Method and device for measuring the temperature in electric power resistors
The present invention relates to a method and device for measuring the temperature in power resistors based on the measurement of the high-frequency circuit parameters of said resistor. The present invention excludes the use of thermocouples, dedicated temperature sensors or thermo chambers.
1. A device for measuring a temperature value of a power resistor through measurement of parasitic parameters of the power resistor, the power resistor having a sheath which circumscribes a resistive element terminating at opposing first and second resistor terminals and adapted to be connected to power terminals of a voltage or electric current source, the device comprising:
an impedance measuring means adapted to measure at least one parasitic parameter of said resistor at a predetermined frequency, said at least one parasitic parameter being chosen in a group comprising L, series inductance of said resistor, G, conductance measured between the two resistor terminals and the sheath of said resistor and C, capacitance measured between the first and second resistor terminals and the sheath of said power resistor;
a connecting means adapted to connect the two resistor terminals and the sheath of a power resistor to said impedance measuring means and to connect the two resistor terminals of said power resistor to said voltage or electric current source;
a control and reading means adapted to read the value of said at least one parasitic parameter of said resistor measured by said impedance measuring means and adapted to carry out conversion between the value of at least one measured parasitic parameter and the temperature value of the power resistor through a predetermined look-up table and/or a predetermined mathematical function defining values of the parasitic parameters of said power resistor to a corresponding temperature value.
2. The device according to claim 1 , further comprising a user interface means including a display, said user interface means being in electronic communications with said control and reading means, and adapted to show the temperature value of the power resistor.
3. The device according to claim 1 , wherein said connecting means comprise coupling means adapted to prevent the low-frequency electric power which supplies the resistor from interfering with the impedance measuring means, thus influencing the measurement.
4. The device according to claim 3 , wherein said coupling means comprise a low-pass filter in combination with a high-pass filter.
5. The device according to claim 1 , wherein said connecting means are further adapted to connect said impedance measuring means between the first terminal and the sheath of said resistor and to connect a load impedance between the second terminal and the sheath of said resistor.
6. The device according to claim 1 , wherein said connecting means are adapted to alternately connect said resistor to said voltage or electric current source and to said impedance measuring means.
7. The device according to claim 1 , wherein said impedance measuring means are adapted to measure the resistance value of a filament of the resistor, R.
8. The device according to claim 7 , wherein said impedance measuring means are adapted to measure the resistance value of the filament of the resistor, R, by means of a measurement of a module of the voltage value and of the current value relative to said filament, and the phase difference therebetween.
9. The device according to claim 1 , wherein said impedance measuring means comprise a capacitance meter.
10. The device according to claim 9 , wherein said capacitance meter comprises a dedicated integrated circuit or a microcontroller adapted to measure capacitance values.
11. The device according to claim 1 , wherein said control and reading means comprise a microcontroller adapted to host tables for the conversion between the measured parasitic parameters of said resistor and the temperature or an experimentally determined conversion function linking said measured circuit parameters and the temperature.
12. The device according to claim 1 , wherein said control and reading means are integrated in a power control circuit of said power resistor.
13. A method for measuring the temperature of a power resistor provided with a sheath and with terminals by using a device as in claim 1 , the method comprising the following steps:
a) providing impedance measuring means,
b) connecting said impedance measuring means between at least one of said terminals and said sheath,
c) reading the value measured by said impedance measuring means at a predetermined frequency,
d) carrying out the conversion between the value measured by said impedance measuring means and the temperature through a predetermined look-up table and/or a predetermined mathematical function defining values of the parasitic parameters of said power resistor to a corresponding temperature value.
14. The method for measuring the temperature of a power resistor according to claim 13 , wherein said step of (a) providing impedance measuring means, (b) connecting said impedance measuring means and (c) reading the value measured by said impedance measuring means comprise:
α) connecting a first load impedance, Z L1 , between a second terminal of said terminals and said sheath,
β) reading the value measured by said impedance measuring means, Z M1 , at a predetermined frequency,
γ) disconnecting the first load impedance, Z L1 ,
δ) connecting a second load impedance, Z L2 , between a second terminal of said terminals and said sheath,
ε) reading the value measured by said impedance measuring means, Z M2 , at a predetermined frequency,
ζ) calculating the impedance, Z, of the resistor between the two terminals, and the admittance, Y, between said terminals and the sheath, through the values Z L1 , Z M1 , Z L2 , and Z M2 ,
η) calculating at least one parasitic parameter L, G or C of said resistor ( 31 ) from the impedance, Z, or admittance, Y, values, and wherein said step (d) of carrying out the conversion comprises:
j) carrying out the conversion between the value of said at least one parasitic parameter L, G or C of said resistor and temperature.
15. The method for measuring the temperature of a power resistor according to claim 14 , wherein in said step (η), the admittance, Y and the impedance, Z are calculated according to the equations:
Y
=
±
-
1
[
Z
L
2
Z
M
2
(
Z
L
1
-
Z
M
1
)
-
Z
L
1
Z
M
1
(
Z
L
2
-
Z
M
2
)
(
Z
L
1
-
Z
M
1
)
-
(
Z
L
2
-
Z
M
2
)
-
(
Z
L
1
Z
M
1
-
Z
L
2
Z
M
2
(
Z
L
1
-
Z
M
1
)
-
(
Z
L
2
-
Z
M
2
)
)
2
]
,
and
Z
=
2
[
Z
L
1
Z
M
1
-
Z
L
2
Z
M
2
(
Z
L
1
-
Z
M
1
)
-
(
Z
L
2
-
Z
M
2
)
-
1
Y
]
.
16. The method for measuring the temperature of a power resistor ( 31 ) according to claim 13 , wherein said impedance measuring means comprises a capacitance meter.