Mike Howell
EASA Technical Support Specialist
 
Insulation resistance (IR) testing is performed on rotating machines for several reasons including evaluation of condition, suitability for service and suitability for additional testing. IEEE Std 43-2000 provides the industry recommended practice. The insulation resistance is defined as follows:

R = E / I T  

where

R is the IR in MΩ
E is the applied direct voltage in V
I_T is the total resultant current in μA

Why is IR temperature correction necessary?
It is well established that there is an inverse exponential relationship between IR and winding temperature. For this reason, minimum acceptable values of IR after one minute are specified at a baseline temperature of 40° C to which test values are corrected for evaluation and/or comparison with previous measurements (see Table 1).

When using IR data for trending, industry recommended practice is that subsequent tests be performed when the winding temperature is near the same temperature as the previous test to minimize correction inaccuracies.

How is IR temperature correction performed?
The insulation resistance is corrected for temperature by multiplying the measured insulation resistance (R_T) by an IR temperature coefficient (K_T) corresponding to the temperature at which the test was performed. The corrected insulation resistance (RC) is calculated as follows:

R_C = K_T R_T

where

R_C is the IR in MΩ corrected to 40° C
K_T is the IR temperature coefficient at T° C
R_T is the measured IR in MΩ at T° C.

IEEE Std 43-2000 recommended practice is to determine the IR temperature coefficient (K_T) by test. This is certainly an appropriate measure in some cases (e.g., large generators, critical motors) but a reasonable approximation for K_T should be sufficient for most machines.

Recent studies (Bibliography 1, 2) found that the relationship between IR and temperature for modern thermosetting insulation systems (e.g., epoxy or polyester) is quite different from that of the thermoplastic systems (e.g., asphaltic) on which early approximation models for K_T are based (see Figure 1). Approximation calculations presented (1) should facilitate more accurate temperature correction for the vast majority of random and form wound machines seen by service centers today. Calculated K_T values (20° C - 60° C) are provided in Table 2.

Depending on the service center and/or customer correction practices, the corrected insulation resistance (RC) could be significantly erroneous if the KT value is inaccurate and the winding temperature is not close to 40° C. If the measured insulation resistance is in the thousands of megohms (e.g. > 5000 MΩ), this may not be much of a concern. However, as results approach pass/fail criteria, the accuracy of K_T becomes increasingly important.

The determination of insulation temperature is also important. Reasonable estimates can be achieved using winding resistance measurements or embedded detectors. In any case, the same measurement method and point should be used for subsequent tests of the same winding if comparisons are to be made. With reasonable K_T and insulation temperature values, calculating R_C is straight forward (see Table 3) and takes very little additional testing time.

Having reliable K_T estimation methods that are valid for the insulation system technologies used today increases the value of the IR test – especially when determining K_T by test is not practical. IR testing is a valuable tool but it is just one tool to help assess the condition of an insulation system. If rewind or downtime costs are significant, it may be warranted to employ additional diagnostic tools when IR data is marginally acceptable or unacceptable.

Bibliography

• 1. Temperature Dependence of the Resistance of Modern Epoxy Mica Insulation of HV Rotating Machines. Lamarre, Laurent and David, Eric. 2008, IEEE Transactions on Dielectrics and Electrical Insulation, pp. 1305-1312.
• 2. Insulation Resistance Measurements Versus Temperature Made on Aged Stator Bars and Coils. Zhu, H. 2007, Electrical Insulation Conference and Electrical Manufacturing Expo, pp. 215-218.

AVAILABLE IN SPANISH

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

Tags: Insulation Insulation resistance IR Testing

Rate this article:

3.0

Print