Gas Turbines with electric motors, more reliable and near maintenance free offshore facilities with overall reductions in C02 emissions would be possible.

In the energy utility business the HERPC could replace the PEC inverter, especially for the power conditioning of fuel cell output.

On land based power grid systems, the likely reduction in costs and increased reliability of the HERPC plant over PECs would encourage underground cable installations in place of overhead power lines.

Acquisition of the resulting narrower and un-intrusive wayleaves by energy utilities would be far easier.

The elimination of overhead power lines is commonly seen as a major benefit to the environment.

The HERPC may alternatively be applied as a brushless Synchro, generally used for position sensing applications, in which case the principal winding would be excited from an AC source.