The instrument that you're using makes a difference on how you will apply a limit. In formal testing generally instruments with accuracy of 1 milliohm or better are specified. IEEE 62.2-2004 mentions an accuracy of 0.25% of reading.
Several motor repair and testing industry documents provide limits. Two terms I use related to comparing three meausurements for a 3-phase machine:
"Range" = (max – min)/average
"Deviation" = (Max Deviation from average)/Average
As a ROUGH rule to translate between these two terms, the range is twice the deviation.
Here are some winding resistance variation limits from various IEEE or EPRI specifications and documents. The 6 digit number list is the EPRI report number:
* Within 5% of each other (5% range, ~2.5% deviation) per EPRI 108773v1.
* Within 5% original value and 1% range (~0.5% deviation) per EPRI 107524
* EPRI Rewind spec 1000897 recommends a maximum 1% winding resistance deviation. Corresponds to approx 2% range.
* EPRI 111195 (MOV’s) – 5% from original factory data and 2% range (~1% deivation).
* EPRI 111196 (form wound) – 1/2% range (~1/4% deviation).
IEEE 1290 (MOV) PAGE E41 – 5% range (~2.5% deviation)
IEEE1415 states "The three values are compared—all readings should be within 3% to 5% from the average of the three readings". i.e. 3-5% deviation.
As you can see there is a spread of limits. The more accurate limits generally apply to larger motors and tested direct at motor. Less stringent limits apply to smaller motors and tested from the switchgear where cable can introduce variability.
Thanks for the info electripete! I am tring to "up" the skills of our technicians by introducing them to the concept of the phase-to-phase short ( I'm tired of the "Megger Test" being the final and only word on motor falures in my plant which thus causes un-needed drive change-outs ) and I found that I didnt have a good answer for what % difference they should look for.
