×
INTELLIGENT WORK FORUMS
FOR ENGINEERING PROFESSIONALS

Log In

Come Join Us!

Are you an
Engineering professional?
Join Eng-Tips Forums!
  • Talk With Other Members
  • Be Notified Of Responses
    To Your Posts
  • Keyword Search
  • One-Click Access To Your
    Favorite Forums
  • Automated Signatures
    On Your Posts
  • Best Of All, It's Free!
  • Students Click Here

*Eng-Tips's functionality depends on members receiving e-mail. By joining you are opting in to receive e-mail.

Posting Guidelines

Promoting, selling, recruiting, coursework and thesis posting is forbidden.

Students Click Here

Jobs

% Difference of DC resistance in AC motor stators

% Difference of DC resistance in AC motor stators

% Difference of DC resistance in AC motor stators

(OP)
Does anyone know what an acceptable "Standard" percent difference of DC resistance one should find in LV 3 phase motor stators?

RE: % Difference of DC resistance in AC motor stators

(OP)
I should have stated the "Phase-to-Phase" DC resistance. Oops!

RE: % Difference of DC resistance in AC motor stators

There should be no difference on a medium quality multi-meter.

Bill
--------------------
"Why not the best?"
Jimmy Carter

RE: % Difference of DC resistance in AC motor stators

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.

=====================================
(2B)+(2B)'  ?

RE: % Difference of DC resistance in AC motor stators

(OP)
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.  

Red Flag This Post

Please let us know here why this post is inappropriate. Reasons such as off-topic, duplicates, flames, illegal, vulgar, or students posting their homework.

Red Flag Submitted

Thank you for helping keep Eng-Tips Forums free from inappropriate posts.
The Eng-Tips staff will check this out and take appropriate action.

Reply To This Thread

Posting in the Eng-Tips forums is a member-only feature.

Click Here to join Eng-Tips and talk with other members!


Resources