GB2033177A - Circuit breaker with undervoltage release - Google Patents

Abstract

A circuit breaker has an under voltage release mechanism including a trip spring, a solenoid coil UVR, and an electrical circuit for resetting the solenoid after the undervoltage release mechanism has been tripped, the circuit supplying coil UVR with an intermittent high power pulse when the voltage is above a first predetermined magnitude to pull the solenoid in against the spring, and also supplying a continuous lower level of power to coil UVR sufficient to retain pull-in when the voltage is above a second predetermined magnitude. <IMAGE>

Description

SPECIFICATION Circuit breaker This invention relates to a circuit breaker having an under voltage release (UVR) facility.

Under voltage releases are arranged so that when the monitored voltage from which they are fed (normally the system supply voltage) is below a certain limit, the breaker is release, thus switching off equipment which presumably would be damaged by continued application of a lower-thannormal supply voltage. In addition, the releases prevent the breaker from being closed when the monitored voltage is below a limiting value. For example, in a widely accepted international switchgear standard an under voltage opening release is required to open the circuit breaker even on a slowly falling voltage at a value between 70% and 35% of the nominal supply voltage and to prevent the closing of the circuit breaker when the supply voltage is below 35% of its nominal value.The UVR must not, however, prevent closing when the supply voltage is 85% or more of its nominal value. These requirements have heretofore been accomplished by means of an arrangement comprising a solenoid energised by the monitored voltage the armature of which acts against a spring having sufficient stored energy to trip the circuit breaker when the solenoid releases. Due to the nature of solenoid force characteristics, they are readily able to retain a far greater spring force than they could pull in against. For example, a solenoid having a stroke of say 12.5 mm and of a typical size for inclusion in a moulded-case circuit breaker, might be able to retain a force of say 1 Kg, but its starting force could be as low as 0.03 Kg.

Obviously, in order to get the necessary trip energy out of a small coil it is desirable to utilise a flat rate spring having a force suitably below the 1 Kg figure which the solenoid would release when the supply voltage dropped. When the supply voltage reestablished, the solenoid could not pull in against the spring and therefore had to be reset mechanically in order that the breaker could close. This reset action has been achieved in a variety of ways, but nearly always by taking advantage of the motion of the circuit breaker dolly between the trip and reset positions in order simultaneously to reset the under voltage release solenoid, along with any catches contained within the breaker mechanism and its tripping elements.This arrangement has presented many problems of a manufacturing nature since each release required basically at least three setting operations; the pick up voltage, the drop off voltage and the setting of the spring in relation to the tripping mechanism. Afourth setting of the reset means was also frequently required and was particularly troublesome since it had to reset positively, but still accommodate a wide variation in tolerances of functional positions and spring forces of both the release assembly and the breaker itself.

It is an object of the present invention to overcome the manufacturing difficulties of the known arrangements and eliminate as far as possible the setting operations required.

Accordingly the invention provides a circuit breaker having an under voltage release mechanism including a trip spring, a solenoid and an electrical circuit for resetting the solenoid after the under voltage release mechanism has been tripped.

Differently expressed, the circuit breaker of the invention has an under voltage release mechanism including a trip spring, a solenoid and associated control circuitry, the associated control circuitry supplying the coil of the solenoid with an intermittent high power pulse when the voltage monitored by the under voltage release is above a first predetermined value to pull the solenoid in against the action of the trip spring, said control circuitry also supplying a substantially continuous lower level of power to the coil of the solenoid sufficient to retain the solenoid in the pulled in condition when the monitored voltage is above a second predetermined value.

In most cases the monitored voltage is preferably the system supply voltage. The first predetermined value of the monitored voltage is preferably 75% of the monimal value of the monitored voltage, and the second predetermined value is between 70% and 35% of the nominal value of the monitored voltage.

The energisation period of the solenoid by the pulse can thus be chosen to be such that problems of either large size or coil heating, that continuous energisation would otherwise cause, do not arise.

The period can be arranged to be just long enough to enable the coil to pull in correctly against the spring, even when energised by a voltage which is below 85% of the nominal supply value. Preferably a resistor is switched in series with the coil to restrict the length of the pulses supplied to the coil to limit the current and thus their heating effect, but still retain the lower level of power through the solenoid to prevent it releasing the spring and tripping the breaker. This current can be chosen also so that on a slowly falling monitored voltage, the solenoid will release the spring at somewhere between the specified limits of 70 and 35% of the nominal value of the monitored voltage.In order to make the system continuously responsive to changes in the monitored voltage, the switching in and out of the series resistor is preferably a cyclic process, the cycle time being a compromise between an upper limit determined by coil heating and a lower limit determined by the minimum time required to reset the circuit breaker. The switching function can be obtained from a relatively simple electronic circuit, which can also have the feature that it will not permit full value energisation of the coil unless the monitored voltage is above the first predetermined value.

The associated control circuitry preferably includes a rectifier feeding a pair of resistors across one of which a timing capacitor and zener diode are arranged in parallel to switch the series resistor in and out. The zener diode, when conducting, preferably activates a transistor switch arrangement which, in its conducting condition, short circuits the series resistor. In this case, preferably the zener diode only conducts when the monitored voltage is above the first predetermined value.

The invention will be described further, by way of example with reference to the accompanying drawing wherein the single figure is a circuit diagram of an associated control circuit for the under voltage release facility of a preferred circuit breaker of the invention.

A monitored voltage Via, which is normally alternating, is applied first to a full wave diode bridge rectifier 10 of sufficient rating to provide the necessary current to energise the UVR solenoid. Its rectified output voltage is fed first to a voltage dividerformed by resistors R1 and R2. A capacitor C1 connected across R2 serves to average out and smooth the rectifier output, and in conjunction with suitably selected values for R1 and R2, provide a time delay for that portion of the cycle when a resistance R6 is in series with the UVR solenoid, i.e.

not short circuited by a transistor TR2.

Across C1 is connected a circuit arranged to switch on and pass current via R3 to discharge the capacitor C1 when its voltage has reached a value determined mainly by the zener voltage of Z1. This zener voltage is chosen to be approximately 75% of the voltage appearing across R2 when V1 is at its nominal value.

Thus current will only flow in Z1 when the average value of V1 is more than 75% of its nominal value and an adequate time has passed to enable C1 to charge up via R1.Transistors TR1 and TR2 are normally held switched off by virtue of resistors R4 and R5 respectively, connected between their bases and emitters. When the capacitor voltage reaches a level at which Z1 starts to conduct, current will flow through it and resistor R5 until the voltage drop across R5 is sufficient to overcome the junction voltage of TR2 which will then commence to conduct.Current will flow from the capacitor via R3, R4 and D1 into the collector of TR2 and the volt drop this current produces across R4 will become sufficient to overcome the junction voltage of TR1 which will also turn on, The collector current of TR1 thus adds to the base current of TR2 and the two transistors are rapidly switched fully on as the process is cumulative. The switching on of TR2 also causes current to flow via the UVR coil and D2 through TR2, effectively short circuiting resistor R6. Thus while TR2 is switched on, the full rectified reference voltage is applied across the UVR coil.The discharge of capacitor C1 via R3 and the parallel circuits comprising TR1, R4, D1, TR2 and R5 continues until its reducing voltage becomes insufficient to sustain adequate current flow to keep the transistors turned on. When this happens, both transistors revert rapidly to an off condition in a similar manner to their switch on and R6 then ceases to be short circuited by TR2. C1 then recharges and the cycle continues.

The time required to charge up C1 is determined by its own value and the values of R1 and R2. This determines the interval between the pulses of full voltage across the UVR coil. The duration of the pulse is the time to discharge C1 and is determined by the value of C1 in conjunction with resistor R3.

Resistors R4 and R5 are relatively high values, their purpose being to hold the base of each transistor at the same potential as its emitter and prevent switch on by spurious leakage currents etc. possible in a power environment. The voltage of Z1 is chosen to set the voltage at which C1 commences to discharge and is such that it is comfortably within the collector/ emitters rated voltage of TR1. Diode Dl prevents flow of current into C1 and Z1 via the UVR coil, D2, R4 and R3, and D2 prevents current flow in the reverse direction from C1 through R3, R4 and D1 into R6.These diodes are both necessary because the voltage across R6 is pulsating rectified Ac, whereas that across C1 is substantially constant during the time of a supply half cycle. Thus there could be unwanted current flows during portions of the mains half cycle, which would cause unwanted switching of the transistors.

Again it will be noted that there is only one timing component in the circuit - Cl. This is chosen to be as small as possible consistent with being able to deliver sufficient current for the pulse duration to keep TR2 fully turned on. It has been found to make R1 and R2 the same value so that the potential to which C1 charges is a maximum of half the rectified reference voltage. Thus as arranged, the resistance R6 remains permanently in series with the UVR solenoid and when the slowly increasing supply voltage reaches 75%, TR2 will be caused to switch on and short circuit R6. After a brief period it will switch off and continue to cycle in this manner as long as the aplied voltage is greater than 75%.The rate at which this cycling occurs is dependent upon the value of V1 above 75% - the higher it is the shorter will be the cycle time. It is therefore inportant to ensure that at the maximum iikely supply voltage when the cycle time is shortest and the current through the UVR solenoid and R6 greatest, the rating of the components is adequate for their expected dissipations. It should be noted that the on period of the two transistors is substantially constant whateverthe supply voltage, since it is determined by the discharge of C1 between fixed limits. Thus, there is no possibility of a short "on" period appearing at high supply voltages with possible problems in the energisation of the UVR solenoid. The remaining functional component in the circuit is D3.This is to prevent any possibiiity of inductive over-voltages appearing across the solenoid due to the switching action of the circuit.

With the invention, the previously necessary setting operations are eliminated, it merely being necessary to select the values of the components of the control circuitry (particularly C1 ) appropriately before manufacture.

In a typical example of a 5W continuously rated circuit breaker the high power pulse supplied to the coil of the solenoid by the control circuitry will be approximately 50 Watts and the continuously supplied lower level of power will be in the range 3 to 4 Watts (at the nominal rated voltage of the circuit breaker). Each pulse lasts between 30 and 40 milliseconds and the interval between the pulses is between 1 and 2 seconds.

The invention is not limited to the precise details of the foregoing and variations can be made thereto.

For example, any convenient alternative circuit could be used to achieve controlled intermittent energisation of the solenoid.

Claims (10)

1. Acircuit breaker having an under voltage release mechanism including a trip spring, a sole noid and an electrical circuit for resetting the sole noid after the under voltage release mechanism has been tripped.

2. Acircuit breaker having an under voltage release mechanism including a trip spring, a sole noid and associated control circuitry, the associated control circuitry supplying the coil of the solenoid with an intermittent high power pulse when the voltage monitored by the under voltage release is above a first predetermined value to pull the sole noid in against the action of the trip spring, said control circuitry also supplying a substantially con tinuous lower level of power to the coil of the solenoid sufficient to retain the solenoid in the pulled in condition when the monitored voltage is above a second predetermined value.

3. A circuit breaker as claimed in claim 2, where in the associated control circuitry includes a resistor which is switched in series with the coil of the solenoid by the circuitry to restrict the length of the high power pulses supplied to the coil to limit the current and thus their heating effect, but still retain the lower level of power.

4. A circuit breaker as claimed in Claim 3, wherein the switching in and out of the series resistor is a cyclic process.

5. A circuit breaker as claimed in claim 4, where in the associated control circuitry includes a rectifier feeding a pair of resistors across one of which a timing capacitor and a zener diode are arranged in parallel to switch the series resistor in and out.

6. A circuit breaker as claimed in claim 5, where in the zener diode, when conducting, acti vates a transistor switch arrangement which, in its conduct ing condition, short circuits the series resistor.

7. A circuit breaker as claimed in claim 6, where in the zener diode only conducts when the moni tored voltage is above the first predetermined value.

8. A circuit breaker as claimed in any one of claims 2 to 7, wherein the first predetermined value of the monitored voltage is 75% of the nominal value of the monitored voltage, and the second predeter mined value is between 70% and 35% of the nominal value of the monitored voltage.

9. A circuit breaker as claimed in any one of the claims 2 to 8, wherein the monitored voltage is the system supply voltage.

10. A circuit breaker substantially as hereinbe fore described with reference to, and as shown in, the accompanying drawing.