LONG-TIME CONSTANT THERMAL ANALOG

DISPOSITIF A GRANDE CONSTANTE DE TEMPS A CARACTERISTIQUES SEMBLABLES A CELLES D'UN DISPOSITIF THERMIQUE

English 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.

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.

Description

S ~
1 CW-1030
LONG-TIME CONSTANT THERMAL ANALOG
9 ~09~1D cr $RI I r~
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.
~L:~
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.