GB2071934A - Changing the frequency of a power supply - Google Patents

Abstract

The frequency of a power supply signal is changed by continuous modulation of each phase of the supply voltage, combining of the modulated signals if there is more than one phase and further processing to produce a supply signal of a single frequency. As shown, the modulated signal is rectified and the rectified signal is inverted to produce the signal of single frequency. In an alternative arrangement, the supply signal is rectified before being modulated. The modulation waveform may be sinusoidal or triangular to provide amplitude modulation. <IMAGE>

Description

SPECIFICATION Improvements in or relating to frequency changers This invention relates to frequency changers and in particular to a new technique of frequency changing using a continuous modulation system.

With the advent of modern semiconductor technology a variety of power frequency changer schemes, such as inverters and cycloconverters, have been evolved to satisfy the requirements and needs of industry. These schemes are based on the principle of modulating a sine wave of power frequency with a rectangular modulating function producted by thyristor elements to form a discrete amplitude modulation. The control circuits of these traditional frequency changers are very complex owing to the relatively involved logic functions that need to be performed and the large numbers of thyristors that require gate signals. Continuous modulation of power signals using sinusoidal and triangular modulating functions has not yet been realised. This type of modulation would eliminate the complexity in the control circuits by the use of diode elements instead of thyristors as modulating elements.

The present invention provides a method of changing the frequency of a power supply, the method comprising continuously modulating the supply voltage with a substantially sinusoidal or substantially triangular continuous waveform signal, and processing the resultant signal so as to produce a signal of a single frequency.

Preferably the modulating signal is of substantially sinusoidal or triangular waveform.

Preferably the modulation is amplitude modulation.

Preferably the processing comprises demodulation.

Preferably the processed power signal is to be fed to a resistive, inductive, or induction motor system.

The use of a frequency changing method in accordance with the present invention thus utilises fewer thyristors than conventional systems, so requiring simpler control circuitry, whilst giving an output power supply signal of a single frequency and with a higher degree of efficiency than many conventional systems. The output is also relatively smooth, so not requiring a high degree of filtering.

The present invention further provides a frequency changer for a power supply, comprising means for continuously modulating the supply voltage with a continuous waveform and means for processing the resultant signal so as to produce a signal of a single frequency.

Preferably the modulation of the supply signal is amplitude modulation.

Preferably the modulation of the supply signal is by a power frequency sinusoidal or triangular waveform signal.

Preferably each phase of a poly-phase supply is separately modulated before being combined so as to give a single signal.

Preferably the demodulation includes rectification.

More preferably the rectification is carried out before modulation of the supply signal.

Preferably the power supply is of more than a single phase. More preferably it is of three or more phases.

Examples of the present invention will now be described, by way of example only, and with reference to the accompanying drawings, in which: Figure 1 is a block diagram of a single phase frequency converter in accordance with the present invention; Figure 2 shows the equivalent circuit of the converter of Figure 1; Figures 3a, b, c, d, e, fand g are representations of voltages in the converter of Figure 1; Figures 3h,iandjshowvoltages in a three phase to single phase frequency converter based on the converter of Figure 1; Figure 4 is a single composite circuit performing the functions of the converter of Figure 1; Figure 5 is a three phase to single phase converter based on the converter of Figure 4; and Figure 6 shows output waveforms obtained from the converter of Figure 5.

A block diagram of a single-phase to single-phase converting system is shown in Figure 1. The modulator, which produces continuous modulation by mixing two power frequency voltage waves is shown in Figures 2(a). The input voltage v5 = V1 sincoSt represents the carrier wave supplied by centretapped input transformer while the modulating voltage vm = V2sin(o,t-pr) is being supplied by the modulating transformer. The equivalent circuit of the modulator, Figure 2(b), consists of two separate circuits A and B with the output voltages V,1 and V02 taken across the two primary windings of the centre-tapped output transformer.

In circuit A rectifier element D1 conducts whenever its anode is positive with respect to its cathode and blocks whenever its cathode is positive with respect to its anode. Output voltage vOa of circuit A is thus governed by the supply voltage (carrier) v,, the modulating voltage vm and diode D1 as shown in Figure 3(a). If v5 is greater than vm, diode D1 conducts and voltage v01 is simply the difference between v5 and vm. When vm becomes equal to or greater than v5, diode D1 blocks and no output voltage appears on the output transformer and the output voltage v0 is thus shown in Figure 3(b). In a similar manner voltage V02 of circuit B is obtained as shown in Figure 3(c), (d).

If the output of the modulator is considered to be the difference between voltages Vo1 and V02 then the output voltage v0 = v0 - v02 is approximately an ideal amplitude modulated wave and corresponds to double side band with suppressed carrier wave in communication engineering (DSBSC), with upper and lower frequency bounds (o)s + )m) and (cu, ( m) respectively, as shown in Figure 3(e). The modulation index m5 of the modulated wave may be defined as the ratio (V2N1).

In power engineering applications it is desirable to produce a voltage at the desired frequency without much loss in power, since high efficiency of conversion is an important factor. The elimination of one sideband of the two generated frequency bounds is impractical since it means loss of nearly half the power generated. An alternative technique for producing a single frequency voltage wave is by using a demodulation approach. If the output DSBSC wave of the modulator circuit is rectified by a full-wave rectifier bridge as shown in Figure 3 (f), and if an inverter stage using thyristor switching is employed to reverse every other half cycle of the output of the full-wave rectifier bridge, the resultant output voltage wave is shown in Figure 3(9).It is obvious that the mean of this wave is a sinusoid of frequency (o)m/2). Neither the upper nor the lower sideband frequency bounds appears in the output demodulated waveform and thus the requirement of producing a single frequency is met.

The modulating voltage can also be a triangular waveform, which is generally easier to produce. In such a case the resultant wave is very similar to that obtained by using sinusoidal modulating voltages.

The output voltage of the single-phase to singlephase converter is a tOm/2 sinusoid with considerable ripple component, Figure 3(9). In order to improve the waveform of the oututvoltage by reducing the ripple distortion component, a three-phase to singlephase conversion may be used. Here, the available three-phase supply is used to feed three similar power modulators to produce three amplitude modulated waves, 120 degrees apart in time-phase. Let each phase of the three-phase output of the modulator circuits be passed through a separate full wave rectifier bridge and the three bridges are connected either in cascade arrangement or tied in parallel. A computer simulation of the three waveforms and the resultant output waveforms for series and parallel rectifier stage arrangements are shown in Figure 3(h), (i) and (j) respectively.The ripple component of the final "summed" or "simulated" output voltage is drastically reduced.

The circuit shown in Figure 4 combines the three stages of the converter circuit in a single composite circuit to meet the same requirement of frequency changing based on a continuous modulation approach. This modified composite single-phase to single-phase converter reduces or eliminates the high circulating currents which are produced in the earlier above-described circuits in the absence of any limiting resitance in the modulator circuit. In the circuit of Figure 4 the rectification process is performed priortothe modulation in an attempt to use the output centre-tapped transformer of the inverter stage also as a modulating transformer. The output resultant modulated wave is exactly the same as that obtained with the three-stage, single-phase conversion, shown in Figure 3(9).

Instead of using a centre-tapped single-phase input transformer, a three-phase transformer may supply the three carrier voltages, 120 apart in time phase. This produces the three-phase to singlephase converter as shown in Figure 5. To produce a three-phase to three-phase converter it is necessary to use three modulating voltages 120 apart in time phase.

The three-stage and modified single-phase converters, shown in Figures 1 and 4 respectively, were designed and tested in the laboratory. Experimental results were found to agree well with the theory.

A poly-phase to single phase converter circuit was constructed on the basis of continuous modulation, as shown in Figure 5. Oscillograms of the resultant output wave with modulation index at 50% obtained from three-phase and six-phase to single phase conversions are shown in Figure 6. It is obvious that the degree of improvement of the output waveform depends largely on the number of phases used in the input carrier. However, the waveforms indicated represent the output voltage without using any filtering equipment.

A three-phase version of three-phase to singlephase converter was also constructed and tested with resistive load, highly inductive load and an induction motor. With resistive load, the converter showed fewest problems since there is no stored energy in the load that may disturb the modulation and commutation processes. With indicative load, unfortunately, the stored energy cannot be returned to the supply through return diode elements since the addition of such diodes disturbs the process of modulation and ruins it completely. A solution to this problem may be achieved by adding another capacitor Cpp, Figure 5, to absorb any energy fed back during the commutation process of the inverter stage. Oscillograms of the performance of this converter with resistive load (VR) and inductive load (vL) are shown in Figure 7.The converter was also tested with a dynamometer loaded 1.5 kwthreephase induction motor. Oscillograms of the waveforms of the 25 Hz stator voltage v2 and current i5for zero modulation, no load and 50% modulation, half full load, are shown in Figures 8 and 9 respectively.

Accordingly the present invention provides a new power frequency modulating system based on a continuous amplitude technique. The system exhibits excellent linear modulation characteristics and gives a good power conversion efficiency since it employs the principle of demodulation instead of single side band suppresion techniques.

The basic modulator circuits may be modified using a continuous modulation approach to establish and construct a new three-phase static frequency changer with 50 Hz input and variable output fundamental frequency. Forced commutation or natural commutation occurs depending on the value of the modulation index.

Afrequency changer in accordance with the present invention has advantage over conventional frequency changers, such as inverters and cycloconverters, in that it is more simple, requires no comples triggering circuits due to the few thyristors used, needs no filtering equipment because the output voltage and current waveforms and very smooth and it is also free from spikes and undesirable discontinuities.

Claims (16)

1. A method of changing the frequency of a power supply, comprising the continuous modulation of the supply signal by a continuous waveform, and processing the modulated signal, so that a singal of a single frequency is produced.

2. A method according to claim 1 wherein the modulation is by a substantially sinusoidal or substantially triangular waveform.

3. A method according to claim 1 or claim 2 wherein the modulation is amplitude modulation.

4. A method according to any of claims 1,2 or 3 wherein the processing of the signal includes rectification.

5. A method according to claim 4 wherein the rectification of the supply signal takes place before the modulation of the signal.

6. A method according to any of the preceding claims wherein the supply signal is of more than one phase.

7. A method according to claim 6 wherein the supply voltage is of three or more phases.

8. A method according to claims 6 or 7 wherein each phase of the supply voltage is separately modulated.

9. Apparatus for changing the frequency of a power supply, said apparatus comprising means for continuously modulating the supply signal by a continuous waveform and means for processing the modulated signal so as to produce a signal of a single frequency.

10. Apparatus according to claim 9 wherein the modulating means are amplitude modulation means.

11. Apparatus according to claims 9 or 10 wherein the modulating waveform is a power frequency sinusoidal or triangular waveform.

12. Apparatus according to claim 9 or claim 10 wherein the processing means includes demodulating means.

13. Apparatus according to claim 12 wherein the demodulating means includes a rectifier.

14. A method according to claim 1 and substantially as described herein.

15. Apparatus according to claim 9 and substantially as described herein.

16. Apparatus according to claim 9 and substantially as described herein and with reference to the accompanying drawings.