Temperature Correction Factor

[danielmurray]

We use several different methods for monitoring the condition of the insulation in our motors. One method in use is a spot check. I am having a difficult time finding the correct temperaure correction factor for Class H insulation in rotating equipment. I have a chart for Class A and B. Can anyone help?

 

[electricpete]

I'm not sure exactly what you mean by spot check.

Per IEEE 43-2000, insulation resistance measurements should be corrected to 40C.

Every 10C is a factor of 2 correction factor.

Is this the type of thing you're looking for? If yes
1- you can derive the correction factor from the above.
2- I can find a link to bureau of reclamation which has chart for temperature correction of insulation resistance. reading.

 

[danielmurray]

By spot check, I mean taking a reading at a specific voltage and time, for comparison over time. For instance 500 volts at 30 seconds. Record this reading for comparison to past readings. However I need to correct this reading for temperature. The method you posted I believe is a generic temperature correction, as opposed to something more specific and thus more accurate.

 

[jbartos]

Suggestion: Generally, any resistance can be adjusted to a different temperature by:
Rt = Ro (1 + alfa x t)
where
Rt is Resistance at Temperature t, degC
Ro is Resistance at Temperature to, degC
alfa is temperature coefficient of resistance
For two different temperatures t1 and t2, there will be two different resistances Rt1 and Rt2 with the relationship:
Rt1/Rt2=(alfa + t1)/(alfa + t2)

 

[edison123]

danielmurray,

what electricpete says is not a generic formula but it is the one you are looking for. All IR values are corrected to 40 deg c. A value of 100 Meg at 30 deg c will be 50 Meg at 40 deg c. A value of 100 Meg at 50 deg C will be 200 Meg at 40 deg C. Refer the std pete has mentioned.

 

[jbartos]

Suggestion: Visit

http://www.geotronixsupply.com/appl...f-Polarization-Index-and-IEEE-Standard-43.pdf

etc. for more info
IEEE Std 43 uses:
Rc=Kt x Rt
Rc is insulation resistance in MOhms corrected to 40degC
Rt is measured insulation resistance in MOhms at temperature t
Kt is insulation resistance temperature coefficient at temperature t
Kt versus temperature plot is obtained by plotting several measurement points. Logarithmic scale is used for temperature.
Once the plot is obtained the Kt may be determined from such plot.
Approximate values for Kt are obtained from the plot by doubling insulation resistance for each 10degC reduction in temperature (above dew point) and has been found typical of some new windings.

However, the testing and the above formula for Rc is essentially following the formula from my previous posting.
There is nothing new theoretically in this IEEE Standard.
Please notice that the original posting does not call for any IEEE Std. Therefore, a general formula, such as one I posted, may be needed.

 

[edison123]

jb,

Your first post is irrelevant to correction factor to insulation resistance question posted here.

Your 2nd post is just a rehash of IEEE 43 std.

It is odd that you should claim IEEE std as irrelevant as against yours.

Your claim that your post is better than referring to a IEEE std is ludicrous.

 

[UKpete]

danielmurray, as you can see from the examples given by edison there is a large variation in insulation resistance with temperature. Because there will be difficulties in correctly measuring this temperature (and it may vary considerably throughout the machine) this means there could be quite a large error in your corrected IRs.

It's a interesting question but I'm not sure how useful this test will be.

 

[edison123]

UKpete,

When you are trending IR values, which vary with temperature, you need to have a constant temperature base (40 deg C in case of IEEE) to have any meaningful analysis of IR. I agree that winding temperature will vary throughout the machine. But, since the winding RTD's are evenly placed around the winding, a reasonable average temp (not differing much from high & low temps) can be arrived at. In my experience, I have found winding RTD's temp variation at a particular point of time is about 2 to 5% from average.

 

[jbartos]

Suggestion to edison123 (Electrical) Nov 2, 2003 marked ///\\jb,
Your first post is irrelevant to correction factor to insulation resistance question posted here.
///Please, notice that the formula is generic. The alfa is normally related to metals; however, in general, it also cover other material, e.g. insulation materials.\\Your 2nd post is just a rehash of IEEE 43 std.
///Not quite, it provides the theoretical background, which IEEE Stds usually do not address. They address references only.\\It is odd that you should claim IEEE std as irrelevant as against yours.
///Certainly Sir, everyone is entitled to once own opinion and expertise.\\
Your claim that your post is better than referring to a IEEE std is ludicrous.
///Certainly Sir, everyone is entitled to once own opinion and expertise. I found your postings unconstructive.\\

 

[UKpete]

danielmurray, I take it back, I've had a read through IEEE43-2000 (some very good info in there) and it does appear to be a useful test.

The value for Kt mentioned in jbartos' post is ideally going to need deriving for your particular motor, the factor of 2 for every 10degC is only an approximation based on very old data. It doesn't follow that it will be the same for all class H insulation systems. That's my interpretation of the above-mentioned standard.

 

[jbartos]

Suggestion: Kt may be taken off from the plot.

 

[danielmurray]

Thank you all for the help. There is a lot off good information here. Exactly what I was looking for. Thanks again.