Avoiding Resistance Measurement Errors

                  
On a cold morning (0°C) the motor winding’s electrical resistance was found to be 1.187 Ohms

Later that day it’s warmer (20°C) the same motor winding was found to be 1.282 Ohms ( a 7.86% increase in resistance)

Bottom Line - The same copper or aluminum component (i.e. motor winding) measured for electrical resistance on a cold day (lower resistance) will have a dramatically higher resistance when measured on a warm day

Measuring the resistance of a (Rcold) copper conductor winding at ambient temperature? How cold/warm is ambient right now?  If you are performing Heat Rise Determination of Motors based on change in resistance, Rcold dramatically changes with ambient or room temperature.

The graph above shows the original copper motor winding was measured to be 1.187 Ohms @ 0 ° C ambient. At 10 ° C the same copper device has now changed its resistance by almost 4%.  At 20° C, the copper’s resistance has increased by 7.86%.  If this ambient temperature effect is not compensated for, the error can be 20 times larger than the accuracy of most ohmmeters.  The AMPTEC 720A micro-ohmmeter’s prevents this measurement error automatically with Option 20C for copper and Option 20A for aluminum.

Ambient Temperature Correction (Option 20C)

Option "20C" available on the AMPTEC 720A Micro-Ohmmeter is for Ambient Temperature Correction of the resistance of copper conductors. This option automatically compensates/corrects for ambient temperature effecting the resistance of the copper device. The temperature coefficient (T.C. = 0.3931% per °C ) of copper is used (option 20A for aluminum) and readings are referenced to a 20°C ambient temperature. Regardless of what the actual ambient temperature is the AMPTEC Option 20C gives you the resistance of the copper winding referenced to 20 degrees C (it's as if the motor is in a 20 degree C oven), the only assumption is that the test item must be at ambient temperature as well. Heat Rise Determination based upon the Change in Resistance Method for Motors, Transformers, and Ballasts can be performed with this option . For the RHOT and RCOLD determination (Copper or Aluminum Ambient Temperature Correction is recommended. "Option 20C" provides you with a very reproducible RCOLD determination.

Formula for Heat Rise Determination of Motors and Transformers 

To calculate the internal heat rise of a motor running on a load for 8 hours, the RCOLD measurement is initially recorded (1.1870 Ohms). It is important to note that ambient temperature correction (option 20C) automatically compensates for varying ambient (see above). After 8 hours under load the motor resistance (RHOT) is measured again and found to be 1.5890 Ohms. The calculation above uses 0.3931% per degree C as the temperature coefficient for copper was used. The final calculation for HEAT RISE IN MOTORS determines there was an 86 degree C increase above ambient when the motor is run continuously for 8 hours under load.