IE52697B1 - Improvements relating to protective systems for electric motors and other electrical apparatus - Google Patents

Description

This: invention relates to an electrical protective system for sensing a fault in associated electrical apparatus (herein called the main apparatus) and for providing an electrical output (herein called the protec5 tive output) in response to such sensing and for initiating remedial action, in relation to the main apparatus. Such system is hereinafter called- a protective system of. the kind specified.

The present invention has been developed primarily in relation to the requirement to provide a protective system of the kind specified for an electric motor.

A fault condition which is required to be sensed in electric motors is overheating of the winding or windings. For; this purpose it is known to provide a sensing means such as a thermistor in thermal communication with the winding to be monitored, e.g. embedded in such winding but insulated therefrom electrically, and to connect the thermistor in a control circuit providing a protective output to operate a relay having contacts in the motor.starter circuit and so connected in the latter as' to cause the motor contactor to trip out in the event of the thermistor sensing a winding temperature above a predetermined value.

Although such protective systems perform- satis25 factorily in most cases they are exposed to malfunctioning by reason of a number of causes.

Thus, in a case where the thermistor controls energisation of a relay discontinuance of the condition of motor operation giving rise to the overheating of the 52687 motor winding may permit the motor to be re-energised without occurrence of the fault having come to the attention of an operator or any supervising personnel and over a period of time repeated incidence of the fault may cause severe damage to the motor.

Further, if the thermistor should be subjected to a short circuited condition, either by virtue of conductive contamination of the thermistor itself or break down of electrical insulation between electrical leads connecting the thermistor to the relay circuit, then the thermistor will not provide a fault signal output (high resistance between its output terminals) and the fault will not be detected.

The object of the present invention is to provide an improved protective system of the kind specified which is b'etter adapted than those heretofore available to avoid occurrence of these faults.

According to the invention, a protective system of the kind specified comprising a fault sensing means for sensing a high tenperature condition and providing a change of resistance from a value producing a normal signal to a higher value producing fault signal, according to the existence of a normal condition or fault condition of the main apparatus, a solid state control circuit comprising two Schmitt trigger circuits responding respectively to said higher value of resistance of said sensing means and a short circuited or low resistance value below that producing said normal signal in either case to produce a changeover of the Schmitt trigger circuit concerned from a normal setting to a triggered setting, in the latter of which it establishes the protective output through the intermediary of a switching circuit furnishing such output, said control circuit including solid state latch means holding at least the Schmitt trigger circuit responding to said high temperature condition in its triggered setting once changeover to its triggered setting has occurred, irrespective of continuance or discontinuance of the 2697 fault condition of the main apparatus, and wherein the latch means includes reset means manually operable in two modes, in the first of which it disables the latch means temporarily, and re-subjects the Schmitt trigger circuit concerned to either the fault signal or the normal signal according to the state of the sensing means at that time, and in the second of which it disables the latch means permanently.

The sensing means may comprise one or more therm10 istors mounted, or adapted for mounting, in thermal communication with the part of the main apparatus to be protected. If; more than one thermistor is provided each of these may be so connected to the control circuit that incidence of a fault signal from any one of the therm15 istors brings about establishment of the protective signal.

Alternatively or in addition the protective output may be utilised to operate an indicator means visual and/or’ audible so that an operator or attendant can manually operate a switch in the control circuit of the main apparatus to bring about de-energisation of the latter'.

The protective output may be used for any of a variety of purposes depending upon the character of the main apparatus. Where the main apparatus is an electric motor and the sensing means is associated operatively with the motor windings, the protective system may . include a relay which is operated in response to the protective output to bring about operation of controlling contacts in the motor starter or control circuit and so de-energise the motor.

The invention will now be described by way of example with reference to the accompanying drawings wherein:FIGURE 1 is a circuit diagram of one embodiment of protective system of the kind specified in accordance with the invention; FIGURE 2 is a circuit diagram of a modified embodiment.

Referring to Figure 1, the circuit is intended primarily to be applied to the protection of an electric motor, for example a three phase motor.

The circuit includes sensing means in the form of three thermistors connected in series and for convenience bearing the single reference Th and mounted, or adapted for mounting, in thermal communication with respective phases of the stator winding of the motor.

The circuit includes a low detector branch 10 for detecting whether the leads to the thermistors have become short circuited or any one of the thermistors as such has become contaminated to an extent such that it registers a short circuit or a low resistance below a predetermined value.

Further, the circuit includes a high detector branch 11 which detects whether any one of the thermistors has attained a resistive value corresponding to a winding temperature above a predetermined value.

The circuit is powered by a low voltage supply, for example a nominal 12 volts applied between a positive terminal ti and a negative terminal t2 and this supply serves to energise a relay A having contacts (not shown) in the starter or controlling circuit of the motor and which, when closed, provide for energisation of the motor.

The relay A, having a protective diode D2 connected in parallel with it, is energised subject to conduction of transistor TR7 which is its normal state. 69 7 .15 In the event of one or more of the thermistors having a high resistance in consequence of the associated winding, or any of them, attaining a temperature above a predetermined value, the voltage at the junction between the thermistors Th and resistor R1 increases.in value and provides an input to transistors TR4, TR5 connected as a Schmitt trigger. The transistor TR4 is normally nonconducting (under the normal voltage.level existing at the junction of R1 and Th) and when the base voltage is raised it;becomes conducting and causes-transistor TR5 to change from conducting to a. non-conducting condition.

- The circuit includes a latching branch comprising transistor TR3. Initially TR3 is non-conducting but when TR4 conducts a conducting path is established through resistor R6, emitter collector of TR3, switch S1 (in the position shown), base-emitter of transistor TR4, and R8. Transistor TR3 and resistor R6 (which has an appropriate low value) thereby hold the junction of R1 and Th a predetermined high voltage irrespective of whether the high fault condition persists or not. When desired, switch S1can.be moved temporarily to break the circuit between upper and lower.right-hand contacts of S1 thereby re-setting the latching branch to its initial state (TR3 non- conducting). If the switch SI is set to bridge the. upper and lower left-hand contacts the latching function is disabled.

When TR5 becomes non-conducting the level of. voltage on the base of transistor TR6 rises to a value near the positive terminal t1 and TR6 is thus cut off interrupting current through resistor R11 and reversing the bias on the base of transistor TR7 to cause this to cut off and de-energise relay A.

The protective output signal furnished at .the contacts of this relay may be used as previously indicated at the starter or control circuit of the motor to cause this to shut down. If desired further contacts, of the relay- A may be utilised to energise a warning light and/or audible warning device..

Capacitor C1 ensures that the base of TR4 remains at a voltage level no higher than its emitter pending conduction of TR5 at initial switch on and thus prevents premature and unwanted latching. When TR5 conducts the emitter of TR4 is held above the voltage of the base by appropriate selection of the values of R1, R2, R8, R9, unless Th reaches a high value, e.g. from the cause already mentioned.

In the.event of short circuit between the leads to any cf the thermistors, the voltage at the junction between R1 and Th which was normally sufficient to maintain transistor TR1 of another Schmitt trigger conducting, may fall below the requisite value so that TR1 becomes non-conducting and transistor TR2 changes from non-conducting to the conducting state.

This effectively places resistor R5 which is of low value in parallel to resistor R12 and reduces the positive bias on the base of transistor TR7 at the point at which this becomes non-conducting. As before, the relay A then becomes de-energised and its contacts operate to provide the effective output.

Although the latch means has been Illustrated in relation only to the high detector 11, it will be understood that if desired a latch means could be provided in association with the Schmitt trigger pair TR1, TR2 together with a reset switch similar to S1.

Variable resistor R2 can be adjusted to set the level of the normal signal to levels of the high and low fault signals as appropriate to meet the protective requirements.

It will of course be understood that instead of using individual transistors and hard-wired connections, the functions performed by the Schmitt triggers and the latching branch of the circuit may be incorporated in one or more integrated circuit units.

Alternatively the circuit may be embodied in a ceramic substrate in which conductors and transistors are Incorporated, the transistors, or the complete assembly, being encapsulated.

In the modified embodiment shown in Figure 2, components corresponding to those of Figure 1 are designated by like reference numerals and the preceding description is to be deemed to apply, the circuit operating generally in the same manner as Figure 1.

The modifications made are for the purpose of eliminating or reducing the effects on the latching transistor TR3 of transient signals generated by opening and closing contacts of the motor starter or controller.

For this purpose, instead of providing a connection to the left-hand lower contact of switch ST from the collector of TR3, the latter is connected to the negative rail through a resistor RT3 and capacitor C2 in parallel therewith. A resistor R14 and capacitor C3 in parallel therewith also connects the base to the emitter of transistor TR3.

The resistive chain R6, R14, R13 provides a bias condition for transistor TR3 and under normal conditions renders it non-oonduoting. The capacitors assist in preventing transience disturbing the operating conditions for TR3 as shown.

To re-set the latch means the switch SI is opened temporarily. If it is required to disable the latching function switch ST is set to the open position.

Claims (7)

CLAIMS:

1. A protective system of the kind specified comprising a fault sensing means for sensing a high temperature condition and providing a change of resistance from a value producing a normal signal to a higher value producing fault signal, according to the existence of a normal condition or fault condition of the main apparatus, a solid state control circuit comprising two Schmitt trigger circuits responding respectively tc said higher value of resistance of said sensing means and a short circuited or low resistance value below that producing said normal signal in either case to produce a changeover of the Schmitt trigger circuit concerned from a normal setting to a triggered setting, in the latter of which it establishes the protective output through the intermediary of a switching circuit furnishing such output, said control circuit including solid state latch means holding at least the Schmitt trigger circuit responding to said high temperature condition in its triggered setting once changeover to its triggered setting has occurred, irrespective of continuance or discontinuance of the fault condition of the main apparatus, and wherein the latch means includes reset means manually operable in two modes, in the first of which it disables the latch means temporarily, and re-subjects the Schmitt trigger circuit concerned to either the fault signal or the normal signal according to the state of the sensing means at that time, and in the second of which it disables the latch means permanently.

2. A protective system according to Claim 1 wherein each Schmitt trigger circuit comprises two transistors connected for mutually exclusive conduction and nonconduction states with the base of one of them receiving the normal or fault signal as the case may be, and the latch means comprises a transistor having its emitter 52697 10 collector circuit connected through . the reset means, comprising a manually operable, normally closed, reset switch, .between an appropriate voltage source and the last .said base to hold that base at a voltage beyond that 5 producing triggering so long as the reset switch is closed.

3. A protective system according to Claim 2 wherein, in the first mode of operation of the reset means, the reset switch is held temporarily in. an open position and in the iq second mode of .operation..the reset switch is- set in a permanently open position to disable the latch means. .

4. A combination of a protective system according to any one of the. preceding claims and an electric motor wherein the sensing means is associated operatively with 15 the motor windings and the switching circuit includes a relay operated in response to changeover of either of the Schmitt trigger circuits- to bring about operation of controlling contacts in the motor starter or control circuit and so de-energise the motor. 20

5. A combination as claimed in - Claim 4 wherein the sensing means comprises a plurality of thermistors associated with respective windings of the motor.

6. A protective system substantially as hereinbefore described with reference to and as shown in Figure 1 of 25 the accompanying drawings.

7. A protective, system substantially as hereinbefore described with reference to and as shown in Figure 2 of ' the accompanying drawings.