Determination of Temperature Rise

A recent topic addressed the problem of "poor" termination connections in a motor. Temperature-rise was given as a parameter to investigate.

It is well known, that, for common power conducting elements, the temperature rise is related to an increase in resistance. One simply
determines the ratio between the "cold" and "hot" states of the circuit in question. Then the ratio is used in a rather tedious formula to calculate temperature (ref 1).

Following is a simple technique (ref 2) but you need a slide-rule. Some of you remember those.... the "white" thing consisting of a ruler and a
medial slide graduated with logarithmic scales.

1) Obtain the R(hot) to R(cold) ratio using a good low scale Ohmmeter. Measurements can be made at the "TEE" terminals of the motor starter.
This method assumes that the cable loss effects are negligible, i.e., its temp rise is insignificant compared to the motor's. If significant then the measurement must be carried out at the motor.

2) Obtain ambient Temp in ° Cel.

3) Align the value "2.6" on the 'C' scale against the left-hand index "1.0" on the 'D' scale.

4) Read the temperature rise in °Cel directly on the 'C" scale, opposite the ratio value on the 'D' scale.

5) Here is the clever part. Each tenth division of the 'C' scale represents a 10° Cel rise. In other words, 2.6 is 0°, 2.7 is 10°, 2.8 is 20°, 2.9 is 30°, etc.

6) Example: for a ratio of 1.13 on the 'D' scale, read 2.9 on the 'C' scale. This corresponds to a 30° Cel rise.

Before those of you with hand held programmable calculators start bombarding me with... "but the accuracy is affected, the relationship of ratio to rise is non-linear, etc" this method is accurate within 1° Cel over the normal range of power component temp-rise, i.e, 40°-90° Cel.

ref 1: obtainable in EE text books or electrical handbooks. If you can't find it contact me.

ref 2: George C. Kent, Chief Engr w/ American Photocopy Equipment Co., Evanston, IL (1964).

Phil Corso, PE
Trip-A-Larm Corp