FR2489621A1 - Electronic load switching circuit with harmonic suppression - has switching pulse generator coupled to asynchronous voltage generator - Google Patents

Abstract

The circuit coupling the load (1) to the mains network (2,3) has an electronic switching element (4) coupled via its control electrode to a pulse generator (5,6,7) synchronised with the mains frequency. This pulse generator (5,6,7) is coupled to a second providing a periodic voltage which is asynchronous with the mains frequency. Pref. the switching element comprises a triac (4) with the control pulse generator (5,6,7) comprising a resistor (5), a capacitor (6) and a diac (7) coupled to the trigger of the triac (4). The other electrode of the diac (7) is coupled to the junction between the resistor (5) and the capacitor (6), receiving the pulses from the asynchronous generator (8). The latter is pref. supplied from the mains via a diode rectifier bridge. The circuit has a reduced level of harmonics upon chopping of the load current to adjust the power.

Description

DEVICE FOR REDUCING THE RATE OF HARMONICS GENERATED BY
THE PHASE CUTTING OF THE CURRENT FLOWING IN A LOAD
The present invention relates to a device for reducing the level of harmonics generated by the phase cutting of the current flowing in a load, said load being connected to the mains by at least one electronic switch, the control electrode of which is connected to a generator. pulses synchronous with the mains frequency.

 If phase cutting is a flexible and efficient system for controlling or regulating the power applied to a load, it nevertheless suffers from the serious defect of generating a harmonic rate of the sector frequency directly proportional to the value of the current control.

However, a high level of harmonics constitutes a hindrance in the operation of an electricity distribution network, which has led certain interprofessional organizations to enact standards fixing a percentage limit of the rate of the different harmonics generated by the users.

 For the phase-cutting system, the limit rate is reached for relatively low powers of use, which limits its range.

 I1 is theoretically possible to remedy this defect by having a low-pass filter between the sector and the controlled device but, having regard to the powers involved, this solution is very expensive and, therefore, inapplicable to "general public" devices. .

 By analyzing the conditions under which the harmonics are generated, it can be seen that the most important rate is reached when the control or regulation regime corresponds to the maximum instantaneous current in the load; this is the case, for example, for harmonic 3, the rate of which is maximum under these conditions for a load supplied with full-wave rectified current.

 To reduce the harmonic ratio, it is therefore necessary to avoid switching the load to the maximum instantaneous current during each half-wave, while maintaining an effective current corresponding to the desired power level.

 The object of the invention is to substantially increase the limit level of power of use of the devices controlled or regulated by phase cutting beyond which the level of harmonics exceeds the recommended standards.

 According to the invention, the device for reducing the level of harmonics generated by the phase cutting of the current flowing in a load, said load being connected to the mains by at least one electronic switch, the control electrode of which is connected to a pulse generator synchronous with the frequency of the sector is notably remarkable in that said pulse generator is coupled to a generator supplying an asynchronous periodic voltage with respect to the frequency of the sector.

 The asynchronous periodic voltage of zero mean value is added algebraically to the control voltage of the pulse generator and thereby displaces the trigger point of the electronic switch at its own frequency during mains alternations, the value of effective current remaining unchanged.

 By judiciously choosing the frequency of the asynchronous generator, the rate of the most troublesome harmonics is greatly reduced.

 It should however be noted that the device according to the invention causes the appearance of harmonics of complex ranks resulting from the combination of the different frequencies involved; however, the level of these harmonics is well below the recommended standards.

 The shape of the asynchronous periodic voltage applied to the pulse generator has little influence on the result, which makes it possible to choose according to needs a generator of rectangular, sinusoidal, sawtooth, trian gular signals, etc.

 The amplitude of the asynchronous periodic signal superimposed on the trigger signal can vary within wide limits without strongly influencing the reduction rate of the harmonics; a value of the order of one third of the tripping voltage has been found to be suitable.

 The field of application of the invention includes all phase-cutting systems controlling or regulating a current in a load, whether this current is alternating or direct, and whether the load is of a resistive or reactive nature such as a motor. for example.

 The description which follows with regard to the appended diagrams will make it clear how the invention can be implemented.

 FIG. 1 represents the block diagram of a phase-cut power dimmer equipped with the device according to the invention.

 FIGS. 2a and 2b respectively represent the form of the output voltage of a signal generator and that of the current flowing in the load of the diagram of FIG. 1.

 Figure 3 shows a practical embodiment of the device of Figure 1 according to the invention.

 FIG. 4 represents the block diagram of an alternative embodiment of the device according to the invention applied only to the adjustment of the speed of rotation of a DC motor.

 FIGS. 5a and 5b respectively represent the form of the signal output voltage and that of the current flowing in the motor of the diagram in FIG. 4.

 FIG. 6 represents the block diagram of the device according to the invention applied to an assembly for regulating the rotation speed of a DC motor.

 FIG. 7 represents the schematic diagram of an assembly for regulating the speed of rotation of the drum drive motor of a washing machine provided with the device according to the invention.

 Figures 8 and 9 show diagrams of two other possible embodiments of the generator of periodic signals fitted to the device according to the invention.

 In FIG. 1, one of the ends of a load 1 is directly connected to the terminal 2 of the sector, the other end being connected to the terminal 3 by a triac 4; at the terminals of the triac is a network consisting of a variable resistor 5 and a capacitor 6, the point common to these two components being coupled, on the one hand to the trigger of the triac 4 by a diac 7, and on the other hand to the output of a generator 8 of periodic signals.

 In the absence of the generator 8, the operation of the power dimmer of FIG. 1 is well known: at the start of each half-wave, the capacitor 6 charges through the resistor 5, and when the breaking voltage of the diac 7 is reached , the trigger of the triac 4 receives an impulse which makes the latter conductive until the end of the alternation, and the process is repeated at the next alternation by modifying the value of the resistance 5, the point of variation is varied priming of the triac with respect to the origin of the alternation, and therefore the effective current flowing in the load 1.

 According to the invention, the generator 8 supplies a periodic signal (FIG. 2a) which alternately increases and decreases the voltage across the terminals of the capacitor 6 at an asynchronous frequency relative to that of the sector, for example 37 Hz.

 Figure 2b, whose time scale is the same as that of Figure 2a, shows in broken lines, the current flowing in the load for a certain adjustment of the resistance 5 when the generator 8 is absent, and in solid lines the same current when said generator is in service, the hatched areas corresponding, in this case, to the conduction regime of the triac.

 In the absence of the generator 8, the conduction of the triac occurs towards the middle of each half-wave at the moment when the instantaneous current is maximum, which results in a maximum rate of harmonics.

 From instant tl to instant t2, the generator 8 supplies a positive voltage which is added to the positive voltage of the capacitor 6 during the first positive alternation API in the sector, thus advancing the triac ignition point at the negative alternation following AN1, the positive voltage of the generator 8 is subtracted from that negative of the capacitor 6, thus delaying the starting point of the triac.

 At the next positive alternation AP2, the ignition takes place at the same place as for the alternation AP1 since the voltage of the generator 8 is always positive.

At time t2, the voltage of generator 8 becomes negative and it is the turn of the following negative half-waves
AN2 and AN3 to see their starting point advanced, while the starting point of the positive alternation AP3 is delayed and so on.

 The frequency of repetition of the signal of FIG. 2a not being in exact relation with that of the sector, the starting points of these groups of alternations move cyclically and only rarely occur at the maximum instantaneous current, which very significantly reduces the level of harmonics; on the other hand, the value of the effective current flowing in the load 1 is unchanged compared to that which exists in the absence of the signal of figure 2a.

 In FIG. 3, the references of which are common with those of FIG. 1, a heel resistor 9 is arranged in series with the variable resistor 5 while a current limiting resistor 10 is disposed between the diac 7 and the trigger of the triac 4.

 Terminal 2 of the sector is connected by a resistor 11 to the anode and cathode joined by a first pair of diodes 12 and 13, while terminal 3 is directly connected to the anode and cathode of a second pair of diodes 14-and 15 between the combined cathodes of the diodes 12, 14 and the combined anodes of the diodes 13, 15 are arranged a Zener diode 16 and an electrochemical capacitor 17.

 The generator 8 includes a comparator circuit 8, the positive input of which is connected to a resistor bridge 19, 20 arranged at the terminals of the capacitor 17; the output of comparator 18 is back-coupled to the positive and negative inputs respectively by a fixed resistor 21 and an adjustable resistor 22, said output being moreover connected to the positive terminal of the capacitor 17 by a resistor 23. A capacitor 24 is disposed between the negative input of comparator 18 and the negative terminal of capacitor 17 while a resistor 25 connects the output of generator 8 to capacitor 6 of the dimmer.

 The diode bridge 12, 13, 14 and 15 of the assembly of FIG. 3 plays a double role: it ensures on the one hand the supply of direct current to the generator 8, and it allows on the other hand to supply the capacitor 6 a signal conforming to that of FIG. 2a, with a value of zero mean voltage, which is necessary to obtain both the same effective current value as in the absence of a signal, and to avoid the appearance of an average component of direct current in the load 1 which would result from a cissymetry in the coraraande of the set of positive and negative alternations.

 The rectangular signal generator 8 is of a known type, the recurrence frequency of which is linked to the values of the resistor 22 and of the capacitor 24; the amplitude of the signal applied to the capacitor 6 can be adjusted by varying the value of the resistor 25.

 As already indicated, the shape of the periodic signal supplied by the generator 8 of the device according to the invention is not critical and can be sinusoidal, jagged, etc., the only condition to be observed being a duty cycle as close as possible to 0.5 in order to avoid an asymmetry generating the faults mentioned above.

 In FIG. 4, the references of which are common with those of FIGS. 1 and 3, the load 100 is constituted by a direct current motor provided with inductors with permanent magnets and arranged in the diagonal of a rectifying bridge comprising two diodes 26 and 27 and two thyristors 28 and 29, the triggers of the latter being connected to the common point of the resistor 5 and of the capacitor 6; the other end of the resistor 5 is connected to the combined cathodes of two diodes 30, 31, the anodes of which are connected to the mains, while the other armature of the capacitor 6 is connected to the combined cathodes of the thyristors 28 and 29.

 At the start of each rectified alternation in the sector, the capacitor 6 charges through the resistor 5 and when the breaking voltage of the diac 7 is reached, the triggers of the thyristors 28 and 29 receive a positive pulse which makes them conductive until the end of the work-study program; the mechanism for adjusting the effective current flowing in the motor 100 is strictly similar to that of the circuit in FIG. 1, except that this current does not change direction.

 FIG. 5a, similar to that of FIG. 2, shows the shape of the output signal from the generator 8, and FIG. 5b, on the same time scale, shows in broken lines the current flowing in the motor in the absence of signal from generator 8, and in solid lines the same current in the presence of the signal, the hatched zones corresponding to the conduction regime of thyristors 28 and 29.

 As in the assembly of FIG. 1, the effective current in the load remains unchanged while the harmonic rate is greatly reduced.

 The two embodiments of the device according to the invention which have just been described apply to "open loop" adjustment systems. The invention also applies to "closed loop" regulating systems where a parameter of the load is kept constant by comparison with a set value, this parameter being able to be an effective current, a rotation speed, etc.

 FIG. 6 shows, by way of example, the application of the device according to the invention to an assembly for regulating the speed of rotation of a direct current motor fitted with inductors with permanent magnets.

 In FIG. 6, the references of which are common with those of FIGS. 1, 3 and 4, the positive input of a comparator circuit 32 is connected to a generator 33 of voltage ramp coupled to the sector by two diodes 34 and 35. The negative input of comparator 32 is connected to an integrator circuit constituted by a resistor 36 and a capacitor 37, the latter being connected to a ground 38 to which the negative terminal of the motor 100 is also connected.

 The output of comparator 32 is coupled to the triggers of thyristors 28 and 29 by a pulse generator 39, while generator 8 is connected either to the positive input or to the negative input of comparator 32, such as it is indicated in dotted lines on the diagram.

 Apart from the presence of the generator 8 according to the invention, the operation of the assembly of FIG. 6 is well known: the voltage ramp generator 33 is triggered at each zero crossing of the alternations of the sector detected by the diodes 34 and 35 ; the ramp voltage is compared to the integrated electromotive force voltage of the motor 100, present during the time intervals when the latter is not supplied, and the comparator 32 switches at each alternation of the sector to make the thyristors 29 and 28 conductive via the pulse generator 39, the delay between the start of each half-wave and the ignition point varies automatically to keep the engine speed constant regardless of variations in its resistive torque.

 For a determined and constant torque value, the starting point at each half-wave can be at the maximum instantaneous current value, thus causing a high level of harmonics; the application of the output signal from the generator 8 to one or the other input of the comparator 32 has exactly the same effect as that shown in FIGS. 5a and 5b by ensuring a reduction in the harmonic rate without harming the regulatory action.

 In FIG. 7, the references of which are common with those of FIGS. 1, 3, 4 and 6, an immersion heater 40 is disposed between the terminal 2 of the sector and the cathodes and anodes combined of the diode 26 and the thyristor 28.

 Between the combined cathodes of the thyristors 28, 29 and the combined anodes of the diodes 26, 27, is arranged a motor 101 for driving the drum, the direction of rotation reverser of which has not been shown in order to simplify the diagram.

 On the alternative inputs of bridge 26, 27, 28 and 29, the anodes of two diodes 41 and 42 are connected, the combined cathodes of which are connected by a resistor 43 to the cathode of a Zener diode 44, the anode of the latter. being connected to the positive terminal of the motor 101.

 The emitters of two transistors 45 and 46, the first of the PNP type and the second of the NPN type, are connected, one to the cathode of the Zener diode 44 by a resistor 47, and the other to the combined triggers of the thyristors 28 and 29 by a resistor 48; the bases and collectors connected head to tail of the transistors 45 and 46 are connected, some by the resistor 25 to the generator 8 and by a variable resistor 49 to the cathode of the Zener diode 44, and the others by a resistor 50 to the positive terminal of the engine 101.

 The emitter of transistor 45 is, moreover, connected to the cathode of a Zener diode 51 whose anode is connected by a resistor 52 to the negative terminal of the motor 101 joined to ground 38, an integrating capacitor 53 being by elsewhere arranged between the positive terminal of the motor and said transmitter.

 The operation of the assembly 3 of FIG. 7 is well known: just as for the assembly of FIG. 6, the thyristors 28 and 29 constitute with the diodes 26 and 27 a rectifying bridge supplying the motor 101 by a pulsed direct voltage formed portions of sinusoid arches, the duration of which varies inversely with the delay in initiation of the thyristors relative to the origin of said arches; the role of the immersion heater 40 consists in limiting the peak current flowing in the armature of the motor, the energy thus dissipated by the Joule effect being recovered during the washing cycle of the machine.

 The triggers of the thyristors 28 and 29 are controlled by the pulse generator constituted by the two tran sistors 45 and 46 of complementary types mounted head to tail comparable to a tetrode transistor whose triggering is caused for a determined ratio between the voltage of emitter of transistor 45 and the voltage applied to collectors and bases together of transistors 45 and 46.

 The pair of transistors 45,46 is supplied from the group of diodes 41,42 with a pulsed positive voltage clipped by the Zener diode 44, said voltage being of course only present during the blocking periods of the thyristors 28 and 29.

 The nominal value used for regulation is the armature electromotive force voltage developed by the motor 101 operating as a generator during the time intervals when the thyristors 28 and 29 are blocked, this voltage being practically proportional to the speed of rotation. , the absence of this at startup means that the triggering of the pair of transistors 45,46 occurs very soon after the start of each alternation of the sector and the motor is practically supplied at full wave.

 As the motor accelerates, the electromotive force increases on the anode of the Zener diode 51, and as soon as the ignition threshold of the latter is reached, the regulation voltage partially integrated by the capacitor 53 is applied to the emitter of transistor 45 making it more negative and also delaying triggering at this time, regulation becomes effective, any variation of FEr1 modifying the delay of triggering of transistors 45,46 in a direction such that it tends to compensate for said variations, and consequently to maintain constant the speed of the motor which, moreover, can be adjusted by varying the value of the resistor 49.

 In the assembly of FIG. 7, the voltage from the generator 8 is applied to the common collector-base connection of the transistors 45 and 46; identical operating conditions can be obtained by applying the signal, either to the end of the resistor 50 connected to the common base-collector connection of the transistors 45 and 46, or again to the emitter of the transistor 45.

 In FIG. 8, an embodiment of the generator 8, of the bridged T-type supplying a sinusoidal voltage, comprises a transistor 54, of the NPN type, the emitter of which is connected to ground 38 and the collector of which is supplied from a source + Vb through a resistor 55, constitutes the output terminal; the collector and the base are back-coupled by three RC networks, one parallel 56, 57, and the other two in T, 58, 59, 60 and 61, 62, 63; this type of generator, the operation of which is well known, is made of discrete components and can be used preferentially with the assembly of FIG. 7 which itself uses these types of components.

 In FIG. 9, another embodiment of the generator 13, supplying a rectangular voltage such as that of FIGS. 2a and 5a, comprises two "NOR" gates 64 and 65 whose joined inputs and outputs are back-coupled by a network consisting of two resistors 66, 67 and a capacitor 68.

 This type of generator will preferably be used with the assembly of FIG. 6 which, in its complete embodiment, has numerous multiple logic circuits in integrated circuits among which it is easy to find the two doors 64 and 65.

 As indicated above, the shape of the periodic voltage supplied by the generator 8 has little influence on the result obtained.

 In the case where this voltage is sinusoidal, the starting point of each alternation moves progressively, while in the example described of implementation of a rectangular tension, this displacement is fixed for a small group of alternations.

Claims (7)

- CLAIMS  1.- Device for reducing the level of harmonics generated by the phase cutting of the current flowing in a load (1), said load being connected to the mains (2-3) by at least one electronic switch (4), the control electrode is connected to a pulse generator (56-7) synchronous with the mains frequency, characterized in that said pulse generator is coupled to a generator (8) supplying an asynchronous periodic voltage with respect to the frequency of the sector.  2.- Device according to claim 1, characterized in that the generator (8) of periodic voltage is connected to the common point with a resistor (5), a capacitor (6) and a diac (7), the other electrode of the latter being connected to the trigger of a triac (4).  3.- Device according to claim 1, characterized in that the generator (8) of periodic voltage is connected to the common point with a resistor (5), a capacitor (6) and a diac (7), the other electrode of the latter being connected to the combined triggers of two thyristors (28-29).  4.- Device according to claims 1, 2 and 3, characterized in that the generator (8) is supplied from a diode bridge (12-13-14-15) connected to the sector (2 -3).  5.- Device according to claim 1, characterized in that the generator (8) is connected to one of the inputs of a comparator circuit (32) whose output is coupled by a pulse generator (39) to the triggers two thyristors (28-29).  6.- Device according to claim 1, characterized in that the synchronous pulse generator is constituted by two transistors (45,46) arranged head to tail.  7. Household appliance, characterized in that it is equipped with the device according to claim 1 and one of claims 2 to 6.