ELECTRONIC AC POWER CONTROLLER

FIELD AND BACKGROUND OF THE INVENTION

The present invention, in some embodiments thereof, relates to a phase control system for supplying power and, more particularly, but not exclusively, to a bridge type system. Prior art systems for controlling power are well known in the art. One popular type of system utilizes phase control to change the power fed to a load. In general, such systems switch the voltage from an input to an output, with the voltage control manifesting itself in which portion of the input sinusoidal voltage is passed. Such systems are generally referred to as "phase control" systems since the control of the system is based on the firing angle of the switches used to switch the voltage.

Fig. 1 shows a typical phase control system of the prior art. In this system an input voltage from line 1 1 is monitored by a controller 61 to determine the phase of the input. The input voltage is fed to a pair of thyristors 41 and 42 whose firing is controlled by controller 61 via control lines 62 and 63. It the absence of a firing voltage, thyristors can stand off a high voltage in both directions. When the thyristor is fired, the voltage across the thyristor falls to a low value and it passes a current. When and if the current goes to zero (for example, when the voltage input crosses zero, for a resistive load), the thyristor extinguishes and reverts to its high impedance state.

In practice, as shown, a pair of thyristors is usually provided so that power is transmitted on both the positive and negative swings of the input voltage.

An example of such prior art implementation given by G. A. Dotto, US 3,394,296, "Synchronous motor stator circuit employing commutator and rectifier during starting". The control circuit for switching on the thyristors taught by Dotto is based on charging capacitors up to a fixed threshold detected by avalanche diodes.

An important characteristic of the circuitry shown in Fig. 1 is that the thyristor is fired when the voltage is high and turns off when the voltage is zero. Thus, for capacitive loads the inrush current is high. In addition, since the current only starts when the thyristor is fired, there will be a substantial phase difference between the input voltage and the current drawn from the line, i.e.., the power factor will be poor. The power factor is even worse since the phase shift between current and voltage is the same as for a motor or other inductive load.

WO 00/029853 describes a current control circuit utilizing pulse width modulation by IGB transistors which provide very fast switching and high standoff voltages.

According to an aspect of some embodiments of the present invention there is provided a circuit that receives a sinusoidal input voltage and produces an AC voltage having a variable value, comprising: an input for connection of a the sinusoidal input voltage; an output for connection to a load; and a switching circuit that transmits a controllable portion of the input voltage starting at a zero crossing thereof to the output.

Optionally, the switching circuit interrupts the transmission at a phase at which the input voltage is not zero.

In an embodiment of the invention, the switching circuit comprises: a bridge connected between said input and output and comprising four arms in which in each arm comprises a rectifier circuit which passes current in only one direction, the diodes in a direct line between input and output passing current in different directions such that no current passes from input to output in said direct line; and a unidirectional switch connected between the diodes of the two arms.

There is further provided, in accordance with an embodiment of the invention, a circuit that receives a sinusoidal input voltage and produces an AC voltage having a variable value, comprising: an input for connection of a the sinusoidal input voltage; an output for connection to a load; and a switching circuit comprising a bridge connected between said input and output and comprising four arms in which in each arm comprises a rectifier circuit which passes current in only one direction, the diodes in a direct line between input and output passing current in different directions such that no current passes from input to output in said direct line; and a unidirectional switch connected between the diodes of the two arms. In an embodiment of the invention, the circuit a controller which receives an indication of phase from the input and closes the switch at substantially zero voltage and opens the switch at a controllable phase of the input voltage.

Optionally, the switching circuit utilizes a transistor as a switching element, said switching element carrying the load current. Optionally, the transistor is an insulated- gate bipolar (IGB) transistor.

In some embodiments of the invention, the load current passes through only a single controllable switching element between the input and the output.

In some embodiments of the invention, the switching element is a unidirectional switching element.

Optionally, the circuit includes a capacitive matching circuit connected between the output and a load. There is further provided, in accordance with an embodiment of the invention a method for controlling voltage transmitted from an input AC source to a load, the method comprising: transmitting a controllable portion of the voltage of the AC source starting at each zero voltage of the input; and turning off the transmitted portion of the AC source voltage at a controllable voltage other than zero.

Unless otherwise defined, all technical and/or scientific terms used herein have the same meaning as commonly understood by one of ordinary skill in the art to which the invention pertains. Although methods and materials similar or equivalent to those described herein can be used in the practice or testing of embodiments of the invention, exemplary methods and/or materials are described below. In case of conflict, the patent specification, including definitions, will control. In addition, the materials, methods, and examples are illustrative only and are not intended to be necessarily limiting.

Implementation of the method and/or system of embodiments of the invention can involve performing or completing selected tasks manually, automatically, or a combination thereof. Moreover, according to actual instrumentation and equipment of embodiments of the method and/or system of the invention, several selected tasks could be implemented by hardware, by software or by firmware or by a combination thereof using an operating system. For example, hardware for performing selected tasks according to embodiments of the invention could be implemented as a chip or a circuit. As software, selected tasks according to embodiments of the invention could be implemented as a plurality of

software instructions being executed by a computer using any suitable operating system.

In an exemplary embodiment of the invention, one or more tasks according to exemplary embodiments of method and/or system as described herein are performed by a data processor, such as a computing platform for executing a plurality of instructions. Optionally, the data processor includes a volatile memory for storing instructions and/or data and/or a non-volatile storage, for example, a magnetic hard-disk and/or removable media, for storing instructions and/or data. Optionally, a network connection is provided as well. A display and/or a user input device such as a keyboard or mouse are optionally provided as well. BRIEF DESCRIPTION OF THE DRAWINGS

Some embodiments of the invention are herein described, by way of example only, with reference to the accompanying drawings. With specific reference now to the drawings in detail, it is stressed that the particulars shown are by way of example and for purposes of illustrative discussion of embodiments of the invention. In this regard, the description taken with the drawings makes apparent to those skilled in the art how embodiments of the invention may be practiced. In the drawings:

FIG. 1 is a prior art embodiment of a phase control system for supplying variable voltage; FIG. 2 is a conceptual general block diagram of a system for supplying variable power to a load, in accordance with an embodiment of the invention;

FIG. 3 (FIGs. 3(A)-3(D)) shows signal waveforms for the embodiments of FIGs. 2 and 5;

FIG. 4 (FIGs. 4(A)-4(C)) shows signal waveforms for the embodiments of FIG. l; and

FIG. 5 is a simplified circuit diagram of a system for supplying variable power to a load, in accordance with an embodiment of the invention.

DESCRIPTION OF SPECIFIC EMBODIMENTS OF THE INVENTION The present invention, in some embodiments thereof, relates to a phase control system for supplying power and, more particularly, but not exclusively, to a bridge type system.

An aspect of some embodiments of the invention is concerned with phase control variable voltage systems in which the output voltage starts at zero phase of the input and turns off at some other phase. This is in contrast to thyristor circuits in which the switch shuts off at zero current and thus must fire at some phase other than zero in order to provide voltage control.

In some embodiments of the invention the system employs a bridge circuit in which a, preferably unidirectional, switch, such as an IGB transistor, is used as the switching element.

An aspect of some embodiments of the invention is concerned with a phase control variable voltage systems in which the output voltage is controlled by a switching circuit comprising a bridge connected between its input and output and comprising four arms in which in each arm comprises a rectifier circuit which passes current in only one direction, the diodes in a direct line between input and output passing current in different directions such that no current passes from input to output in said direct line. For purposes of better understanding some embodiments of the present invention, as illustrated in Figures 2, 3 and 5 of the drawings, reference is first made to Fig. 2 which shows a conceptual general block diagram of a system 10 for supplying a variable voltage to a according to an embodiment of the invention.

As shown in Fig. 2, a load 31 (shown as a motor in this example) is fed by a voltage switching circuit 12 from a line source 11. In some embodiments of the invention a matching circuit 81 is provided to adjust and reduce the power factor seen by the line. As will be explained below, such matching is generally not possible for the circuit of Fig. 1.

Switching circuit 12 comprises a controller 66 which receives information on the phase of the input voltage via a line 13 and provides switching signals to a switching unit 44 which can be a unidirectional switch via a line 67. The switch is connected between two opposite junctions of a bridge circuit whose other two junctions are connected to the input and output of circuit 12, respectively.

Four diodes 441-444 are placed in the arms of the bridge oriented such that if the switch 44 is open, there is no current in the arms. Use of a bridge as shown in Fig. 2 allows the alternating current between the source and the load to flow in a unidirectional manner over the unidirectional switch 44.

It should be understood that the unidirectional switch can be any unidirectional switch which can open at a high voltage. For example, when used to control power from the line, the element must be able to reliably open at instantaneous voltages in excess of 200, 300, 400 or 500 volts and carry currents suitable for the loads used, which may be in the range of 1 - 10, 10- 100 or more amperes.

Fig. 3 shows the voltage waveforms of the circuit of Fig. 3 in operation, in accordance with a preferred embodiment of the invention.

Four waveforms are shown in Fig. 3 and are indicates as Figs 3(a)-3(d). Fig. 3(a) shows the input voltage, namely the voltage supplied to circuit 12 of Fig. 2. Fig. 3(b) shows a trigger voltage supplied by controller 66 (assuming that the switch is normally open). Fig. 3(c) shows the current waveform through switch 44 and Fig. 3(d) shows current fed from a particular matching circuit 81, in accordance with an embodiment of the invention, to the load 31.

Fig. 3(b) illustrates the switch control signal 67. The switch controller 66 closes the switch whenever the generator 11 voltage crosses zero, i.e. on phases zero and π . The switch controller 66 opens the switch twice in each cycle on predetermined time phase depending on the amount of power desired to deliver to the load.

Fig. 3(c) illustrates the current flow through the unidirectional switch 44. If the load is properly matched the current will approximately follow the generator voltage. Due to the bridge, both negative and positive voltage polarities will generate a positive current flow in the unidirectional switch 44. The current on the unidirectional switch 44 will be cut off when the controller switched off, however as illustrated in Figure 6(d) the current flow through the load 31 might continue for a while until the capacitors in the matching circuits 81 will be discharged. The nature of the current on the load 31 is obviously alternating.

It is noteworthy that for the embodiment described by Fig. 3, the switch closes at the zero crossing of the input voltage and opens at some later phase as compared to the prior art system of Fig. 1 in which the switch opens at some phase and closes at the zero crossing. Furthermore, as indicated above, the circuit Fig. 1 is not amenable to a very common type of matching for heavy loads such as motors. Most motors present inductive loads at their input. In an effort to reduce the power factor of the input it is

common to provide a large capacitance at the input to the motor so that the power factor is near 1. Since the circuit of Fig. 1 provides a large step in voltage to the load when the thyristor switches, the use of a large capacitor would cause a large inrush of current at switching. Thus, capacitive matching of motors is generally precluded when thyristor type phase voltage control is employed. Thus, in addition to the power factor reduction of the voltage control, the inherent power factor can not be corrected.

Fig. 4 shows the voltage and current waveforms for the prior art circuit of Fig. 1. It is presented as a contrast to Fig. 3 to clarify the substantial operational differences between the prior art and the embodiment of Fig. 2 of the present invention. Again, the input voltage is shown at 4(a). 4(b and (c) show the trigger waveforms for the two thyristors 41 and 42. As indicated above the thyristors are triggered in the middle of the cycle and extinguish at the zero crossings. The resulting voltage/current waveform (for a resistive load) is shown at 4(d).

The following is evident from Figs. 3 and 4 and especially from the lowermost graphs:

1) The current always leads the voltage for the circuit of Fig. 2 and always lags the voltage for the circuit of Fig. 1.

2) The phase angle between current and voltage is greatest for very short on-times and approaches π/2 and 0 when the voltage output is the same as the voltage input.

3) For the same percentage on time, if the angle for circuit of Fig. 2 is -β then the angle for the circuit of Fig. 1 is +β.

It is interesting to note that if the circuits of Figs. 2 and 1 are used for starting a motor, then for starting voltages (at which the current for a motor is usually highly inductive) the circuit of Fig. 1 adds lag to the current drawn from the line, while the circuit of Fig. 2 reduces the motor lag as seen at the line.

This, taken together with the fact that capacitive matching can be applied at the output of circuit 12 of Fig. 2, makes this circuit and any phase control circuit that has a zero start very powerful for this use. Fig. 5 shows a circuit of the type shown in Fig. 2, with two differences. Firstly the generic unidirectional switching circuit 44 of Fig. 2 has been replaced by a transistor r I-»! _

since such transistors switch very quickly and can handle high currents and voltages. IGBT has become a popular switch transistor for power application in recent years Secondly, the generic matching circuit 81 has been replaced by a power factor reducing capacitor 82. It is understood that possibly other switching elements having an ability to turn off at high voltage can be used.

It is understood that the control systems of the invention are applicable to motor control and also to providing a electric motor soft start controller that gradually increases the power delivered to the motor to avoid large peaks in current consumption occurring when the motor starts at once. This is done by switching the power on and off during each cycle while increasing the on period duty cycle with time. During soft start scenario the controller will shift the switching off phase from near zero initially to π eventually.

The invention is also applicable as a voltage control system for an air conditioner. As taught in co-pending US Provisional Patent application filed on February 25, 2008, entitled POWER SYSTEM FOR AIR CONDITIONING SYSTEMS and bearing attorney docket number 42250, the disclosure of which is incorporated by reference, the performance of an air conditioner can be significantly improved by controlling the input voltage to the air conditioner so that it is kept at a reasonably constant voltage even when the line voltage varies. The present invention is suitable for use in the control of the voltage of an air conditioner, in accordance with the teaching of that application.

The embodiments of Figs. 2, 3 and 5 have been presented as single phase control. It should be understood that the circuitry of the present invention can be utilized to control the voltage of a plurality of phases, for example by placing a circuit 12 in each phase. It is expected that during the life of a patent maturing from this application many relevant switches will be developed and the scope of the term unidirectional switch is intended to include all such new technologies a priori.

The terms "comprises", "comprising", "includes", "including", "having" and their conjugates mean "including but not limited to". The term "consisting of means "including and limited to".

The term "consisting essentially of means that the composition, method or structure may include additional ingredients, steps and/or parts, but only if the

additional ingredients, steps and/or parts do not materially alter the basic and novel characteristics of the claimed composition, method or structure.

It is appreciated that certain features of the invention, which are, for clarity, described in the context of separate embodiments, may also be provided in combination in a single embodiment. Conversely, various features of the invention, which are, for brevity, described in the context of a single embodiment, may also be provided separately or in any suitable sub-combination or as suitable in any other described embodiment of the invention. Certain features described in the context of various embodiments are not to be considered essential features of those embodiments, unless the embodiment is inoperative without those elements.

Although the invention has been described in conjunction with specific embodiments thereof, it is evident that many alternatives, modifications and variations will be apparent to those skilled in the art. Accordingly, it is intended to embrace all such alternatives, modifications and variations that fall within the spirit and broad scope of the appended claims.

All publications, patents and patent applications mentioned in this specification are herein incorporated in their entirety by reference into the specification, to the same extent as if each individual publication, patent or patent application was specifically and individually indicated to be incorporated herein by reference. In addition, citation or identification of any reference in this application shall not be construed as an admission that such reference is available as prior art to the present invention.