Nearly all electrical conductors perform a fiddle taking into consideration in resistance once a alter in temperature. A rise in temperature increases the amount of molecular warning in a conductor hindering the seize of deed through the same conductor. To an observer, the measured resistance of the conductor has increased to the lead the temperature alter. This implies that meaningful comparisons of resistance for conductors of various sizes or materials must be performed at the same temperature Temperaturepro DFW.
Experimentation has shown that for each degree of temperature adjust above or under 20 degree C, the resistance of a impinge on serve on conductor changes as a percent of what it was at 20 degree C. This percentage regulate is a characteristic of the material and is known as the ‘temperature coefficient of resistance’. For copper at 20 degree C the coefficient is unchangeable as 0.00393; that is, each fine-appearance of one degree in the temperature of a copper wire results in a resistance alter equal to 0.393 of one percent of its value at 20 deg C. For narrow temperature ranges, this relationship is almost linear and can be expressed as:
R2 = R [1 + a(t2 – t1)]
R2 = resistance at temperature t2
R = resistance a 20 degree C
t1 = 20 degree C
a = temperature coefficient of resistance at 20 degree C
Given the resistance of a length of copper wire is 3.6 ohms at 20 degrees C. What is its resistance at t2 = 80 degrees C?
R2 = R [1 + a(t2 – t1)]
R2 = 3.60 [1 + 0.00393(80 – 20)]
R2 = 3.6 X 1.236 = 4.45 ohms
Using the above method, the heat rise (degrees C) in a transformer or relay winding can be nimbly certain by measuring the winding resistance and every second the gone adding together happening:
1) Measure the winding resistance cool (at room temperature approx. 20 deg); call it R (i.e. 16 ohms).
2) Measure the unadulterated resistance at the fade away of a heat control; call this R2 (i.e. 20 ohms)
3) Calculate the resistance ratio of the affectionate winding to that of the winding detached: R2 / R = 20 / 16 = 1.25
4) Subtract 1 from this ratio: 1.25 – 1 = 0.25
5) Divide this figure (0.25) by 0.00393: 0.25 / 0.00393 = 63.20 degrees C
In summary, we have shown that a alternating in temperature will play a part the measured resistance of a firm conductor. We have plus shown that this property can be exploited to calculate the heat rise in a winding from admiring and distant resistance measurements.