CA1169501A - Long-time constant thermal analog - Google Patents

Abstract

ABSTRACT OF THE DISCLOSURE
This device provides electrical characteristics which are the analog of the thermal characteristics of a device whose thermal behaviour is being monitored. This device utilizes a resistor-capacitor network to provide an analog circuit which equates the thermal conditions in the rotor of a dynamoelectric machine to the voltage build-up in the capacitor of the analog circuit. In order to keep the size of the capacitor within reasonable limits, a sampling technique is used to connect the capacitor into the electrical circuit for short periods only. This has the effect of multiplying the size of the capacitor.

Description

S ~

LONG-TIME CONSTANT THERMAL ANALOG
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At times it is neces~ary to provide an accurate indication o~ the internal temperature o~ a body whe~re it is pract~cally impossible to physically mea~ure the temper-ature of the body. In situations such as these ~here for instance one might wish to have an accurate indicaticn o~
the temperature of a transformer winding or core, or the rotor temperature of a large motor~ it is sometimes convenient to build an analog circuit whose electrlcal characteristics bear a close resemblance to the thermal characteristics o~
the device who~e internal temperature may not be directly measured, S~MARY OF THE INVENTION
. .
This invention comprises an analog circuit whose electrical characteristics closely approximate the thermal characteristics an electric motor, the inte~nal temperature of which is for purpose of protection, most desirous to accurately approximate~ The analog circuit uses a resistor-' .

O i capacitor network where the ~ollowing quantiti2s are analogs:
ELECTRICAL THERMAL
voltage temperature resistance thermal ~mpedance current heat capacitance ~peci~ic heat mrough proper choice of the electrical compo~ents ~n accurate analog of the physical device may be represented.
For an electric machine which may have an unduly long thermal lQ time constant; this invention provides a method of ~ampling whereby the capacitor is co~nected into the analog circuit ~or short periods only.
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Figure 1 is an electrlcal model of a ther~al system;
Figure 2 is an exten~ion o~ the circuit of Figure l;
F~gure 3 is a circuit utilizing a sampling technique;
Figure 4 is a graph of duty cycle and capacitor voltage;
Flgure 5 is a voltage-to-current converter; and Figure 6 is a typical analog circuit of a thermal device utilizing a sampling circuit.
DESCRIPTION OF THE PREFERRED EMBODrMENT
Figure 1 shows the electrical analog of a thermal device. The de~ice may be a transformer or motor and the choice o~ the various parameters may be made by those skilled ~ 1 69~0 1 in the art to accurately represent the thermal de~ice characteristicsO
Other considerations may cause changes to the basic circuit. In this particular invention two character-istics are deemed important to the invention.
It is therefore important that:
(1) the electrical analog have an ad~ustable time constant to facilitate matching the analog and real device characteristics;

(2) the capacitor in the resistor capacitor combination should be kept to a small value (for convenience o~ size).
In order to reduce the size o~ the capacltor CT
the circuit oP Figure 3 will be utilized to switch the capacitor ~nto and out o~ the analog circuit. Figure 3 provldes a circuit which switches the capacitor at a predetermined sampling Prequency into the analog clrcuit.
Figure 4 shows the di~Perence in VOUt for the curve under-going sampling and the sampled curves. I~ the sampling rate is increased such that the sampling ~requency is much higher the sampled curve will appear to be stepless.
It is prePerred that the sampling gate oP Figure 3 be a CMOS transmission gate which exhibits a very high impedance in the OFF state and a very low impedance ln the ON state in the presence of a control signal.
To complete the circuitry of this invention it is necessary to provide a circuit which will produce an 9'i~ i ~nalo~ current signal from an an~log voltage ~nput s~gnal.
To do this, the circu~t o~ Figure 5 m~y be utilized.
In thi~ clrcuit ampllfier A1 ampliPies the input current il created by VIN, ~ and the virtual ground at Al'3 invert~ng input by the ~actor R~ so th~t:

IR = VIN (R2 1 1) ~ ~3 mi~ curre~t ~s reflected into ~ OAD by~he 'tcurrent mirrorl~ ~ormed ~y ma~ched tran~istors Q?, Q3 and the impedance bu~er ~ormed by Q4~ In this in~tance Io tracks VIN regardle~ o~ the impedance ~ RLoAD-A typical complete circuit with all ~he c.trcuit value~ hown ln Figure 6.
A~ will be ~een, the input portlon o~ the olrcuit c~rre~pond~ to the circult o~ Figure 5 with ampli~ler A1 ampl~fying ~nput current. The equ~tion ~or iR becomes the IN R
(~+ 1) Thi8 current iR is ren ected into R5 by transistors Q2 and Q3 and the i~pedance bu~er ~ormed by Q4. With this arrangement, Io tracks VIN regardless of the impedance o~ R5. The capacitor designated CT in Figure 2 i5 C1 in Figure 6 and to reduce its capacity a COMS
transmission gate ~ i8 placed in ser~es with the capacitor and switched by means of the oscillator circu~t which ~s a standard oscillator uslng amplifier A2 ~nd its as30ciated I .1 ~ 0 ~

components.
The fre~uency o~ the oscillator is adjuætable by means o~ R7. I~ this way~ the capacitor Cl ls switched into the circuit at a ~requenoy determlned by the ~requency o~ the oscillator and charged at a rate dependant upon the current and the frequency o~ applicat:Lon of the cuI~ent.
VOUt there~ore becomes a mea~ure o~ the temperature.
Figure 4 show~ the effect of the sampllng period on the output voltage and shows that the capacitor can be substantially smaller and st~ll pro~lde ~n accurate result i~ the 3amp1es of the current are re~uced in time duration by mean3 o~ the switching cir~uit described.
With the arrangement shown :Ln Figure 6 and the component value 3hown therein, the duty cycle will b~ about .33 a~ ~hown in Fi~ure 4 ~nd there~ore the ~oltage a~tained by the capacitor will obey the law ~llu~trated in Flgure 4 in dotted lines.

Claims

Claims:

1. A method of producing an analog circuit with selectively adjustable time constant to facilitate matching of the selected characteristics of a real device with those of said analog circuit comprising:
a resistor representative of a thermal impedance of said device, a capacitor representative of a specific heat of the device, an input current representative of the heat input to the device, an oscillator arranged to periodically sample said current and apply it to said capacitor to charge said capacitor, sensing means for sensing the voltage on said cap-acitor having an impedance which is sufficiently high to prevent loading of a resistance network and to obtain there-from an electrical signal which is the analog of the temper-ature of said device, and means to utilize said voltage sensed on said capacitor to control a protective circuit protecting said electrical device.