Tom Bishop, P.E.
EASA Senior Technical Support Specialist

The Institute of Electrical and Electronics Engineers (IEEE) standard for insulation resistance testing of mo­tor and generator windings that was published in 2002 has been revised. The 2013 edition was published in March 2014. 

The first change in the new docu­ment is a slight change in the title. It has changed from “IEEE Recom­mended Practice for Testing Insulation Resistance of Rotating Machinery” to “Recommended Practice for Testing Insulation Resistance of Electric Ma­chinery.” The reason for the change was to use the more prevalent IEEE term for motors and generators. Significant changes to clauses of the standard that affect service center repairs and testing are described in this article.

Polarization index
A major change in clause 5.4, titled “Polarization index readings,” affects random winding tests. The relevant text now states, “This test may not ap­ply to small random winding machines since the absorption current IA becomes negligible in a matter of seconds (see Annex A for further discussion).” In Annex A, the standard acknowledges that in random windings, “the absorp­tion current may decay to nearly zero in 2 min–3 min,” still far short of the 10-minute duration prescribed for the polarization index (PI) test. The prior edition of the standard focused on form coil windings in this clause and did not specifically address random windings. The significance of the change is that it makes clear that in many if not most cases, the PI test is not applicable to random windings. There­fore it would not provide useful information, could create confusion between end users and those per­forming the test, and would essentially be a time-waster if applied.

Regarding the PI of DC machine armature windings, a sentence in clause 12.2.1 has the following statement: “A polarization index test is not applicable to DC armatures that have an exposed cop­per commutator that is by necessity not encap­sulated in insulation.” Thus the PI test does not apply to conventional armatures.

Note: The minimum PI value remains 2.0 for windings with an insu­lation system thermal rating of class B (130° C) or higher. Similarly, the 5000 megohm rule remains unchanged. That is, a winding with an insulation resistance value of 5000 megohms does not need to be PI tested.

Temperature correction
For well over half a century, the insulation resistance (IR) versus tem­perature characteristic given in IEEE 43 has followed the simple rule that the IR value doubles for every 10° C decrease in winding temperature, and conversely that the IR value is halved for every 10° C increase in winding temperature. However, clause 6.3 of this new edition provides two tem­perature correction factors, one of which has two distinct formulas for the correction. Windings are now differen­tiated as being either “thermoplastic” or “thermosetting.” Thermoplastic insulation windings are those with asphaltic systems and other insulation systems that were used prior to the early 1960s. Thermosetting insula­tion windings appeared beginning in the late 1960s and include epoxy and polyester based systems.

Unfortunately, the prior “10 degree” rule applies to thermo­plastic windings, which are relatively rare since they date back more than 5 decades. The “rule” for the much more prevalent thermosetting insu­lation systems is expressed by two slightly complicated formulas. One formula covers insulation tempera­tures from 10° C to less than 40° C, and the other formula addresses in­sulation temperatures from 40° C to less than 85° C. These formulas are given below.

Formula for temperatures from 10° C to less than 40° C:

K_t =  exp [-1245 {(1/(T+273) - (1/313)}] (Equation 1)

Formula for temperatures from 40° C to less than 85° C:

K_t =  exp [-4230 {(1/(T+273) - (1/313)}] (Equation 2)

Where:
T = Temperature (degrees C) at which insulation resistance was measured.
K_t = factor to multiply T by to obtain insulation resistance corrected to 40° C.

Table 1 illustrates the variation in the Kt factor versus a range of tempera­tures. Use of the table is a faster and simpler process for estimating Kt than the exact method of using a formula.

Note that Table 1  has a range from 10° C to 60° C, whereas the range given in the formulas is from 10° C to less than 85° C. The explana­tion for this apparent inconsistency is provided in a note in IEEE 43 that reads as follows:  “[Equation 2 above] and [Equation 1 above] are approximations and could lead to significant errors if used to calculate insulation resistance at temperatures outside the range from 10º C to 60º C.”

To illustrate the effect of the tem­perature correction factor using the new standard versus the prior version, here is an example. The insulation resistance of a winding is 160 megohms with a winding temperature of 20° C (68° F).  The base temperature for insulation resistance is 40° C (104° F). Using the old method, we would halve a lower value to obtain the IR value at a temperature that is 10° C higher. In this case we need to do that twice, halving the 20° C value to obtain the 30° C value, and halving the resulting 30° C value to obtain the value at the 40° C base temperature. Mathematically we are multiplying ½ by ½, meaning that we are multiply­ing the 20° C value by ¼ to obtain the equivalent 40° C value. Thus the 160 megohm value at 20° C is equivalent to 40 megohms (160/4) at 40° C. 

Next, we convert the measurement using the new standard. The measure­ment temperature is 20° C and from Table 1 (on Page 4) the conversion factor is 0.76. The measured insulation resistance of 160 megohms multiplied by 0.76 is equal to 122 megohms. Thus the insulation resistance corrected to 40° C is 122 megohms. Note that this value is much higher than the result using the old method. Table 2  illustrates the difference in results from each method based on a final result of 100 megohms at 40° C.

For more details on temperature correction, see the July 2013 Currents article, “Revisiting insulation resis­tance temperature correction.” 

Minimum insulation resistance
Included in clause 12.3 is a table labeled “Recommended minimum insulation resistance values at 40° C (all values in MO).” The significant change in the table is that the minimum insulation resistance for armatures has changed from 100 megohms to 5 megohms. The rationale for the change was recognition that the exposed bare copper of the commutator has a lim­iting effect on insulation resistance, regardless of winding type.

Table 3  provides a comparison of the IEEE 43 2013 edition and the 2000 edition minimum insulation resistance values for various winding types. Note that the levels for minimum insulation resistance listed in the first column are the same for both editions.  Also, the changes related to armatures are further illustrated by the use of blue text. 

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Categories: AC motors, Stators/windings, Winding insulation, Field coils, Interpoles, Testing, Motor testing

Tags: IEEE Insulation resistance Standards

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