GB2062322A

GB2062322A - Temperature Monitoring

Info

Publication number: GB2062322A
Application number: GB8027900A
Authority: GB
Original assignee: Loher GmbH
Current assignee: Loher GmbH
Priority date: 1979-08-29
Filing date: 1980-08-28
Publication date: 1981-05-20
Also published as: AR231772A1; NL8004683A; BE884940A; IT8024172A0; FR2464465A1; ES8104559A1; DE2934926A1; ES494581A0; IT1132565B

Abstract

In a temperature sensor, for an electric machine, employing a probe 2 of which the electric resistance increases with a rise in temperature, a switching signal is produced when a limiting temperature value is reached, said value being adjustable by means of a variable resistor 5. <IMAGE>

Description

SPECIFICATION Temperature Monitoring The invention relates to an arrangement for detecting the temperature in and on electric machines.

Electric machines exhibit a certain amount of inherent heating depending on the load. This heating occurs at different points, for example in the coil, in the rotor, in the bearings and/or on the surface. In some cases the temperature rise caused by the inherent heating can assume such high values that the machine and the surroundings are endangered.

By means of measuring probes, the temperature can be measured. The probes to be used for such methods should be small so as to respond rapidly. These measuring probes generally consist of electric resistance materials of which the electric resistance changes with temperature. Such measuring probes are known.

There are so called hot conductors of which the resistance drops with an increase in temperature and cold conductor sensors of which the resistance increases with a rise in temperature.

The measuring probes are disposed at positions of critical temperature rise. The resistance of the probe is measured in a separate electronic circuit as a measure of the temperature. When necessary, a signal or a plurality of signals are emitted which, depending on the temperature, serve as a warning for switching on cooling fans or/and for switching the installation off.

To enable a plurality of measuring points to be scanned, a corresponding number of probes is necessary. To avoid the need for bringing the leads of each separate probe to the evaluating circuit, a selection circuit is advisable which can be accommodated in the vicinity of the probes, for example in the terminal box of the machine.

A difference is made between continuous probes with a constant pulse of the resistance in relation to the temperature and discontinuous probes exhibiting a certain change in the characteristic curve at the so called "Curie temperature". Whereas no selection circuits are necessary for discontinuous sensors, particularly in the case of cold conductor probes, in which case a series circuit is generally sufficient, suitable selection circuits are necessary in the case of continuous sensors.

Continuous probes have to be monitored for lead breakages in circuits which are particularly suitable for this purpose. Discontinuous cold conductor probes are self-monitoring with regard to lead breakages. This means that when such cold conductor probes are employed and a break occurs in the lead, a notification of "excessive temperature" is given automatically, This is why the use of cold conductor probes is much simpler.

Circuits for detecting and selecting are adequately known and in practical use.

Discontinuous probes have the disadvantage that subsequent adjustment of the response temperature is not possible after the probes have been installed. For different temperature limiting values, sensors with different response values have to be incorporated. This makes it more expensive to have them available. Continuous hot conductor probes involve a high capital expenditure in respect of circuitry for the evaluation.

These disadvantages are avoidable by using continuous cold conductor probes.

The invention is based on the problem of providing an arrangement with which, by using continuous cold conductor probes, the aforementioned disadvantages are avoided and a selection is made from a number of different measuring locations, even if the critical temperatures in the spatial arrangement are at differently high levels. A selection circuit so acts on one or more evaluating stages that an appropriate signal is emitted each time a critical temperature is reached.

According to the invention, this problem is solved in that the incorporation of balancing resistors in a selection circuit makes it possible to balance differently high sensor temperatures to a respective one voltage level, a very particular voltage level corresponding to each critical temperature at the different measuring locations.

Other embodiments of the invention are given in the subsidiary claims.

The invention will now be described in more detail with reference to the drawing, wherein Fig. 1 shows the relationship in principle between the resistance and temperature in the case of continuous hot conductor sensors; Fig. 2 shows the relationship in principle between the resistance and the temperature in the case of continuous cold conductor sensors; Fig. 3 shows the relationship in principle between the resistance and temperature in the case of discontinuous cold conductor sensors; Fig. 4 is an electric circuit diagram of a selection circuit for continuous cold conductor sensors; Fig. 5 is an electric circuit diagram of a selection circuit with balancing possibility for continuous cold conductor sensors;; Fig. 6 is an electric circuit diagram of a selection circuit consisting of one or more elements with and without balancing possibility for continuous cold conductor sensors; Fig. 7 is an example of an electric circuit diagram for a signal generator which is excited by way of a selector and includes lead breakage monitoring; Fig. 8 is an example of an electric circuit diagram with a plurality of evaluating stages and common lead breakage monitoring.

The characteristic curve shown in Figs. 1, 2 and 3 are known and here serve for clarification.

The present invention is concerned with the selection and evaluation for continuous cold conductor sensors according to Fig. 2. Such sensors are also provided in the arrangement of Fig. 4. The sensor with the highest temperature also prossesses the highest electric resistance.

Thus, the highest voltage Us occurs at the appropriate junction between the preresistor 1 and the sensor resistance 2. The diodes 3 lead to the common output conductor and it always only the highest voltage that reaches the output of the selector from the location having the highest temperature. The output of the selector of one of Figs. 4, 5 or 6 leads to the input of the evaluating circuit according to Fig. 7.

The selector circuit according to Fig. 4 consists of the measuring voltage divider 1 and 2, wherein 2 is in each case the resistance of the cold conductor sensor and 1 is a resistor having a predetermined fixed value. By way of a grid consisting of the diodes 3, the voltage reaches the base of the transistor 6 which is operated as an emitter follower. At the emitter of the transistor, the voltage is derived for the evaluating circuit described hereinafter. In essence, the transistor 6 operates as an impedence converter and the source resistance at the output is low. The resistor 8 is provided as a basic load for the supply voltage. A current flows through this resistor 8, whereby the positive supply can be monitored for lead breakage.

Fig. 5 shows a selection circuit in which the measuring voltage applied to the sensor reaches the output transistor by way of a respective emitter follower 4 with a connected potentiometer 5 and a coupled diode 3. The intent and purpose of this arrangement is to adapt the output level at differently high temperature criteria to the individual measuring locations by means of the potentiometers 5. When using sensors 2 of standardized values, the potentiometers may have adjustment scales graduated in degrees of temperature.

Fig. 6 illustrates an example of a selection circuit with a mixed arrangement of measuring voltage preparation. This circuit can be used mainly where similarly fixed predetermined temperature limiting values and subsequently adjustable values are to result in initiation.

The voltage emanating from the selector of one of Figs. 4, 5 or 6 reaches the input of the evaluating circuit of Fig. 7. Basically, the relay 1 8 is attracted when the base of the transistor 1 6 is positive. If the temperature at the sensors is low, the input voltage is very small and the transistor 14 has not operated. Consequently the voltage at the base of the transistor 1 6 is high corresponding to that at its collector. The transistor 1 6 is operative and the relay 1 8 attracted. In the case of a temperature rise up to operation of the transistor 14, the voltage at the base of the transistor 1 6 becomes small and the relay 1 8 drops off.A feedback effect is set up by way of the resistor 1 5 which gives the circuit a trigger behavious if the resistance of the resistor 13 is higher than the coil resistance of the relay 1 8. Similarly, the voltage for the base of the transistor 1 6 becomes small when the transistor 20 is no longer operated as a result of a lead break in the positive lead of the selector.

Similarly, transistor 14 is operated when the connection between the measured voltage lead is undone. In that case the transistors 10 and 14 are operated, the transistor 1 6 becomes deenergised, and the relay 18 drops off.

It is possible to connect a plurality of evaluating stages to the selector. Fig. 8 shows a two-stage embodiment by way of example. In order that the relay circuit is operated in steps, an adaptation is necessary and can be made by way of the potentiometers 22 and 23 associated with each stage.

Claims

1. An arrangement for detecting and evaluating temperatures with cold conductor sensors, particularly in and on electric machines, characterised in that electric switching signals are produced at one or more limiting values and that the temperature levels at which the switching signals appear are adjustable by means of variable resistors.

2. An arrangement according to claim 1, characterised in that lead breakage monitoring is provided which, by way of the current flow to the selector unit, monitors the lead for a breakage.

3. An arrangement according to claim 1, characterised in that the potentiometers for adapting to the temperature switching value are provided with scales graduated in degrees of temperature.

4. An arrangement according to claim 1, characterised in that evaluating stages and selectors with only three leads are connected.