SINGLE TO THREE-PHASE VOLTAGE CONVERTER

Abstract: A rotary phase converter adapted to supply a three-phase power signal to a three-phase electrical load (5) from a single-phase power signal, comprising: a dynamoelectric machine (3) having a rotor which rotates at constant high speed and three stator windings; a phase changing means (6); a load sensor means (14) adapted to monitor a load signal; and a stabilising means which acts to minimise variations in said load signal, comprised of either an autotransformer (44) or a series of capacitors (10).

Description: SINGLE TO THREE-PHASE VOLTAGE CONVERTER The present invention relates to a single to three-phase converter, and more particularly to such an apparatus which includes a rotary dynamoelectric machine. It is well known that the generation and transmission of electrical power is more efficient in polyphase systems than in single-phase systems. Three-phase motors are also more efficient and less expensive to operate than single-phase motors, the latter having several drawbacks including limitations in their horsepower rating. The potential applications for a single to three-phase converter are widespread, from farm machinery, to industrial uses as well as commercial and domestic applications. The development of a single to three-phase converter is particularly important in agriculture, where a great deal of farm machinery operates with three-phase motors. The installation of three-phase lines from the local county council supply is generally too costly for the average farmer to afford, so, until now, a farmer has had to install separate diesel run generators to operate pumps and other machinery. This involves considerable expense in fuel and labour in refilling and maintaining such generators in different parts of the farm. For small jobs, not requiring continuous power generation, three-phase tools have been run off a tractor motor. Much industrial equipment also requires a three-phase supply source, for example, welding tools, air compressors, paper balmers, machine tools, pumps, cream separators, silo unloaders, etc. Commercial and domestic uses for a three-phase voltage source are also widespread, for example, fans, refrigeration, battery chargers, air conditioners and swimming pool heaters. In most of these applications, on-line three-phase voltage is not readily available except at great expense, and its cost is not justified by its limited use. There are several known attempts to create a rotary phase converter which is capable of supplying three-phase power indefinitely as long as the electrical load does not exceed the rated capacity of the converter. A major problem with these known converters is that, as load conditions vary, electrical inbalances are created which result in unbalanced phase currents in the three-phase load. Some of these approaches are shown in U.S. Patent No. 3,271,646 to A.J. Lewis and U.S. Patent Nos. 4,079,446 and 4,249,237 to CM. Hertz. While these approaches attempt to improve the three-phase power inbalances somewhat, they are far from an adequate and economical solution. Such approaches include adding additional capacitance to the systems, internally tapping the Stator windings of the converter, and providing a tapped autotransformer to compensate for load inbalances. The present invention seeks to substantially overcome the above problems by providing a novel and effective single to three-phase converter, with a variable autotransformer. The present invention also seeks to provide a three-phase power supply which has features comparable to a normal three-phase line, and provides a . dependable power source to operate three-phase motors and other equipment. The present invention also seeks to provide a single to three-phase converter with a power factor close to unity - indicating high efficiency. • The present invention also seeks to provide a mobile three-phase supply which is easily installed by doing away with pole transformers, poles and wiring, which also requires minimum maintenance and supervision, and which provides quiet running. The present invention also seeks to provide a durable three-phase supply which will handle tough applications such as welding equipments, air compressors, battery chargers, water pumps, refrigeration and other equipment, which enables motors of varying horsepower, speed, etc. to operate simultaneously or individually from a single-phase source. The present invention also seeks to provide a three-phase power source which does not require capacitor starts as in single-phase motors, which operates with no difficulty on motors which require long starting cycles, frequent stops and starts, instant reversing, or multi-speed motors, or Y or connected motors. The present invention also seeks to provide a single to three-phase converter which features safety protection devices and which has no regenerative feedback effects back into the council's single-phase system from which the converter is operated which through wave distortion could cause undesirable harmonic effects. The present invention in its broadest aspect provides a rotary phase converter for connecting to a single-phase a.c. power signal and adapted to supply a three-phase signal to a three-phase electrical load, said converter comprising, a dynamoelectric machine having a rotor which rotates at constant high speed and three stator windings connected to a first, second and third output terminals, said single-phase a.c. power source being connected across said first and said second output terminals, a phase changing means connected across said first and said third output terminals, and a load sensor means adapted to monitor a load signal drawn by at least one of said output terminals and to supply a feedback signal to a stabalising means which acts to minimise variations in said load signal. These objects of the present invention will become apparent from the following detailed description of the invention in connection with the accompanying drawings, in which: Fig. 1 is a circuit diagram of a preferred embodiment of the present invention; Fig. 2 details the operation of a variable autotransformer in conjunction with the current/voltage sensors; Fig. 3 shows the phase generation of the three-phase voltage from a single-phase voltage; Fig. 4 shows the simplified relationship in which a load may be connected to the single to three-phase converter. Fig. 5 shows the connection diagram by which several three-phase loads may be connected to the rotary phase converter of the present invention; and Fig. 6 illustrates how safety switches are preferably incorporated into the rotary phase converter of the present invention. In Fig.l, a single-phase disconnect switch 1 is provided with fuses where applicable before the tapped transformer 2. The tapped transformer 2 may be selected to be of any desired turns ratio. For example, in Australia, normal council-supplied mains voltage is 240V and three-phase equipment is generally designed to run off a 415V three-phase supply. In this case, a 240V to 415V step-up transformer is selected. Thus, the desired step-up/step-down voltage can be chosen by selecting the turns ratio of the transformer or by selecting an appropriate tap of the transformer. The step-up transformer 2 is constructed preferably from top grade transformer steel and all copper windings for minimal losses. This provides maximised efficiency by reduction of losses, either due to heat developed in the resistance of the windings known as copper losses, or due to heat build-up due to the eddy currents in the core resulting in hysteresis losses known as iron losses. The 240V to 415V step-up tapped transformer 2 may be earthed on its secondary side by the earthing line 29 or, at the converter motor star point connection, by the earthing line 30 if required by local safety standards. The required stepped-up voltage is then applied to a star (Y) connected rotating machine 3 which is preferably built with all copper windings and incorporates a tapped winding in the stator 4 and a segmented rotor design. The outputs of the rotating machine 3 provides the desired three-phase power signal to be connected to the load 5. Also connected to the star (Y) connections of the rotating machine 3 is the circuitry which provides the phase-shifting to obtain the required 120 phase shifts between the three voltage signals. The phase-shifting is more clearly explained later with reference to Fig. 3. A first capacitor bank 6, shown in the diagram as comprising of four oil-filled or polypropylene-filled capacitors Cl, C2, C3 and C4, is connected across two of the three-phase voltage lines 7 and 8, providing rigid phase shifting. When the machine 3 is rotating, an electrical field helped by the capacitive action from the capacitor bank 6, causes a leading current to occur within this field, and the constant high speed of the rotor of the machine 3 generates the third phase. The range of rotor speeds at which the three-phase generator will operate is between 1280 r.p.m. to 3800 r.p.m. and its optimum speed is 1500 r.p.m. The mechanism of this invention may, therefore, be more accurately defined as a phase generator. The generation of one voltage, which, when paralleled with the other two voltages generated from a single-phase line, achieves the production of true three-phase power. A second capacitor bank 10, consisting of three oil-filled, polypropylene-filled or electrolytic capacitors C5, C6 and C7, may be used to provide overall balancing, when large individual loads are connected to the single to three-phase converter. The single to three-phase converter is designed to withstand the expected load power required to operate several motors and equipments. The manner in which various loads may be connected to the single to three-phase converter is shown in Fig. 5. The converter is typically designed to supply the necessary power to operate larger farms and machine shops. When several three-phase machines are either simultaneously or individually operated in any sequence or duty cycle as required, the capacitor bank 10 is extremely useful to dampen any under or over-current effects produced. The remainder of the circuitry of Fig. 1 consists of a current and/or voltage sensor 14, a 415V to 12V voltage transformer 18, a 12V or 24V dc rectifier 21, a magnetic contacter 19, and a contact transfer switch 20. If a voltage sensor is to be used instead of a current sensor, a 450V to 12V voltage transformer 18 should be used, the secondary terminals of which are connected to terminals 11, 12 and 13 of the current and/or voltage sensor 14, in place of the wires 15, 16 and 17 as shown in Fig. 1. In this case, the potential transformer 18, will be used to supply A.C. for rectification to the D.C. system supplying a D.C. voltage to the sensor unit 14. Fig. 2 shows a diagram of a more preferred embodiment illustrating the operation of the variable autotransformer in conjunction with the voltage/current sensors. Two current transformers 41 and 42 are provided around two of the three-phase output lines 7 and 8 to sense variations in the current drawn by the load. The variable autotransformer 44 consists of the winding 45, the threaded shaft 46 which moves the contact 47 electrically connecting the winding 45, and the coupling 48 which connects the autotransformer to the bidirectional motor 43. The contact 47 is preferably a carbon stud contact. As the current transformer sensing devices 41 and 42 senses a change in the current drawn by the load 5, a signal is sent to the transducer 40 which interprets a change and sends it to the amplifier 50. The amplifier 50 amplifies its received input to a magnitude suitable to drive the coil of the relay 51. When the line voltage falls below a nominal voltage with the increase in current drawn by the load, the normally open contacts of the relay 51 close, thus energising the magnetic contact or 52 which consequently activates the bidirectional drive motor 43 driving the threaded shaft 46, to change the position of the contact 47 with the autotransformer winding. This operation thus changes the position of the winding until the required nominal voltage are reached and the current drawn by the load is stabalised. At this moment, the relay coil 51 is then de-energised, the relay contacts returning to the normally open position, to consequently deactivate the drive motor 43. Fig. 3 illustrates in more detail the generation of the three-phase signals from the single-phase supply. The first signal 23 at 0O phase shift is obtained directly from the secondary of the transformer, i.e. 415 volts through the step up transformer 2. The secondary signal 22, which has a 120o phase shift, is obtained when the rotating magnetic field of the converter motor is formed which, cutting its own conductor being spaced electrically at 120o develops the second phase shift. The third signal 24, which is 120o out of phase with the second signal 22 and the first signal 23, is obtained by means of the constant high speed of the rotor of the rotating machine 3, causing the electrons to flow through the third conductor which is spaced electrically 120o apart from the first and second conductors. The capacitor, connected to lines A and C or the first and third signal lines 23 and 21, maintains this constant phase shift partiularly when a change of power factor occurs. The exact phase relationship between the three components of the three-phase system is also shown in Fig. 3, which clearly demonstrates that each component is exactly 120o apart from the adjacent components, resulting in a complete 360o angle. The sinusoidal waveform shown in Fig. 3, demonstrates that if 415V single-phase is connected to Phase A of the rotating machine, then at 30 on the signal-phase waveform, the voltage is 207.5V, and on the three-phase waveform likewise. At 60°, the single-phase voltage is 359.39V, again likewise on the three-phase waveform. At 90 ' , the single-phase voltage is at its maximum of 415V, also likewise on the three-phase output waveform. B and C phases follow respectively, that is, at 207.5V, B phase position is 150° which corresponds to a 120° seperation from A phase, so too is C phase, at 207.5V itsposition is 270 , here again, a seperation from B phase byl20°. The formula given for this calculation is V = maximum Voltage x Sin &. This indicates that the first phase, i.e.Phase A is in phase with the single-phase input signal supplying this converter. In order to neutralise the impedance drops of the windings, the third leg of the converter motor winding is connected to the capacitive circuit at all time. Together with the balancing unit, this third leg will act as a rigid phase converter and passing a fixed amount of power to the three-phase supply line serving the other motors as required on the load end, and maintain this phase shift position at all time. Fig. 4 shows the relationship in which a load may be connected to the single to three-phase converter. The diagram illustrates the single to three-phase converter 24, incorporating a secondary pilot motor 25 and a capacitor 26. These features act to stabalise and produce a more rigid three-phase supply. The diagram shows the connections of these components to the three-phase lines 7, 8 and 9, and the connections to star or delta connected motor 27. The figure thus shows that the motors and equipments connected to the single to three-phase converter at the present invention require no additional components or modifications to that required for standard three-phase supplies available from the council lines. Therefore, if the council three-phase supply is connected at a later date, no modification is required to the equipment. Fig. 5 shows the connection diagram by which several three-phase loads may be connected to the single to three-phase converter. The single to three-phase converter is preferably constructed such that an operator is not required to switch-in or add additional capacitance to compensate for the particular load. The provision of the correct value of capacitance, depending upon voltage input and output may be calculated and incorporated into the single to three-phase converter. Thus, it is not left up to the skill of the operator to select the correct value, which eliminates the danger of possible selection of the incorrect value. In Fig. 5, two three-phase outlets 53 and 54 are shown, with different sized loads 55 and 56 connected thereto. Capacitor 57 is chosen depending on the load 55 and input/output voltages, and connected to the three-phase outlet 53. Similarly, capacitor 58 is chosen depending on the load 56 and the input/output 54. Additional componets also shown in Fig. 5 are the active and neutral links of the single-phase input signal 61 and 62 the single to three-phase converter stop/start switch 63, the stop/start switches 64 and 65 for each of the three-phase outlets 53 and 54, the converter magnetic contacter 66, a magnetic contacter balancing unit 67, control relay 68, and a MIG welding outlet 69, control relay 68, and a MIG welding outlet 69, also provided with it's phase-balancing capacitive circuit 70. Additional safety features, incorporating separate start/stop switches, are also preferably incorporated as part of the single to three-phase converter. In Fig. 6, the connections for incorporating separate start/stop, switches are shown. Switch 59 is connected to the three-phase outlet 53 to start load 55, and switch 60 is connected to the three-phase outlet 54 to start load 56. The single to three-phase converter of the present invention is preferably provided as a package which separate outlets for loads of different sizes, safety cut-out devices, thermal overload protection, magnetic contactors to the motors and earth leakage. For welding applications, a special safety plug and socket is preferably provided, such that the plug cannot be removed from the socket when the switch is in the on position. This avoids causing current and voltage inbalances if higher than necessary values of capacitance remain in the circuit after a particular loaded condition, The advantages of this invention of a single to three-phase converter provided with a variable autotransformer are numerous and include the following. County Council line voltages can be regulated to provide the voltage required to operate motors at their rated full load current. Proper earthing is provided such that the possibility of receiving electric shocks is reduced, a much needed safety feature. The converter will always be portable in as much as 240V will normally be available on a given installation. 240V normally consists of one active wire and the other neutral to ground wire. This in effect means that any supply voltage having one active wire may be used to convert to a three-phase line. This is unlike many other converters which convert the same voltages as the incoming supply, in the majority of cases two active wires from the council transformer which also incorporate a centre tapped neutral, providing 240, 240 and 480V. The 480V is not true single-phase as two active wires are required and in many cases not available. A number of County Councils do not favour a 480V line, as many problems are encountered with the 480V supply, especially motors. When starting load motors, the inrush current is limited, which is favourable to the power supplier. The inrush current of a three-phase motor connected to the converter, is about 30% less than the same motor operating on a conventional three-phase solid system. In effect, reduced voltage is accomplished. The efficiency of the system and the motors is comparable to that of a three-phase line; it is much better than a single-phase motor. The control cubicle comes as a package with the converter and includes safety cut out devices. It also provides thermal overload protection, magnetic contactors to the motors plus earth leakage if required. It is inexpensive, requires little maintenance compared with the cost of installation of a conventional three-phase council line, which is often not economically feasable nor readily available. Additionally for short seasonal loads or small loads, it is generally impractical. The converter cost is normally just a fraction of the cost of extending a three-phase line. Since this converter operates on a single-phase line, the unit rate costs are less, with no demand or power factor penalties. Three-phase equipment to be used with the converter would be numerous, ie: welders, air compressors, paper bales, machine tools, pumps, cream separators, silo unloaders, fans, refrigeration, battery chargers and air conditioning to name a few. It will be appreciated that the present invention provides a novel and effective method of single to three-phase conversion. It will be recognised by persons skilled in the art, that variations and modifications can be made to this invention without departing from the overall scope of the invention as described herein.

Claims: THE CLAIMS ; 1. A rotary phase converter for connecting to a single-phase a.c. power signal and adapted to supply a three-phase signal to a three-phase electrical load, said converter comprising, a dynamoelectric machine having a rotor which rotates at constant high speed and three stator windings connected to a first, second and third output terminals, said single-phase a.c. power signal being connected across said first and said second output terminals, a phase changing means connected across said first and said third output terminals, and a load sensor means adapted to monitor a load signal drawn by at least one of said output terminals and to supply a feedback signal to a stabalising means- which acts to minimise variations in said load signal. 2. A rotary phase converter as claimed in claim 1 wherein said stabalising means comprises variable autotransformer which is electrically connected to said stator windings. 3. A rotary phase converter as claimed in claims 1 or 2 wherein said stabalising means comprises at least one capacitor electrically connected between said first and said third output terminals. 4. A rotary phase converter as claimed in claim 3 wherein each of said capacitors may be switched by a switching means in response to variations in said load signal. 5. A rotary phase converter as claimed in any one of claims 1 to 4, wherein said load sensor means comprises a current or voltage sensing device which electrically co-operates with at least one of said output terminals to detect variations in the current or voltage of said output terminal. 6. A rotary phase converter as claimed in any one of claims 1 to 5, wherein said load sensor means further comprises a contact driven by a bidirectional motor on a threaded shaft, said contact being electrically connected to one of said variable autotransformer, such that said contact may be moved by said bidirectional motor in response to said variations in the current or voltage of said output terminal detected by said current or voltage sensing device to change the position of said contact relative to said variable autotransformer and to consequently counteract said variations. 7. A rotary phase converter as claimed in claim 6, wherein said contact is a carbon stud contact. 8. A rotary phase converter as claimed in claims 6 or 7, wherein said load sensor means further comprises a transducer/amplifier means and a switching means, such that when said current or voltage sensing device detects said variations, said transducer/amplifier means energises said switching means, and said switching means activates said bi-directional motor. 9. A rotary phase converter as claimed in claim 8, wherein said switching means comprises a relay and a magnetic contactor, such that when said transducer/amplifier means energises said switching means, said relay is activated to control said magnetic contactor, and said magnetic contactor operates said bidirectional motor to move said carbon stud contact relative to said variable autotransformer in a direction which would counteract said variations. 10. A rotary phase converter as claimed in any one of claims 1 to 9, wherein said phase changing means comprises at least one capacitor, the capacitance of which is selected to provide substantially a 12QO electrical phase difference between each of said output terminals.' 11. A rotary phase converter as claimed in any one of claims 1 to 10, further comprising a voltage transformer wherein a single-phase a.c. power source is connected to a primary winding of said voltage transformer, and said single-phase a.c. power signal is produced at a secondary winding of said voltage transformer, wherein said primary and said secondary windings are selected such that the magnitude of said single-phase a.c. power signal is chosen to be substantially equivalent to the magnitude of said three-phase signal. 12. A rotary phase converter as claimed in claim 11 wherein said voltage transformer is a 240V:415V step-up voltage transformer, the primary winding of said transformer being connected through a safety fuse and switch to a 240V input source, and the secondary winding of said transformer providing a 415V output voltage. 13. A rotary phase converter as claimed in claims 11 or 12, wherein said secondary winding is provided with an earthing line. 14. A rotary phase converter as claimed in any one of claims 1 to 13, wherein said dynamoelectric machine is a star (Y) - connected dynamoelectric machine. 15. A rotary phase converter as claimed in claim 14, wherein said star (Y) - connected rotating machine is provided with an earthing line at it's star-point connection. 16. A rotary phase converter as claimed in any one of claims 1 to 13, wherein said dynamoelectric machine is a delta ( ) - connected dynamoelectric machine. 17. A rotary phase converter as claimed in any one of claims 1 to 16, wherein said rotor of said dynamoelectric machine rotates between 1280 r.p.m. to 3800 r.p.m. 18. A rotary phase converter as claimed in any one of claims 1 to 16, wherein said rotor of said dynamoelectric machine rotates at approximately 1500 r.p.m. 19. A rotary phase converter as claimed in any one of claims 1 to 18 wherein said three-phase electrical load is comprised of at least one three-phase machine and said rotary phase converter further comprises at least one phase-balancing means, each of said phase-balancing means being connected between said first and said third output terminals and having reactance which is substantially matched to characteristic reactance of each of said at least one three-phase machine respectively. 20. A rotary phase converter as claimed in claim 19, wherein each of said phase-balancing means comprises at least one capacitor. 21. A rotary phase converter as claimed in claims 19 or 20, wherein said rotary phase converter further comprises at least one outlet socket such that each of said three-phase machines may be electrically adapted by a plug to each of said outlet sockets, and wherein one of said phase-balancing means is electrically connected to one of said outlet sockets respectively. 22. A rotary phase converter as claimed in claim 21, wherein each of said outlet sockets is provided with a safety switch, such that each plug of each of said three-phase machines cannot be connected or disconnected whilst said three-phase machine is in an operative state. 23. A rotary phase converter as claimed in any one of claims 1 to 22 in use with welding equipment. 24. A rotary phase converter substantially as herein described with reference to the accompanying drawings.

Inventors: LING PING, LING, PING
Applicants: INNOVATIVE ELECTRICS PTY LTD, INNOVATIVE ELECTRICS PTY. LIMITED
Application year: 1986
Main classification: H02M5
IPC codes: H02M5