EP1009004B1 - Control device for an electromagnet, with detection of accidental movement of the movable core of the electromagnet - Google Patents

Description

The invention relates to a control device and an electromagnet, the assembly comprising a core. mobile, with at least one call phase, during which the electromagnet receives a inrush current, and a holding phase, during which it receives a holding current lower than the inrush current, and comprising at least one coil connected in series with an electronic switch at the terminals of a supply voltage, means for measuring the current flowing in the coil and the means for controlling the electromagnet, connected to the current measurement means and to a control electrode the electronic switch and comprising means for regulating the current in the coil during the holding phase.

EPO 411 903 shows an assembly according to the preamble of claim 1.

For the control of an electromagnet, it is known (FR-A-2 .133.652) to supply it temporarily a relatively high inrush current, followed by a more holding current low. This can be achieved as well with a device comprising a single coil, in which the current is chopped to constitute the holding current, only with a double winding consisting of a take-up coil and a holding coil. he is also known to enslave inrush and holding currents to setpoint levels predetermined (FR-A-2,568,715).

An electromagnet conventionally comprises a movable core whose displacement towards a position in which the electromagnet is actuated is caused by the circulation of the inrush current in the inrush coil. It is then held in this position by the passage of the holding current in the holding coil, which can be the same as the call reel. To reduce the heating of the electromagnets we seek to reduce the holding current. In certain cases, this reduction in the holding current poses problems due to the existence of shocks, mechanical in particular, likely to cause a untimely displacement of the core towards the rest position of the electromagnet.

This type of problem arises in particular in contactors or auxiliaries circuit breakers, for example with opening electromagnets (MN or MX) or closing (XF) of circuit breakers.

More generally, the search for a reduction in the volumes of the electromagnets leads to a decrease in the power which can be dissipated by the coils and makes the electromagnets more sensitive to shocks.

The invention aims to eliminate these drawbacks.

According to the invention, this object is achieved by the fact that the control means include detection means, for detecting an untimely movement of the mobile core of the electromagnet during a holding phase depending on the value of the circulating current in the coil when said current is greater than the setpoint during the phase holding, and control means for switching to the call phase when a unexpected movement is detected.

Rapid shock detection allows you to return to the call phase and pick up the core mobile before its movement had an impact on the device it controls.

According to a first development of the invention, the detection means include means for detecting the direction of variation of the current flowing in the coil, a inadvertent displacement being considered as detected when, during the hold, the current is simultaneously greater than the set value and increasing.

The means for detecting the direction of variation of the current then preferably comprise means for determining a quantity representative of the derivative of the current by relative to time, an untimely movement being considered as detected when, during the holding phase, the current is greater than the set value and said greatness is positive.

According to a second development of the invention, untimely displacement is detected by the detection means when, during the holding phase, the current is greater than the setpoint for a predetermined period.

La figure 1 représente un dispositif de commande d'un électro-aimant selon l'art antérieur.

La figure 2 représente, de manière schématique, en coupe, un électro-aimant de type connu.

Les figures 3a et 3b représentent respectivement les variations, en fonction du temps, pendant une phase de maintien, des signaux B et Ib d'un dispositif selon la figure 1 dans lequel la régulation se fait à partir d'échantillons de courant.

Les figures 4a et 4b représentent respectivement les signaux B et Ib d'un mode de réalisation d'un dispositif selon l'invention avant et après la détection d'un choc.

Les figures 5a et 5b représentent respectivement les signaux B et Ib d'un dispositif selon la figure 1, dans le cas où la régulation est mauvaise.

La figure 6 illustre schématiquement des éléments additionnels du dispositif selon la figure 1 dans un mode de réalisation comportant une bobine d'appel.

La figure 7 représente un mode de réalisation particulier d'un sous-programme correspondant à une phase de maintien dans un dispositif selon l'invention.

La figure 8 représente une variante du sous-programme de la figure 7.

Other advantages and characteristics will emerge more clearly from the following description of particular embodiments, given by way of nonlimiting examples and represented in the appended drawings in which:

FIG. 1 represents a device for controlling an electromagnet according to the prior art.

Figure 2 shows, schematically, in section, an electromagnet of known type.

FIGS. 3a and 3b respectively represent the variations, as a function of time, during a holding phase, of the signals B and Ib of a device according to FIG. 1 in which the regulation is made from current samples.

Figures 4a and 4b respectively represent the signals B and Ib of an embodiment of a device according to the invention before and after the detection of a shock.

Figures 5a and 5b respectively represent the signals B and Ib of a device according to Figure 1, in the case where the regulation is poor.

FIG. 6 schematically illustrates additional elements of the device according to FIG. 1 in an embodiment comprising a take-up coil.

FIG. 7 represents a particular embodiment of a subroutine corresponding to a maintenance phase in a device according to the invention.

FIG. 8 represents a variant of the subroutine of FIG. 7.

The device according to FIG. 1, which is of the type described in document FR-A-2,568,715 comprises a coil 1 connected in series with a transistor T1 and a measurement resistor R1 at the terminals of a supply voltage Va. Conventionally, a freewheeling diode D1 is connected in parallel on the coil 1. An output S1 of a circuit 2 for controlling and regulation is connected to a control electrode of transistor T1 to which it provides control signals B. An input E1 of circuit 2 receives signals A of solenoid control. Circuit 2 is also connected to the terminals of the resistor R1 so as to receive, on an input E2, signals Ib, representative of the current Ib flowing in the coil 1 when the transistor T1 is conductive. Circuit 2 thus allows both to control the device and to control the current in the coil to predetermined values, independent of the supply voltage Va. A circuit 3 supply, connected across the voltage Va supplies circuit 2 with a voltage stabilized auxiliary power supply.

The electromagnet, of known type, represented in FIG. 2, comprises an armature 4 to the interior of which the coil 1 is disposed. The coil 1 surrounds a fixed core 5, integral with the frame, and a movable core 6. A spring 7 is disposed between the fixed cores and movable so as to separate the movable core 6 from the fixed core. In the rest position of the electromagnet, shown in Figure 2, a plunger 8 secured to the movable core made protrusion outside the frame 4.

When a control command A is applied to the input E1 of circuit 2, it passes first by an appeal phase. During the call phase, signals B control the conduction of transistor T1, i.e. closing of the electronic switch constituted by the transistor, so that a relatively high current Ib, or current call, flows in the coil 1. The passage of the inrush current in the coil 1 causes the displacement of the movable core 6 in the direction of the fixed core 5, against the action of the spring 7. When the cores are glued, the plunger 8 no longer projects out of the frame 4. Conventionally, the position of the plunger 8 makes it possible to control the opening or closing a device, for example a contactor or a circuit breaker.

The call phase lasts long enough to allow complete movement of the movable core 6 and its bonding against the fixed core 5. Subsequently, the passage of a current high calling capacity is no longer necessary to maintain the mobile core in position actuation of the electromagnet and, conventionally, the circuit 2 for controlling and regulation goes to a maintenance phase. During the maintenance phase, signals B control the conduction of transistor T1 so that a holding current Ib, weaker, circulates in coil 1.

In a preferred embodiment, the holding current Ib is regulated by the circuit 2 so as to be close to a setpoint Icm of the holding current. Of in known manner, circuit 2 can be constituted by an analog circuit or by a circuit digital, for example, microprocessor. In the prior art, regulation is carried out by pulse width modulation (PWM) of a frequency control signal B fixed high.

FIGS. 3a and 3b illustrate the signals B and the current Ib during a holding phase and the consequences of a shock in a device according to FIG. 1 in which the regulation is carried out from samples Ib _i of the current taken at a fixed predetermined sampling frequency.

At an instant t1, the current Ib being lower than the set value Icm, the signal B is at a logic value 1, and the transistor T1 conducts. In FIG. 3b, the current Ib is sampled with a sampling period Te by the circuit 2. As long as the holding current Ib is less than the set value Icm, the signal B remains at 1 and the current in the coil increases. When, at an instant t2, a sample Ib _i of the current Ib reaches or exceeds the set value Icm, the signal B goes to 0, thus controlling the passage to an opening phase of the electronic switch constituted by the transistor T1 . This being blocked, the current in the coil then begins to decrease. During the opening phase, the circuit 2 periodically sends sampling pulses Bi on the base of the transistor T1, so as to make it conductive and to allow a measurement across the resistance R1 of a sample Ib _i of the current Ib flowing in the coil. These periodic pulses Bi, of period Te, have a very short duration, so as not to influence the value of the current Ib in the coil. These pulses are represented in FIGS. 3a, 4a and 5a. Throughout the following description, the transistor T1 is considered to be in an opening phase of the holding phase as long as the signal B remains at 0 outside the instants sampling. In normal operation, the current Ib again becomes lower than the set value Icm after one or two sampling periods, for example at time t3 in FIG. 3b. This is then detected by the circuit 2 which ends the opening phase by returning to the logic value 1 the signal B, again controlling the conduction of the transistor T1 and the growth of the current Ib during at least one sampling period.

A mechanical shock exerted on the electromagnet or on the device it controls can cause the moving core 6 to move away from the fixed core 5 when the current is too low.

The consequences of a shock of this type are shown in Figures 3a and 3b at a time t4. The shock causes the mobile core to begin to move. This displacement causes in the coil producing an electromotive force which results in the generation of a additional current which is added to the regulated holding current. There are, therefore, after the instant t4, increase in the current Ib in spite of the regulation, that is to say in the particular embodiment shown, although the transistor T1 is in a phase opening (B = 0 outside the sampling pulses). If no action particular is not taken, the current Ib can take the form shown in Figure 3b, between instants t4 and t5: growth, then passage through a maximum and decrease until it goes below the set point Icm.

At time t5, the microprocessor starts again to control the regulation of the current of holding in the coil. However, this holding current is insufficient to pick up the movable core 6 against the fixed core 5. The shock thus leads to a de-excitation nuisance of the electromagnet. As an example in the case where the electromagnet is part an undervoltage release (MN) of a circuit breaker, the electromagnet can be of the type shown in Figure 2. In the energized position of the electromagnet, cores 5 and 6 glued, the plunger 8 is in the withdrawn position. When the voltage applied to his device control drops below a predetermined value, current flow is interrupted in the coil and the movable core 6 moves away from the fixed core 5 under the action of the spring 7. The plunger 8 then projects outwards, causing immediate opening of the circuit breaker. Subsequent closing of the circuit breaker is only possible when the undervoltage release being supplied, the movable core 6 is glued against the fixed core 5. An untimely shock as described above, can therefore lead to opening of the circuit breaker. The holding current supplied to the electromagnet after time t5 being insufficient to re-glue the cores, it is then impossible to close the front circuit breaker cut the power supply to the trigger and then re-energize it, which causes a phase call and bonding of the cores.

According to the invention, an untimely displacement of the mobile core of the the electromagnet during a holding phase and the transition to a phase is controlled as soon as such a movement is detected. Thanks to the rapid transition to the call phase, effects of nuisance shock are either completely eliminated or reduced. For exemple, in the case of an undervoltage release (MN), if the untimely shock is detected fairly early in the movement of the mobile core (6) and the call phase occurs before that the plunger 8 could not cause the opening of the circuit breaker, the shock has no result. If the shock is detected later, the circuit breaker may open under the action of the plunger 8. However, the automatic transition to the call phase upon detection of the shock automatically leads to re-bonding of the core and allows reclosing of the breaker. In this case, the drawbacks related to nuisance shock, if they are not completely deleted, are however reduced.

Figures 4a and 4b illustrate the signals B and Ib in a device according to the invention. Until time t4, the device ensures, as before, the regulation of the holding current around the set value Icm. At time t4, when an untimely shock occurs, the current Ib increases. In a first variant, if, during an opening phase of the holding phase, the control device detects four successive samples Ib _i greater than the set value Icm, it considers that this is due to a shock and it causes the transition to call phase. This variant is illustrated in Figures 4a and 4b. At time t6, four successive samples greater than Icm have been detected since time t4. The control and regulation circuit 2 then causes the conduction of the transistor T1 (B = 1) until the current Ib in the coil 1 reaches a call setpoint value Ica. It then regulates the current in the coil so that it is equal to the Ica value during the call phase. The Ica value is much higher than the Icm value (10 to 20 times) and causes the mobile and fixed cores to stick together. Conventionally, after a predetermined time (80 ms for example), the control circuit again switches to the holding phase.

In certain cases, this criterion for detecting a shock is however insufficient. Such case is illustrated in Figures 5a and 5b. In these figures, the electromagnet is in the phase of hold, with regulation of the current Ib in the coil at the setpoint Icm. he can happen, as represented between instants t7 and t8, that when closing the transistor T1 (B = 1), the current Ib in the coil increases rapidly. This can particular be the case if the supply voltage Va, which is generally a voltage alternating rectified double alternation, occasionally has a too high peak voltage. After having detected, at time t8, a first sample greater than the set value Icm, the control circuit normally causes blocking of transistor T1 (B = 0) and the transition to the opening phase. The current Ib then goes down to the value of Icm setpoint. However, the maximum value reached by the current Ib at time t8 being relatively high, it requires a period greater than two sampling periods Te to fall back below Icm. In FIG. 5b, the current Icm again becomes lower than the Icm setpoint at time t9 only, after five successive samples of the current Ib was greater than the setpoint. In the variant described above, in with reference to FIGS. 4a and 4b, this is interpreted by the control circuit 2 as being due to a shock which caused an untimely displacement of the mobile core. However, there is none nothing and it is actually poor regulation. In the first variant, the circuit of command would then go to the call phase, when this is unnecessary. Gold use too frequent of the call phase would lead to significant energy dissipation in the coil, which could lead to the destruction of the device.

According to a development of the invention, it is sought to eliminate such untimely passages during the call phase. For this, it is possible to increase the duration of the observation window minimum during which the current Ib must be greater than the setpoint for conclude that there is an untimely displacement of the mobile nucleus. At frequency fixed sampling, this amounts to increasing the number of successive samples higher than Icm necessary to conclude that a shock exists. But this leads to decrease the reaction speed of the device and allow greater movement of the moving core before reacting.

The comparison of FIGS. 4b and 5b makes it possible to see that, if in both cases, during the periods t4-t6 and, respectively t8-t9, the current Ib remains above the threshold Icm, by however, the variations in Ib are completely different. In the event of a shock (fig. 3b and 4b, from t4), the displacement of the mobile nucleus causes the appearance of a force in the coil and, consequently, an increase in current despite the blocking of transistor T1 outside the sampling instants. However, in the case of poor regulation (fig. 5b, from t8), the current Ib decreases as soon as the transistor T1 is in an opening phase.

In a preferred embodiment, a shock is detected when, the transistor T1 being in an opening phase, the current Ib is greater than the set value Icm and, simultaneously, the current Ib in the coil is increasing. To detect such growth, the control circuit 2 can determine a quantity representative of the derivative of the holding current versus time, dIb / dt. When this quantity is positive, this means that the current Ib increases and this growth, when B = 0 outside the instants sampling and Ib> Icm, is interpreted as corresponding to a displacement untimely nucleus which must lead to the transition to the call phase.

In FIGS. 1 and 2, the control device comprises a single coil and on the Figures 4a and 4b, the control circuit 2 regulates the current in the coil either to the value Icm during a hold phase, i.e. at Ica value during a call phase.

The invention applies in the same way if the current is not regulated during the phase call. Then, the transistor T1 remains conductive (B = 1) throughout the call phase. She also applies if the device comprises a double coil, the coil 1 constituting then the holding coil and another coil constituting the take-up coil which is not traversed by a current, regulated or not, only during the call phase.

FIG. 6 illustrates the complementary elements of a double embodiment winding. A call coil 9 is connected in series with a transistor T2 and a measurement resistance R2 across the supply voltage Va. A wheel diode free D2 is connected in parallel on the call coil 9. The control electrode of the transistor T2 is connected to an output S2 of the control and regulation circuit 2. If the current in the call coil 9 must be regulated during the call phase, the common point at R2 and T2 is connected to an input E3 of circuit 2.

The control and regulation circuit 2 can be produced by any suitable means, analog or digital. In a preferred embodiment, it comprises a microprocessor which performs, with a sampling period Te, the sampling of signals applied to its inputs E2 and E3, their analog / digital conversion, their comparison with the set values Icm and Ica, respectively during the phases of hold and call, and control of transistors T1 and T2.

FIG. 7 illustrates a particular subroutine corresponding to a maintenance phase and implementing the variant of the invention described with reference to Figures 4a and 4b, it is say detecting a shock when Ib is greater than Icm for at least 4 samples successive of an opening phase of transistor T1 (B = 0 outside the instants sampling).

During a first step F1 of initialization of the holding phase, the signal B is set to 1 (conduction of T1) and an indicator i is set to zero. Then the microprocessor of circuit 2 goes to a step F2 of measuring a sample Ib _i of the current flowing in the coil 1. If B is zero, a sampling pulse Bi is applied transiently on the basis of transistor T1, the value of B not changing in the program. In a step F3, the microprocessor compares the sample Ib _i with the set value Icm. If Ib _i is not greater than the set value (NO output of F3), the microprocessor loops back to the input of step F1. The transistor T1 therefore remains conductive and the current Ib continues to rise. This happens, for example, between the instants t1 and t2 of figure 3b. On the other hand, if, in F3, Ib _i is greater than Icm (YES output of F3), then the microprocessor checks, in a step F4, if B = 1. If B = 1 (instants t2 or t3 in FIG. 3b), then the microprocessor goes to a step F5 where B is set to zero, controlling the passage to an opening phase of the transistor T1, before returning to the input from step F2. The subroutine described so far corresponds to a regulation of the current Ib at the value Icm during the holding phase. If, in F4, B = 0 (NO output of F4), then the indicator i is incremented (i = i + 1), in a step F6. Then in a step F7, the microprocessor checks if i = 4. If this is not the case (NO output of F7), it returns to the input of step F2. On the other hand, if i = 4, this means that four successive samples Ib were greater than Icm during the opening phase of the transistor T1. This is considered representative of an untimely shock having led to an untimely start of displacement of the mobile core 6 of the electromagnet. The microprocessor then passes (YES output of F7) to a step F8 corresponding to a call phase.

The number of samples retained in step F7 can be modified depending in particular on the desired sampling frequency and reaction speed. The number samples greater than Icm when B = 0 must at least be greater than or equal to 2. This corresponds to more than two successive samples greater than Icm during the phase of maintenance, the first sample leading to blocking of T1. The value 4 is a value preferential which gives satisfactory results when the supply voltage Va is a full-wave rectified voltage from an electrical network at 50 or 60Hz and for a sampling period of the order of a few hundred microseconds.

FIG. 8 represents a variant of the subroutine of FIG. 7, in the case where the decision criterion retained is no longer the number of successive samples greater than Icm, but the direction of variation of the holding current when Ib> Icm during a phase opening of transistor T1.

The subroutine corresponding to the maintenance phase remains identical until step F6. In the variant of FIG. 8, at the output of step F6, the microprocessor, in a step F9, checks whether the indicator i is equal to or greater than 2. If this is not the case, (output NO of F9), or if i = 1, that is to say if a single sample Ib _i greater than Icm was measured during an opening phase of the transistor T1, this sample is stored in a step F10 in a location Ib _i-1 (Ib _i-1 = Ib _i ). Then, the microprocessor returns to the input of step F2 for the measurement of the following sample Ib _i . On the other hand, if, in F9, i is greater than or equal to 2, then (YES output of F2), the microprocessor proceeds to a step F11 of determining a quantity ΔIb = Ib _i -Ib _i-1 . The quantity ΔIb is representative of the direction of variation of the current Ib after blocking of the transistor T1 at the start of the opening phase, and, more particularly, representative of the derivative of the holding current with respect to time between two successive samples during this phase. Then, in a step F12, the microprocessor checks the sign of ΔIb. If the quantity ΔIb is negative or zero (NO output from F12), it goes to step F10, storing the last sample before measuring the next. On the other hand, if in F12, the quantity ΔIb is positive (YES output of F12), then, the current being increasing, the microprocessor considers that there has been an untimely shock causing the displacement of the mobile core and passes to phase d 'call (F8).

In the description above, the regulation is carried out from a sampling periodic of the current Ib in the coil.

The invention is also applicable when the regulation is carried out by modulation of pulse width (PWM) as in the aforementioned prior art. In this case the transistor T1 works as a chopper with a fixed chopping frequency and a duty cycle. variable. During a period Th corresponding to the chopping frequency, the transistor T1 is returned conductor (B = 1) during a variable Th1 (Th1 <Th) period. The length of the period Th1 is a function of the difference between the measured current Ib and the setpoint (Icm during the maintenance phase). When the current is equal to the setpoint, the conduction period Th1 takes a predetermined value Th1c corresponding to a duty cycle ratio Nc = Th1c / Th, which is for example equal to 0.5. When the current Ib in the coil is less than the set value Icm, the conduction period Th1 increases and, consequently, the duty cycle N = Th1 / Th is greater than Nc.

When the current Ib in the coil is greater than the setpoint, the period of conduction Th1 is less than Th1c, and therefore the N at Nc.

To detect an inadvertent displacement of the mobile core, it is possible, in a way analogous to the embodiment described in FIG. 7, to determine if the current Ib remains greater than the set value Icm for a predetermined period. For this the circuit 2 compares, during each cycle or chopping period Th, the value of the ratio cyclic N to the setpoint duty cycle Nc. If N remains greater than Nc for a predetermined number of successive cycles (at least 2, preferably 4), then circuit 2 considers that there is an untimely displacement of the mobile core and commands the passage into call phase.

In a preferred embodiment, circuit 2, analogously to the mode of embodiment described in Figure 8, takes into account the direction of variation of the holding current when Ib> Icm. To do this, it compares the successive duty cycles when N is lower than Nc (Ib> Icm) and considers that there is untimely displacement of the mobile core when the duty cycle N being less than Nc for at least two successive cycles, this duty cycle is decreasing. This indeed means that the current is both increasing and greater than the Icm setpoint for more than one chopping period. As before, it then commands the transition to the call phase.

In all cases, the detection of an inadvertent displacement of the mobile core during a holding phase is linked to monitoring the current in the coil during a holding and detecting such a movement causes the transition to the call phase.

Claims (10)

1. A control device and electromagnet , the assembly comprising a movable core (6), with at least one inrush phase during which the electromagnet receives an inrush current, and a holding phase during which it receives a holding current weaker than the inrush current, and comprising at least one coil (1) connected in series with an electronic switch (T1) to the terminals of a supply voltage (Va), means for measuring the current (Ib) flowing in the coil and means (2) for control of the electromagnet, connected to the means for measuring the current (Ib) and to a control electrode of the electronic switch (T1) and comprising means for regulating the current in the coil (1) to a preset value (Icm) during the holding phase, characterized in that the control means comprise detection means for detecting an unscheduled movement of the movable core (6) of the electromagnet during a holding phase according to the value of the current (Ib) flowing in the coil (1) when said current is greater than the setpoint value (Icm) during the holding phase, and means for commanding switching to inrush phase when an unscheduled movement is detected.
2. Assembly according to claim 1, characterized in that the detection means comprise means for detecting the direction of variation of the current (Ib) flowing in the coil, an unscheduled movement being considered as being detected when, during the holding phase, the current (Ib) is both greater than the setpoint value (Icm) and increasing.
3. Assembly according to claim 2, characterized in that the means for detecting the direction of variation of the current (Ib) comprise means for determining a quantity (ΔIb) representative of the differential of the current with respect to time, an unscheduled movement being considered as being detected when, during the holding phase, the current (Ib) is greater than the setpoint value (Icm) and said quantity is positive.
4. Assembly according to claim 1, characterized in that an unscheduled movement is detected by the detection means when, during the holding phase, the current (Ib) is greater than the setpoint value (Icm) during a preset time.
5. Assembly according to claim 4, characterized in that the means for measuring the current comprise means for sampling the current, with a preset sampling period (Te), and that an unscheduled movement is detected if more than two successive samples (Ib_i) of the current are greater than the setpoint value (Icm) during the holding phase.
6. Assembly according to claim 5, characterized in that an unscheduled movement is detected when more than four successive samples of the current are greater than the setpoint value (Icm) during the holding phase.
7. Assembly according to one of the claims 5 to 6, characterized in that the sampling period (Te) is about a few hundred microseconds.
8. Assembly according to any one of the claims 1, 2 and 4, characterized in that, the regulating means controlling turn-on of the electronic switch (T1) with a fixed chopping period and a variable duty cycle factor (N) depending on the difference between the value of the current (Ib) flowing in the coil and the setpoint value (Icm), the detection means compare the duty cycle factor (N) with a setpoint duty cycle factor (Nc) at each chopping period.
9. Assembly according to claim 8, characterized in that an unscheduled movement is considered as being detected when, during a holding phase, the duty cycle factor (N) is lower than the setpoint duty cycle factor (Nc) during at least two successive chopping periods.
10. Assembly according to claim 8, characterized in that an unscheduled movement is considered as being detected when, during the holding phase, the duty cycle factor (N) is at the same time decreasing and lower than the setpoint duty cycle factor (Nc) during at least two successive chopping periods.

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