GB2053596A

GB2053596A - Circuit for Mains Monitoring

Info

Publication number: GB2053596A
Application number: GB8019454A
Authority: GB
Original assignee: BBC Brown Boveri AG Switzerland; BBC Brown Boveri France SA
Current assignee: BBC Brown Boveri AG Switzerland; BBC Brown Boveri France SA
Priority date: 1979-06-16
Filing date: 1980-06-13
Publication date: 1981-02-04
Also published as: FR2459568A1; IT8022706A0; IT1131304B; DE2924390A1

Abstract

A circuit for monitoring a single phase or three-phase mains voltage supply comprises a full-wave bridge rectifier (4 to 9), a first comparator (13) which detects when a first threshold is exceeded by the rectified mains voltage and a second comparator (14) which detects when the rectified voltage falls below a second lower threshold. The outputs of the comparators are connected in common by way of a bias resistor (15) to a first voltage source (U1) and by way of a diode (16) and a low pass filter (17, 18, 19) to a second voltage source (U2). Voltage changes at the common output of the comparators, indicating a fault, are applied to a logic circuit (24) which may automatically disconnect the mains supply. <IMAGE>

Description

SPECIFICATION Circuit for Mains Monitoring The invention relates to a circuit for monitoring a single-phase a.c. or three-phase mains for under-voltage and over-voltage as well as for phase failure.

The invention can be used, for example, in regulating the speed of a d.c. motor. In such case, a power transistor, operating under chopper conditions, is frequently used as a d.c. regulator for a d.c. motor and therefore functions as a speed regulator. This kind of use of a transistor as a switch is described in "Einführung in die Elektronik" by W. Bitterlich, published by Springer-Verlag, Vienna-New York, 1976, pages 390 to 396.

If the d.c. supply of the motor is obtained from a three-phase mains and a preceding bridge rectifier, any voltage fluctuations of the supplying three-phase mains or any phase failure will have a substantial effect on the operation of the power transistor. For example, in the event of phase failure the transistor will be supplied with an excessively low control curre resulting in an increase of the forward resistance of the conductor in the conductive state. This results in high peak losses which can lead to damage of the transistor. The operation of the speed regulating unit cannot therefore be guaranteed, for example in the event of phase failure, i.e. the speed regulating device must be removed from the mains.

The invention seeks to provide a circuit for monitoring a single-phase a.c. or three-phase mains for over-voltage, under-voltage and phase failure, and adapted to protect sensitive loads in a sufficiently rapid manner against changes of the mains voltage and where appropriate to isolate such loads from the mains.

According to the present invention, there is provided a circuit for monitoring a single-phase or three-phase mains supply for over-voltage, undervoltage and for phase failure, comprising two comparators, each phase of the mains supply to be monitored being connected via a bridge rectifier to the negative input and the positive input, respectively, of the two comparators the other inputs of the comparators being connected to receive respective reference voltages and the outputs of the comparators being interconnected and being further connected to a voltage source and by way of a series combination of a diode and a resistor to a capacitor and to a further resistor which is biased by a voltage source.

The advantages to be achieved by the invention are due more particularly to the fact that the mains monitoring system very rapidly detects any breakdown in the mains supply, a feature which is important for loads which are sensitive to mains voltage fluctuations. The circuit is of uncomplicated construction, can be economically produced and is operationally reliable, not being very trouble prone in use.

Preferably, the resistors and the capacitors are dimensioned such that the time constant for the discharge of the capacitor via the diode and the resistor connected in series is small in relation to the time constant for the charging of the capacitor via the said further resistor. This provides a particularly reliable means of detection and evaluation in the event of failure of one phase of the mains voltage which is to be monitored.

In another embodiment of the invention, the interconnected outputs of the comparators are connected to a logic part which, advantageously, is provided for the evaluation or storing of any occurring mains disturbances.

According to a second aspect of the invention, there is provided a circuit for monitoring a mains supply having one or more supply phases which comprises means for full-wave rectifying the or each phase to be monitored, a first comparator for determining when the peak value of the rectified mains voltage exceeds a first threshold and a second comparator for determining when the minimum value of the rectified mains voltage drops below a second threshold value, by a bias comparator having a common output connected resistor to a first voltage source and by way of a diode and a low-pass filter to a second voltage source.

The invention will now be described further, by way of example, with reference to the accompanying drawing, in which: Figure 1 is a basic circuit diagram of a circuit for mains monitoring, and Figure 2 is a voltage waveform showing the change of voltages with respect to time of a rectified three-phase mains under conditions of under voltage, normal operating voltage, over voltage and phase failure.

Figure 1 shows a basic circuit diagram of a circuit for mains monitoring. A three-phase mains supply with phases R, S, T to be monitored is connected by protective or series resistors 1, 2, 3 to a three-phase bridge rectifier comprising diodes 4, 5, 6, 7, 8 and 9. On the d.c. side the three-phase bridge rectifier is connected across resistors 10 and 11 and a zener diode 12 is connected in parallel with the resistor 12.

The anode of the zener diode 12 is connected to the bridge rectifier d.c. output at the "ground" reference potential while the cathode of the zener diode 1 2 is connected to the junction of the resistors 10 and 11 which form a voltage divider.

The negative input of a comparator 13 and the positive input of a comparator 14 are also connected to the last-mentioned junction.

The interconnected outputs of the two comparators 1 3 and 14 are connected to a resistor 1 5 and to the cathode of a diode 1 6. The other terminal of the resistor 1 5 is connected to a voltage source having a voltage U1. The anode of the diode 1 6 is connected to a resistor 1 7 the other terminal of which is connected to one terminal of a resistor 1 8 and by way of a capacitor 1 9 to ground. The other terminal of resistor 18 is connected to a voltage source having a voltage U2.

The positive input of the comparator 13 is connected to the common junction of two resistors 20 and 21 of which the resistor 20 is biased by a voltage source with the voltage U3 and the resistor 21 is connected to ground potential. The negative input of the comparator 14 is connected to the common junction of two resistors 22 and 23 and the resistor 22 is supplied with the voltage UA from a voltage source and the resistor 23 is connected to ground potential. The comparators 1 3 and 14 are therefore biased with a reference voltage via their positive or negative input respectively and the reference voltages can be adjusted in accordance with the selected voltage value U3 or U4 and the selected resistance values of the resistors 2023.

The supply input of the comparator 13 is biased by a voltage U5 while the supply input of the comparator 14 is connected to ground potential. The interconnected outputs of the comparators 13, 14 are supplied to a logic module 24.

The method of operation of the circuit for mains monitoring will now be described.

Figure 2 shows the change of voltage of a rectified three-phase network with respect to time, region I showing the "undervoltage" case, region 11 the "normal d.c. operating voltage" case, region III the "over-voltage" case and region IV the "failure of one phase of a three-phase system" case.

According to definition, "under-voltage of the feeding three-phase mains always occurs if the minimum values (= voltage collapse) of the undulating d.v. voltage resulting from the rectified three-phase voltage just reach or fail to reach a defined voltage threshold Uu (region 1). "Over voltage of the feeding three-phase mains" according to definition occurs when the peak values of the undulating d.c. voltage just reach or exceed a defined voltage threshold Uu (range III).

According to definition, the case of "normal operating voltage of the feeding three-phase mains" always occurs if minimum values as well as peak values of the undulating d.c. voltage are within the voltage band bounded by the defined voltage thresholds Uu and Uü (range II).

In the case of range IV "failure of one phase of a three-phase system" the peak values of the half wave will be within the voltage band but the minimum values of the undulating d.c. voltage fail to reach the voltage threshold Uu so that under voltage also occurs in range IV.

The circuit system for mains monitoring responds to "over-voltage" as well as to "undervoltage" or "phase failure". Adjustment of the voltage thresholds Uü or Uu is obtained by means of the voltages U3 or U4 via the appropriate resistors 20/21 or 22/23. In the present embodiment the comparator i3 therefore monitors the mains for over voltage and the comparator 14 monitors the mains for under voltage and phase failure.

The resistors 1, 2, 3 are provided to reduce the three-phase voltages which are to be monitored The d.c. voltage obtained downstream of the three-phase rectifier (4, 5, 6, 7, 8, 9) is subdivided by the voltage divider 10/11 and supplied to the comparators 13, 14, and the zener diode 12 protects the comparators 1 3, 14 against excessive voltage peaks.

The voltages U, and U2 as well as the resistors 15, 17 and 18 are selected so that in the presence of a normal operating voltage of the three-phase mains (i.e. the comparators 1 3 and 14 have not come into operation and are in the cut-off state) no current will throw via the diode 1 6 (i.e. the diode 1 6 is also driven to cut off). In the event of a mains disturbance, i.e. over voltage or under voltage or phase failure, the corresponding comparator 13 or 14 will be driven into the conductive state. If the undulating d.c.

voltage reaches or exceeds the voltage threshold Uu, the comparator 13 will be driven into the conductive state and if the undulating d.c. voltage reaches or fails to reach the voltage threshold Uu the comparator 14 will be driven into the conductive state. In both cases the capacitor 19, which is charged by the voltage source U2 via the resistor 18, discharges via the resistors 1 7 and the diode 16, which is conductive, into the corresponding output of the comparator 1 3 or 14, which is in the conductive state, namely with a time constant T,=C,9x R", where C19 corresponds to the capacitance of the capacitor 1 9 and R,7 corresponds to the resistance value of the resistors 1 7.

The appropriate comparator 13 or 14 is driven into the conductive state when "over-voltage" or "under-voltage" states occur whenever the maximum or minimum instantaneous values of the rectified mains voltage just reaches the corresponding limit values Uu or U,. The resultant periodic signal thus produced is converted by the components 17, 18, 16 and 19 into a constant signal.

In the event of "phase failure" the value of the undulating d.c. voltage fluctuates constantly at the rhythm of the mains frequency between a peak value, within the normal operating voltage band, and a minimum value which is below the threshold voltage Uu. In this case, the comparator 14 switches constantly at the rhythm of the mains frequency from cut off condition into the conductive condition and vice versa. While the comparator 14 conducts, the capacitor 19 is discharged via the resistor 1 7 as already described. While the cut-off condition of the comparator 14 persists, the capacitor 1 9 is recharged via the resistor 1 8. The resistance value R18 of the resistor 18 must be made sufficiently large to ensure that the time constant T2 of the charging operation T2=CrgxR,a is large in relation to the time constant T, of the discharging operation, i.e. T2 must be greater than one cycle of the three-phase voltage system. This ensures that a "phase failure" alarm signal is constantly maintained and does not appear and is then extinguished at the rhythm of the threephase mains frequency.

"Phase failure" as well as "under-voltage" or "over-voltage" signals are detected and evaluated by the logic module 24. The logic module 24 processes the voltage levels which appear at the interconnected outputs of the comparators 1 3 or 14, i.e. in the simplest case, the logic module 24 can differentiate between the voltage obtained from the conductive comparator 1 3 or 14 against the voltage obtained when the comparators 13 and 14 are driven to cut off, so that the module can supply corresponding output pulses. If a mains breakdown occurs the logic module 24 provides mains isolation for the load which is sensitive to changes of mains voltage. At the same time the module will store the mains breakdown event and display it.

No automatic reconnection of the isolated load is provided when the mains breakdown is at an end. The renewed connection of the mains is possible only by manual operation of a key (not shown). This ensures that any breakdown of the supply mains is notified to the operating personnel. However, if necessary it is of course also possible to provide for automatic reconnection of the disconnected load after the disappearance of a mains breakdown.

Claims

1. A circuit for monitoring a single phase or three-phase mains supply for over-voltage, undervoltage and for phase failure, comprising two comparators, each phase of the mains supply to be monitored being connected via a bridge rectifier to the negative input and the positive input, respectively, of the two comparators, the other inputs of the comparators being connected to receive respective reference voltages and the outputs of the comparators being interconnected to a voltage source and by way of a series combination of a diode and a resistor to a capacitor and to a further resistor which is biased by a voltage source.

2. A circuit as claimed in Claim 1, in which the resistors and the capacitors are dimensioned such that the time constant for the discharge of the capacitor via the diode and the resistor connected in series is small in relation to the time constant for the charging of the capacitor via the said further resistor.

3. A circuit as claimed in Claim 1 or 2, in which the interconnected outputs of the comparators are further connected to a logic module.

4. A circuit for monitoring a mains supply having one or more supply phases, which comprises means for full-wave rectifying the or each phase to be monitored, a first comparator for determining when the peak value of the rectified mains voltage exceeds a first threshold and a second comparator for determining when the minimum value of the rectified mains voltage drops below a second threshold value, by a bias comparator having a common output connected resistor to a first voltage source and by way of a diode and a low pass filter to a second voltage source.

5. A circuit for monitoring a single phase or three-phase mains supply constructed substantially as herein described, with reference to and as illustrated in the accompanying drawings.