GB2094576A - Electric protective system - Google Patents

Abstract

A protective system, primarily for use in detecting over-heating of electric motor windings, comprises a sensing means e.g. thermistors (Th), a first detector branch (11) responsive to a high thermistor resistance (signifying overheating of the motor), and a second detector branch (10) responsive to a low thermistor resistance (signifying a short-circuited thermistor). The first detector branch includes a Schmitt trigger circuit associated with transistors (TR5 and TR6) controlling current through transistor (TR7) and a relay (A). A larching circuit is associated with transistor (TR3) for maintaining switch off of transistor (TR7) irrespective of continuance of the fault condition. The second detector branch includes a further Schmitt trigger circuit associated with transistors (TR1, TR2) which render transistor (TR7) non-conducting if any of the series connected thermistors short circuits. <IMAGE>

Description

SPECIFICATION Improvements relating to protective systems for electric motors and other electrical apparatus 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 protective 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 contactorto trip out in the event of the thermistor sensing a winding temperature above a predetermined value.

Although such protective systems perform satis factoriiy 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 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 shou Id 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 better adapted than those heretofore available to avoid occurrence of these faults.

According to a first aspect of the invention a protective system of the kind specified comprises a fault sensing means providing a normal signal and a fault signal, according to the existence of a normal condition or fault condition of the main apparatus, a solid state control circuit providing the protective output and responsive to the normal and fault signals from the sensing means respectively to withhold or establish the protective output, said control circuit including solid state latch means for holding the protective output of the circuit in being once it has been established irrespective of continuance or discontinuance of the fault condition of the main apparatus.

From a second aspect the invention resides in the provision of a protective system of the kind specified comprising a fault sensing means providing a normal signal and a plurality of fault signals each pertaining to a respective fault condition of the main apparatus and sensing means in combination, and one of which fault conditions is a short circuited or low resistance condition of the sensing means, and a solid state control circuit providing the protective output and responsive to a normal signal from the sensing means to withhold the protective output and responsive to any of the fault signals to establish the protective output.

Preferably both aspects of the invention are combined in a single protective system of the kind specified, latch means being operative to hold the protective output in being once it has been established by incidence of a selected one or more of the fault conditions as may be required. The latch means may include manually operable reset means. If desired it may also include manually operable disabling means.

The sensing means may comprise one or more thermistors 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 thermistors brings about establishment of the protective signal.

Alternatively or inaddition 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 ofthe 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 lisa 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 primarilyto 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 co@nected ir @ @@es and for convenience bearing the single reference Th and mounted, or adap@@d for mounting, in therm@@ communication with respective phases of the stator winding of the motor.

The circuit includes a low detector branch 1 'or detecting whether the oads to the thermistors have become e short circuited or any one of the thermistors as such has become contaminated to en 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 t1 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.

In the event of one or more 0g 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 non-conducting (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 condi tion.

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. TransistorTR3 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 S1 can 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 S1 is set to bridge the upper and lower left-hand contacts the latching function is disabled.

When TR5 becomes non-conducting the level af voltage on the base of transistor TR6 rises to a value near the positive terminal t1 and TR6 is thus cut off interruptingcurrentthrough resistor R11 and revers ing 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 con tacts of this relay may be used as previously indi cated 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 end/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 inital 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 erection 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 of the thermistors, the voltage at the junction between R1 and Th which was normally sufficient to maintain resistor TR1 of another Schmitt trigger conducting, may fall below the requisite value so that TR1 becomes non-conducting and tra@@istor TER2 changes from non-conducting to the ca-lducting 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 relayAthen 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, orthe 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 Fig urge 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 S1 on the collector of TR3, the latter is connected to the negative rail through a resistor R13 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 fortransistorTR3 and under normal condi tions renders it non-conducting. The capacitors assist in preventing transience disturbing the operat ing conditionsforTR3 as shown.

To re-set the latch means the switch S1 is opened temporarily. If it is required to disable the latching function switch S1 is set to the open position.

Claims (12)

1. A protective system of the kind specified com prising a fault sensing means providing a normal signal and a fault signal, according to the existence of a normal condition or fault condition of the main apparatus, a solid state control circuit providing the protective output and responsive to the normal and fault signals from the sensing means respectively to withhold or establish the protective output, said control circuit including solid state latch means for hold ing the protective output of the circuit in being once it has been established irrespective of continuance or discontinuance of the fault condition of the main apparatus.

2. A protective system of the kind specified comprising a fault sensing means providing a normal signal and a plurality of fault signals each pertaining to a respective fault condition of the main apparatus and sensing means in combination, and one of which fault conditions is a short circuited or low resistance condition of the sensing means, and a solid state control circuit providing the protective output and responsive to a normal signal from the sensing means to withhold the protective output and responsive to any of the fault signals to establish the protective output.

3. A protective system according to Claim 2 further including latch means operative to hold the protective output in being once it has been established by incidence of a selected one or more of the fault conditions.

4. A protective system according to either of Claims 1 and 3 wherein the latch means includes manually operable reset means, and/or a manually operable disabling means.

5. A protective system according to any one of the preceding claims wherein the sensing means comprises one or more thermistors mounted, or adapted for mounting, in thermal communication with the part of the main apparatus to be protected.

6. A protective system according to Claim 5 wherein more than one thermistor is provided and each of these is so connected to the control circuit that incidence of a fault signal from any one of the thermistors brings about establishment of the protective signal.

7. A protective system according to any one of the preceding claims including a visual and/or audible indicator means connected to the control circuit to be operated by the protective output.

8. Acombination of a protective system according to any one of the preceding claims and an electric motor wherein the sensing means is associated operatively with the motor windings and the protective system includes a relay 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.

9. A combination as claimed in Claim 8 wherein the sensing means comprises a plurality of thermistors associated with respective windings of the motor and wherein one of the fault conditions sensed is high temperature producing high resistance of the thermistor concerned and another of the fault conditions is low resistance due to the short circuiting of one of the thermistors.

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

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

12. A protective system including any novel feature or novel combination of features disclosed herein and/or shown in the accompanying drawings.