Zero crossing circuit

Abstract

 A microprocessor-based zero crossing circuit produces in response to an input command provided by closure of a switch (18) an output signal (13, 14) which is synchronised with the zero crossing points of an a.c. supply and includes a predetermined delay. By this means the output signal (13, 14) may be used to operate or control operation of, for example, a circuit breaker (Figure 2; Figure 3) for the a.c. supply in order to minimise contact arcing. The circuit is useful in an electricity supply meter when the input command on input 12 is generated by a credit or pre-payment mechanism which acts to cut-off an a.c. supply on expiry of credit. Following a cut-off command on input 12 the circuit produces the synchronised output signal (13, 14) appropriately delayed to take into account the operating time of the circuit breaker (Figure 2; Figure 3) so that its contacts (20, 21) open as nearly coincident as possible with a zero crossing point of the supply.

Description

[0001] The invention relates to a zero crossing circuit particularly, but not exclusively, for the purpose of operating an a.c. circuit breaker in electricity supply apparatus.

[0002] In any electric circuit breaker arcing occurs between circuit contacts, at the point of make or break, resulting in damage to the contacts which will eventually necessitate their replacement. In an a.c. circuit it is possible to minimise arcing damage and thereby prolong contact life by arranging for the contacts to make or break at or, at least, as near as possible to the a.c. current zero-crossing point.

[0003] An object of the present invention is to provide a zero crossing circuit which continually monitors the a.c. current zero-crossing points and acts automatically to synchronise operation of a circuit breaker, for example, with a zero crossing point.

[0004] According to the present invention there is provided a zero crossing circuit for producing an output in response to an input command in which the output is synchronised with the zero-crossing points of an a.c. supply comprising, means for sensing the waveform of the a.c. supply connected via full wave rectifying means to the input of switching means having a low voltage switching threshold level so as to produce a stream of relatively narrow pulses centred on the zero-crossing points of the a.c. waveform and, gating means operative in response to an input command to gate the next of said pulses to actuate a variable delay output generator which produces after a predetermined delay an output synchronised with the a.c. waveform zero-crossing points.

[0005] The invention and how it may be carried into practice will now be described, by way of example only, with reference to the accompanying drawings, in which:

Fig. 1 shows the circuit diagram of a zero crossing circuit, and

Figs. 2 and 3 show general views of alternative circuit breaker arrangements for use in conjunction with the zero crossing circuit of Fig. 1 in electric supply apparatus.

[0006] Referring now to the drawings, there is shown in Fig. 1 at reference 1 a schematic a.c. electric supply circuit and at 2 a transformer for sensing the a.c. current in circuit 1. The secondary winding 3 of the transformer forms together with rectifier diodes 4 and 5 a full wave rectifying means which supplies rectified but unsmoothed half-cycle pulses via a step-down potentiometer arrangement 6 to the input 7 of an amplifier 8.

[0007] The step-down potentiometer 6 comprises essentially a pair of resistances having values in the ratio 100:1 to provide in effect a very low switching threshold level at the input 7 of amplifier 8. The amplifier possesses a high gain so that its input 9 switches between alternative output saturation levels as the input voltage level passes through said threshold switching level. As the threshold is close to the zero level which is also the steepest part of the a.c. current curve the amplifier output 9 consists of a stream of relatively narrow pulses each temporarily centred on a zero-crossing point of the a.c. waveform.

[0008] The pulse stream output 9 is connected to a first input 10 of an integrated circuit block 11 which also receives an input command signal at a second input 12. The circuitry within block 11 is operative to gate the first input 10 by the second input 12 and at the next narrow pulse an input 11 following an input command on input 12, provided by closure of switch 18 to commence timing a predetermined delay at the end of which an output is produced on either output line 13 or 14, as will be further described below.

[0009] In the embodiment being described the integrated circuit block 11 comprises a microprocessor having an internal program memory store (not shown) in which is stored a control program adapted to carry out the functions described above. For the purposes of its internal functioning and timing the microprocessor has connected to it an external timing crystal 15 of natural resonant frequency 6MHz. Power for the. microprocessor is provided via the terminals marked 0_v and +5_v, in the drawing, from a power supply unit 17 which may conveniently be connected to the secondary winding 3 of transformer 2.

[0010] Where the invention is for use in electric supply apparatus it may be used in conjunction with a circuit breaker of the type shown in either Fig. 2 or Fig.' 3. In Fig. 2 the circuit breaker shown comprises an adjustable, but otherwise fixed, contact 20 and a movable contact 21 carried at one end of a pivoted contact arm 22. A contact operating arm 23 pivoted together with arm 22 about an axis 24 is arranged to bear against arm 22 through a contact pressure spring 25. The operating arm 23 has a yoke 26 at one end in which is eccentrically journalled at 27 a spindle 28 which is attached through a toggle link 29 with one arm of a rocking toggle lever 30 mounted on a pivot 31 secured to the fixed base plate of the circuit breaker. The lever 33 has two arms 32 and 33 on opposite sides of the pivot 31 which may be tipped against respective stops 34 and 35 by solenoid plungers 36 and 37. The positions of stops 34 and 35 is set to provide the toggle mechanism with an over centre action in both its alternative positions each of which is maintained by stored energy in partially compressed toggle spring 29a.

[0011] Solenoids 38 and 39 are arranged to operate plungers 36 and 37 respectively and are mounted above either arm of the toggle lever 30, with their axes parallel, as shown in the drawing. The plungers 36 and 37 are normally withdrawn into the illustrated position by the restoring force of return springs 40 and 41, therefore solenoids are pulse energised. That is, to extend the plunger of a solenoid its winding requires an energising pulse of sufficient power and duration to overcome restoring spring pressure and drive the plunger to its fullest extent.

[0012] In Fig. 2 the toggle lever 30 is shown tipped one way in which arm 33 rests against stop 35 and a form is transmitted through toggle spring 29a to the contact moving arm 23 holding the contacts 20 and 21 closed. In order to open the contacts the solenoid 38 is energised and plunger 36 depresses arm 32 of the toggle lever against stop 34 breaking the over-centre position of toggle link 29 and resetting it in its alternative position. This action withdraws operating arm 23 opening conacts 20 and 21.

[0013] The arrangement of the circuit breaker of Fig. 3 is basically the same as that shown in Fig. 2 but the toggle link arrangement is replaced by a shaped cam, otherwise like parts have like references.

[0014] In Fig. 3 the contacts are mounted vertically below the operating solenoids 38 and 39. The movable contact arm has a degree of inherent resilience and is arranged to be moved by a shaped cam 50 formed on the underside of toggle lever 30. This cam has a rounded toe portion 51 which rocks which permits the resilience of arm 22 to break contacts 20, 21 when lever 30 is tipped one way and, has a heel portion 52 which urges arm 22 towards the contacts closed position (illustrated) when lever 30 is tipped as shown.

[0015] Referring now to operation of the circuit of Fig. 1; consider the outputs 13 and 14 to be connected to solenoids 38 and 39 of a circuit breaker of the type shown in Fig. 2 or 3 fitted to an electricity supply apparatus. In use electricity is to be supplied when switch 18 is closed and disconnected when it is opened. Suppose the switch is initially open and at some instant is closed, when this switch changes state the next pulse on input 10 is gated to start timing the predetermined delay period at the end of which a solenoid energising pulse is produced on output 14. This pulse operates solenoid 39 and causes contacts 20 and 21 to be closed as described above.

[0016] At some later instant switch 18 opens and the same series of events occurs but on this occasion results in a pulse on output 13 to operate solenoid 38 and open the contacts 20 and 21.

[0017] The predetermined delay period is chosen to include compensation for the operating time of the solenoids and circuit breaker contact mechanism so that the actual instant when contact arcing can be expected coincides as closely as'possible with the zero crossing point of the a.c. waveform. The delay period may also be chosen to take into account a high power factor which causes the actual zero crossing point of the current to lag somewhat behind the voltage zero-crossing point.

[0018] In an alternative embodiment the microprocessor may be adapted to provide a single output which is alternatively energised or de-energised in order to operate a circuit breaker'of the continually energised to close type.

[0019] In one embodiment of the invention the switch 18 is connected to be operated by the credit or pre-payment mechanism of an electric supply meter, e.g. the well known clockwork coin mechanism is but one example. When a coin is inserted switch 18 is closed until a predetermined amount of electricity has been supplied whereupon the switch is opened.

[0020] The invention may be used in other embodiments to control the make and break of a.c. supply circuits in any situation where contact damage due to arcing is a problem.

Claims

1.. A zero crossing circuit for producing an output in response to an input command in which the output is synchronised with the zero-crossing points of an a.c. supply comprising, means (8) for sensing the waveform of the a.c. supply connected via full wave rectifying means (4, 5) to the input (7) of switching means (8) having a low voltage switching threshold level so as to produce a stream of relatively narrow pulses (9) centred on the zero-crossing points of the a.c. waveform and, gating means (11) operative in response to an input (12) command to gate the next of said pulses to actuate a variable delay output generator which produces after a predetermined delay an output (13, 14) synchronised with the a.c. waveform zero-crossing points.

2. A zero crossing circuit as claimed in Claim 1 wherein the gating means and the variable delay output generator comprise a microprocessor (11) responsive to receipt of an input (10) command to measure from the next zero-crossing pulse a predetermined delay and, at the end of said delay to generate the output (13, 14).

3. Electricity supply apparatus including a zero crossing circuit as claimed in either Claim 1 or Claim 2 the output of which is connected to actuate an electric supply circuit breaker (Fig. 2; Fig. 3).

4. Electricity supply apparatus as claimed in Claim 3 wherein the predetermined delay includes compensation for the operating time of the circuit breaker (Fig. 2; Fig. 3) so that make or break of the electricity supply is substantially coincident with zero crossing of the supply.

5. Electricity supply apparatus as claimed in Claim 3 or Claim 4 wherein the circuit breaker (Fig. 2; Fig. 3) includes an energise to make operating solenoid (39) operable by a zero crossing circuit output (14) arranged to occupy a first state to energise the solenoid and a second state to de-energise the solenoid and wherein a change between said states constitutes the synchronised output.

6. Electricity supply apparatus as claimed in Claim 3 or Claim 4 wherein the circuit breaker includes a make operating solenoid (39) and a break operating solenoid (38) and the zero crossing circuit as arranged produces a first output (14) to energise the make solenoid (39) and a second output (13) to energise the break solenoid (38).

7. Electricity supply apparatus as claimed in Claim 6 wherein the circuit breaker is self-latching and the first and second outputs (13, 14) of the zero crossing circuit comprise pulses for energising the respective solenoids (38, 39).

8. Electricity supply apparatus as claimed in any of preceding claims 3 to 7 wherein the input command is provided by a credit meter apparatus.

9. Electricity supply apparatus as claimed in Claim 8 wherein the input command is provided by voltage switch means (18) arranged to be actuated by the credit meter apparatus.

Drawing