WO1987007787A1

WO1987007787A1 - Ac to ac converter

Info

Publication number: WO1987007787A1
Application number: PCT/AU1987/000169
Authority: WO
Inventors: Ewart Trevor Osborne
Original assignee: Uni-Power International Pty. Ltd.
Priority date: 1986-06-11
Filing date: 1987-06-10
Publication date: 1987-12-17

Abstract

An AC to AC converter comprising a multiphase AC power supply (12), a rectifier means (16) connected to the multiphase AC power supply (12) to rectify each of the phases (A, B, C) thereof, a switch means (14) connected to cause the phases (A, B, C) to be switched in and out of circuit, the rectifier means (16) or the switch means (14) having two outputs (20, 22) to which is connected an inverter (24) having an AC output (58, 60) for a load (L) and a control means (70) connected to the inverter (24) to cause switching thereof at a desired frequency to produce a relatively fixed frequency AC supply at the AC output (58, 60).

Description

TITLE

AC TO AC CONVERTER

DESCRIPTION

The present invention relates to an AC to AC converter particularly envisaged for use in providing an AC output whose frequency is substantially independent of an AC supply thereof.

FIELD OF THE INVENTION In general AC to AC converters and DC to AC converters comprise electronic switch means to periodically switch a supply of AC or DC power to produce an AC supply at a desired voltage and frequency at an output.

However, in such converters there is generally an isolation between the supply of AC or DC power and the output, the isolation being created by the electronic switch means. Such isolation results in undesirable supply characteristics at the output. In particular the output, by virtue of said isolation, is usually unable to provide good transient supply to a load. Whereas, the supply of AC or DC power may be capable of good transient supply the transient supply at the output is limited^'by the transient capability of the electronic switch means. Accordingly, it is necessary to overdesign the power handling capacity of the electronic switch means to attempt to overcome the problem. However, increase in power handling capability does not necessarily produce a better speed of response at the output.

Another solution to the isolation problem is to use an alternator wired to produce a mains voltage and mains frequency output. However, such requires a dedicated motor with a throttle to set the motor speed to produce the required voltage and frequency. Such dedicated motor/alternator supplies are generally inefficient in operation at light loads. SUMMARY OF THE INVENTION

The present invention provides an AC to AC converter in which the frequency at an output thereof is substantially independent of the frequency of an AC supply thereof. In accordance with one aspect of the present.invention there is provided an AC to AC converter characterised in that it comprises a multiphase AC power supply, a rectifier means connected to the multiphase AC power supply to rectify each of the phases thereof, a switch means connected to cause tne phases to be switched in and out of circuit, the rectifier means or the switch means having two outputs to which is connected an inverter having an AC output for a load and a control means connected to the inverter to cause switching thereof at a desired frequency to produce a relatively fixed frequency AC supply at the AC output. It is envisaged that a multiphase transformer could be interposed between the multiphase AC power supply and the switch means. For the purposes of the present invention high frequency is a frequency substantially above mains frequency, such as, for example, greater than about lOOftz.

For the purposes of the present invention low frequency is a frequency substantially the same as or less than mains frequency, such as for example, less than about 50Hz or 60Hz.

The present invention will hereinafter be described, with particular reference to a multiphase AC power supply derived from a vehicular alternator although it is to be understood that it is of general applicability. For example, the multiphase AC power supply could be derived from a DC to AC converter having a corresponding multiplicity of output transformers.

BRIEF DESCRIPTION OF THE DRAWINGS The present invention will now be described, by way of example, with particular reference to the accompanying drawings, in which:- Figure 1 is a circuit diagram of a multiphase AC power supply, a rectifier means, a switch means and^"an inverter of anAC to AC converter in accordance with the present invention.

Figure 2 is a block diagram of a control means of the AC to AC converter of Figure 1;

Figure 3 is a circuit diagram of a leg of the inverter of the AC to AC converter of Figures 1 and 2; Figures .4 to 6 are circuit diagrams of alternative wiring arrangements of the multiphase AC power supply, the rectifier means and the switch means for star configured phases of Figure 1; and Figure 7 is a graph showing a typical step wise increasing and decreasing switching sequence for the phases of the AC to AC converter of Figures 1 and 2. DESCRIPTION OF THE INVENTION

In Figure 1 there is shown a part of an AC to AC converter 10 in accordance with the present invention and comprising a multiphase AC power supply 12. In the present embodiment the power supply 12 is in the form of 3 alternator winding phases A, B and C. For the purposes of the present invention multiphase is considered as being two or more phases. However, it is to be understood that the power supply 12 coulα be in the form of 2 or more transformer secondaries. The frequency of the supply at each phase A, B or C is dependent upon the speed of the engine, to which the alternator is attached and the consequent speed of the alternator rotor. Typically the frequency of each of the phases A, B and C is in the range from 100Hz to 20kHz, more typically in the range from about 300Hz to 6kHz, such as 1kHz to 2kHz. The AC to AC converter 10 also comprises switch means 14 in the form of two SCR's per phase A, B and C. Each of the SCR's SCR1. to SCR6 has an associated, voltage divider and an opto coupled TRIAC DCl to DC6 to control it. Each of the TRIAC's DCl to DC6 has a driver L.t-D LDl to LD6 (see Figure 2) optically coupled thereto.

The driver LEDs LDl and LD2 are configured to operate the TRIACS DCl and DC2 to operate the SCR's SCRl and SCR2 respectively. The SCR's SCRl and SCR2 are converted to switch the phase A into and out of circuit and to alter the direction of current flow through the phase A.

The pairs of components, LEDS LD3 and LD4 and TRIACS DC3 and DC4 and SCR's SCR3 and SCR4 are connected to switch the phase B into and out of circuit and to alter the direction of current flow through the phase B. Similarly, for phase C mutatis mutandis .

The switch means 14 has connected to it a rectifier means 16 with the phases A, B and C connected therebetween. The recitifier means conveniently, in the present embodiment, comprises a series arrangement of diodes Dl to D6 with two of the diodes Dl to D6 per phase A, B and C.

The diodes Dl and D2 are in a full wave bridge arrangement with the SCRs SCRl and SCR2 with the phase A connected between nodes Al and A2 of the bridge. The phases B and C have a similar arrangement of the diodes D3 to D6 and the SCRs SCR3 to SCR6 with nodes Bl, B2, Cl and C2.

In the present embodiment the voltage produced by each of the phases A, B and C is added together by the operation of the SCRs SCRl to SCR6. For example, with all of the SCRs SCRl, 3 and 5 and 2, 4 and 6 set active the ouputs of each of the phases A, B and C are added together in circuit. However, if all of the SCRs SCRl to 6 are inactive all of the phases are disconnected from circuit. In the present embodiment the switch means 14 forms part of the rectifier means 16. The arrangement of the switch means 14 and the rectifier means 16 has a high potential node 20 and a low potential node 22. Conveniently, the phase B has one of its ends connected to ground potential so that the high potential node 20 is at a positive potential and the low potential node 22 is at a negative potential, although such is not essential. It is to be understood that half wave rectifier bridges or a combination of half wave and full wave bridges in series connection or parallel connection or in a switch series combination could be used, as shown in Figures 4 to 6.

The nodes 20 and 22 are connected to an inverter 24. The inverter 20 comprises two legs 26 and 28 connected between the nodes 20 and 22. Each of the legs comprises a first switch SWl and a second switch SW2 as shown in Figures 1 and 3. The first switch SW1 conveniently comprises an SCR30 controlled by an opto coupled TRIAC 32 with bias resistors 34 and 36. The first switch SW1 is intended to be operated oncethere is little or no current flowing through the SCR30.

The second switch SW2 conveniently comprises one or more MOSFETS 38 controlled by an opto coupled transistor 40 via a power supply 42 derived and regulated from either thenodes 20 or 22. The power supply 42 comprises a rectifier 44, a regulator 46 connected thereto to power a schmitt trigger 48. An output 50 of the schmitt trigger 48 drives a gate of the or each MOSFET 38. The second switch SW2 is intended to be com- mutated at any load up to full current load. A free wheel diode 52 and a snubber circuit 54 are provided as shown in Figure 1 to protect the or each MOSFET 38.

The legs 26 and 28 are connected in a bridge arrangement with nodes 58 and 60 arranged to have a load L connected therebetween. Preferably, a capacitor 62 of about 5 to1000/.f, such as, for example, about 15/Uf is connected across the nodes 58 and 60 to provide power factor correction and filtering for the inverter 24.. The switch SW1 of leg 26 and the switch SW2 of leg 28 are controlled by a control means 70 (see Figure 2) viaphoto diodes 72 and 74 to allow current to flow through the load Lin one direction in one mode and similarly for switch SW1 of leg 28 and switch SW2 of leg 26 and photo diodes 76 and 78 to allow current flow through the load L in an opposite direction in another mode. That is to say the inverter 24 is controlled to periodically alternate the direction of flow of current through the load L.

The AC to AC converter 10 also comprises a capacitor C connected in parallel with the nodes 20 to 22. Alternatively a capacitor (not shown) would be connected in parallel with each pair of diodes Dl, D2 and D3, D4 and D5,D6. Preferably, the capacitor has a value of between 10 andlOOOjF, such as for exampl , about10O^uF. It has been discovered that the provision of the capacitor C produces an improved power output _for the AC to AC converter 10 of about 160%. The capacitor C also leads to a reduction in generation of r.f.i. The AC to AC converter 10 also comprises a voltage regulator 80 connected between the nodes 20 and 22 and configured to control the supply to a field winding of the alternator to control the voltage generated by the phases A, B and C.

^'The control means 70 conveniently comprises an EPROM 82 connected to a binary counter 84 having a clock input 86 derived from a high frequency oscillator 88 via a divider 90. The binary counter 84 sets an address in the EPROM 82 whereat there is programmed a particular segment of output conditions. A buffer 92 couples the outputs of the EPROM 82 to the LEDS LDl to LD6.

A flip flop 94 and two AND gates 96 and 98 are connected to the EPROM 82 and via a buffer 100, having outputs 102 and 104 to the inverter 24. The flip flop 94 operates to switch the inverter 24 between its two modes of conduction.

The EPROM 82 is programmed to produce a set wise sequence by connection of the phasesA, B and C as described hereinabove, and which sequence is typified by that shown in Figure 7. The sequence causes a half of a sinusoidal type wave to be produced between the nodes58 and 60. Once a sequence is completed the flip flop 94 is operated to switch the inverter from one mode to the other to produce a supply of power at the load L which is low frequency and substantially free of harmonics and distortion, such as for example a relatively smooth 50Hz mains supply of power.

It has been found to be particularly important to be able to produce a current waveform free of harmonics and distortion at the load L. The AC to AC converter 10 of the present invention has been found to substantially provide such load supply. It is envisaged that the power supply 12 could comprise a low voltage AC supply, such as a low voltage multiphase alternator and a multiphase transformer to produce the high voltage.

It is envisaged that snubber circuits could be placed across all of the SCRS.

In use, the AC to AC converter 10 has its nodes 58 and 60 connected to the load L, which load may be resistive, capacitive and/or inductive. The alternator is operated in known manner to produce power in the phases A, B and C and a relatively high frequency such as, for example, between 100 Hz and 20kHz, typically between 300 Hz to 6 kHz, particularly between 1kHz to 2kHz.

The EPROM 82 by virtue of the counter 84 activates its outputs to activate the LEDS LDl to LD6 to control the SCRs SCRl to SCR6 in a step wise manner, such as that shown in Figure 7. In Figure 7 the SCRl is first activated. Then whilst active, the SCRs SCR2 to 6 are sequentially activated, the delay in actuation represented by the displacement of the graph to the right. In reverse manner the sequence steps down. During the above operation high frequency power^" is supplied from the connected phase or phases A and/or B and/or C rectified by the rectifier means 16 and the switch means 14 to the inverter 24. With the switch SWl of leg 26 and SW2 of leg 28 active current flows through the load L from left to right. The power being substantially cosine squaredand with power factor correction by the capacitors C and 62.

Once the sequence of Figure 7 (conveniently referred tα as a half cycle) is completed the flip flop 24 operates to deactivate the switch SWl of leg 26 and SW2 of leg 28, then delays and then activates switch SWl of leg 28 and SW2 of leg 26 to set current flow in the load from right to left.

Simultaneously, an active signal is generated on output 106 of the EPROM 82 to a reset input 108 of the counter 84. The active signal at the reset input 108 restarts the counter 84 to replay the sequence of Figure 7. The frequency of the AC output between nodes 58 and.60 is set by the frequency at input 86 to the counter 84. Conveniently, the frequency at input 86 is 6kHz, which frequency generates 60 steps in the counter to produce the 12 steps shown in Figure 7 and therefore a 50Hz alternating supply of power at the AC output 58, 60. The instantaneous voltage appearing between nodes 20 and 22 during the half cycle is the instantaneous sum of the rectified voltage wave forms appearing at the outputs of the rectified phases A, B and C. This is an instantaneous addition of cosine squared waveforms and can lead to a relatively smooth signal at the nodes 20 and 22, particularly where switching of the SCRs .SCRl to 6 is effected using zero crossing detectors. Such also reduces r.f.i. It has been found that due to the cosine ≤quar-ednature of the rectified waveforms the power supplied to the load L has a substantially smooth and clean sine wave appearance. It has been found preferable, and beneficial to the shape of the current waveform at the load L, to allow a period of dead time at the end of a half cycle shown in Figure 7.

Where the timing means 70 is an EPROM or the like the frequency of the power supply to the load may be altered by altering a clock frequency of the EPROM. The arrangement of Figure 4 may be operated with the switching sequence of Figure 7 to operate substantially the same as that of Figure 1. The arrangement of Figure 5 comprises a gate turn off device 109 and driver 109a to produce up to 8 steps in the switch sequence.

The arrangement of Figure 6 comprises two gate turn off devices 109 and drivers 109a to produce only four steps in the switch sequence. It has been found that since the load L is virtually connected to the phases A, B and/or C the AC to AC converter 10 has an inherent ability to accommodate heavy short term loads.

Also the output at the nodes 58 and 60 is substantially independent of the alternator frequency of operation. It is envisaged that a feed back network could be arranged to control the alternator speed in dependence of the load L requirement. For example, a ramp up ramp down facility could be provided to align the voltage at the load L with the frequency. It is also envisaged that three of the AC to AC converters 10 could be arranged to give a 3 phase voltage at the load L. Such arrangement could be provided with a frequency and voltage ramp up and ramp down to facilitate easy starting of 3 phase motors at low load. It is also envisaged that means could be provided to alter the sequence used to control the SCRs SCRl to 6 in dependence of the nature of the load L. It is also envisaged that an under voltage/over voltage sensor 110 could be connected to alter the EPROM 82 sequence in the event of detection of an under or over voltage condition of the AC output 58, 60. It is also envisaged other switching devices could be used in place of the MOSFETS 38, such as for example Gate Turn Off devices (GTO's) . In the context of the present invention inverter is considered to mean a DC to AC converter.

Preferably the SCR's SCRl to 6 are switched in a sequence such that there is equal sharing of load between the phases A, B and C. Preferably, the multiphase AC power supply 12 has an end of one of its phases connected to ground potential so as to substantially halve the potential of the nodes 58 and 60 above ground potential and thus reduce the likelihood of electricution thereby.

In the context of the present invention the number of steps in the sequences, such as that of Figure 7, are counted as the total number of steps up and down. It is envisaged that the number of steps in the sequence could be greater than the 12 steps shown in Figures 7, such as, for example, 24 steps. It has been found that since the multiphase AC power supply 12 is a relatively high frequency, small low less transformers could be used to step up the voltage thereof prior to rectification.

Accordingly, a relatively low voltage multiphase AC power supply 12 could be used and said transformers could be used to increase the voltage thereof prior to. rectification.

The capacitor 62 has been found to aid in filtering of the output power together with power factor correction for the load. It is to be understood that the amount of power factor correction required may vary with the nature of the load.

Modifications and variations such as would be apparent to a skilled addressee are deemed within the scope of the present invention. For example, a microcomputer could be used in the control means 70. Accordingly the waveform could be altered in dependence of the load.

Claims

1. An AC to AC converter characterised in that it comprises a multiphase AC power supply, a rectifier means connected to the multiphase AC power supply to rectify each of the phases thereof, a switch means connected to cause r.e phases to be switched in andout of circuit, the rectifier means or the switch means having two outputs to which is connected an inverter having an AC output for a load and a control means connected to the inverter to cause switching thereof at a desired frequency to produce a relatively-fixed frequency AC supply at the AC output.

2. An AC to AC converter according to claim 1, characterised in that the control means is connected to the switch means to cause each of the phases to be switched in and out of circuit for desired periods of time to produce step wise increases and decreases in the voltage at two outputs, the control means being configured to cause repetition in the step wise increases and decreases.

3. An AC to AC converter according to claims 1 and 2, characterised in that the number of step wise increases and decreases in one repetition total between 1 to 2 .

4. An AC to AC converter according to any one of the preceding claims, characterised in that the multi- phase AC power supply is configured to operate at a relatively high frequency and the inverter is controlled by the control means to operate at a relatively lower frequency.

5. An AC to AC converter according to any one of the preceding claims, characterised in that the multiphase

AC power supply is in the form of a multiphase alternator.

6. An AC to AC converter according to any one of the claims 1 to 4, characterised in that the multiphase AC power supply is in the form of a multiphase transformer secondary.

7. An AC to AC converter according to any one of the preceding claims, characterised in that the phases of the multiphase AC power supply are wound electrically independently of each other.

8. An AC to AC converter according to any one of the preceding claims 1 to 6, characterised in that the phases of the multiphase AC power supply are wound in a star configuration.

9. An AC to AC converter according to claim 8, characterised in that a star point of the star configured phases is connected to ground potential.

10. An AC to AC converter according to any one of the preceding claims 1 to 9, characterised in that a capacitor is connected in parallel with each of a full wave bridge of the rectifier means or with all of the full wave bridges.

11. An AC to AC converter according to any one of the preceding claims, characterised in that the multiphase AC power supply is coupled to a transformer having a corresponding number of phases.

12. An AC to AC converter according to claim 7, characterised in that one end of one of the phases of the multiphase AC power supply is connected to ground potential.

13. An AC to AC converter according to any one of the claims 1 to 9, characterised in that a capacitor is connected in parallel with the AC output.