POWER SUPPLY CIRCUIT

Abstract: A load is supplied by transformer 10 supplied via a voltage regulator and thyristor network. The thyristor network and regulator are simultaneously controlled to respond to changes in the load and in practice the thyristors respond rapidly and are re-adjusted as the regulator is inherently more slowly adjusted. The result is that the thyristors produce minimum reflected harmonics overall in the mains and an optimum power factor can be achieved for each power setting.

Description: The invention relates to electrical power supply circuits. BACKGROUND TO THE INVENTION: There is often a need to control the power supplied to an electrical load, especially if the power has to be limited or varied in accordance with operating requirements. In French Patent Specification No A 2375755 there is proposed an alternating current supply circuit in which the voltage of the output is rectified for a variable electrical load. The circuit comprises a power transformer having a secondary winding for connection to the load, a controllable thyristor network for supplying the primary winding, and a load responsive circuit arranged to provide control signals to control the operation of the thyristor network. In European Patent Application No EP A 026866 there is proposed a circuit for charging a battery in which the firing angle of thyristors in a network controlling the voltage applied to the battery is kept at a low value. However, in both these proposals the voltage of the output is controlled. The present invention has been developed to control the power being supplied to an ohmic heater, such as described for example in UK Patent Specification A 2068200, but can be used for supplying a wide variety of electrical loads. In many cases the supply must be controlled to respond quickly and evenly to changes and also to comply with various regulations of power utility companies so as not to interfere with the power being supplied to other consumers from the same mains supply by the generation of unacceptable harmonics in the mains power supply. SUMMARY OF THE INVENTION: According to the present invention, there is provided an alternating current power supply circuit for a variable electrical load, which circuit comprises a power transformer (10) having a secondary winding for connection to the load, a controllable thyristor network (15) for supplying the primary winding, and a load responsive circuit (16) arranged to provide control signals to control operation of the thyristor network (15), characterised in that there is provided an adjustable voltage regulator (13, 14) connected to the primary winding of the transformer (10), and in that the load responsive circuit (16) is also arranged to provide control signals to adjust the voltage regulator (13, 14) in a manner to tend to maintain the thyristors of the said thyristor network (15) in their conductive states for a predetermined maximum period of each cycle of respective half-wave voltage of the incoming supply (12) so that overall inherent harmonic reflections are kept to a minimum. DESCRIPTION OF THE PREFERRED EMBODIMENTS: Three alternating voltage power supply circuits according to the invention will now be described by way of example with reference to the accompanying drawings which show in Figures 1 and 2 circuit diagrams of the circuits. Referring to the drawing, a power transformer 10 has terminals 11 on its secondary winding for connection to a variable electrical load. The primary winding is connected at one end to a mains supply terminal 12 and its other end is connected to a movable sliding contact 13 on a voltage regulator 14, in the form of an autotransformer arrangement. The regulator 14 is connected between the output side of a thyristor network 15 and the mains supply terminal 12. A load responsive circuit 16 is connected to receive signals from a load sensing transducer 17 and arranged in use in response to signals from the transducer to vary the operation of the thyristors and simultaneously to adjust the position of the contact 13. The described circuit is arranged to vary the supply to alter the output of the transformer 10 as the load or the requirement for supplying the load vary. For example in a food processing plant as described in UK Patent Specification No: 2068200, the flow rate of the medium being processed and its temperatures are continuously monitored by one or more transducers and the output of the transformer 10 varied to keep the effective amount of heating of the medium within a desired predetermined range. The overall control made possible by use of the described circuit enables the inherent rapid response of the thyristor circuit to be used to make rapid adjustments for any rapidly required changes in the power output of the transformer 10. This is in contrast, certainly, to the inherently much slower response of the described regulator 14 which incorporates mechanical moving parts. However, by using the regulator 14 to vary the primary voltage, the thyristors can be maintained to turn on always in each cycle close to the zero crossings of the mains voltage so that resultant harmonics generated in the system are always of low power. Clearly, the thyristors must always be turned on somewhat after the crossing, even when at supposed optimum adjustment for any load, to allow the control circuit to respond to all changes in the load, that is changes in both directions, as required. In practice, the control circuit is arranged to simultaneously and continuously control the operation of the thyristor circuit 15 and the movement of the contact 13 to achieve the best conditions for minimum power harmonic generation and maximum power factor. This can be achieved for example by making, when necessary immediate, changes to the output of the transformer 10 using the facility of rapidly changing the effecting mark space ratio of conduction of the thyristors. However, the control circuit is arranged to also adjust, but inherently more slowly, the regulator 14 to alter the primary voltage in response to this load change while controlling the mark space ratio to compensate for the regulator alterations as the contact 13 is moved. Thus for example, if the load reduces the power can be rapidly reduced by delaying the switch- on of the thyristors and then as the regulator reduces the voltage the thyristors are progressively switched on earlier until the optimum balance is achieved for the new lower load conditions. In Figure 2, the supply circuit is generally the same and controlled in a similar manner to the supply circuit in Figure 1 except the voltage regulator is in the form of an auxiliary transformer 20 supplied by a second thyristor network 21. The load sensing circuit in Figure 2 controls the operation of both thyristor networks 15 and 21 simultaneously. In figure 3 the supply circuit is generally the same and controlled in a similar manner to the supply circuit in figure 1 and the auxiliary transformer 20 and thyristor network 21 as in fig 2, but in this circuit the main transformer has secondary taps and a tapping switch. The taps are arranged for load impedance ranges. With this system the auxiliary transformer and its thyristor primary control are considerably reduced in rating and hence size because the range of control can be much reduced by selecting the correct tap in anticipation or precalibration of the overall load to be supplied by the circuit. It is possible to place the thyristor network between the voltage regulator and the transformer primary winding although simultaneous control of the thyristor network and regulator is then somewhat more complex in practice.

Claims: An alternating current power supply circuit for a variable electrical load, which circuit comprises a power transformer (10) having a secondary winding for connection to the load, a controllable thyristor network (15) for supplying the primary winding, and a load responsive circuit (16) arranged to provide control signals to control operation of the thyristor network (15), characterised in that there is provided an adjustable voltage regulator (13, 14) connected to the primary winding of the transformer (10), and in that the load responsive circuit (16) is also arranged to provide control signals to adjust the voltage regulator (13, 14) in a manner to tend to maintain the thyristors of the said thyristor network (15) in their conductive states for a predetermined maximum period of each cycle of respective half-wave voltage of the incoming supply (12) so that overall inherent harmonic reflections are kept to a minimum. A circuit according to Claim 1, characterised in that the voltage regulator (14) includes a slidable contact (13). A circuit according to Claim 1, characterised in that the voltage regulator (14) comprises an auxiliary transformer (20) supplied in use by a second thyristor network (21). A circuit according to any of Claims 1-3, characterised in that the power transformer (10) is provided with a tapped secondary winding.

Inventors: SOAN, BASIL
Applicants: APV UK LIMITED
Application year: 1989
Main classification: H02M5
IPC codes: G05F1, H02M1, H02M5