Nondestructive Testing of Insulation

E. L. B R A N C A T O M E M B E R A I E E

Τ USER O F E L E C T R I C APPARATUS is con­stantly in need of information concerning the condi­

tion of insulation. This information, at the present time, is supplied by such tests as insulation resistance, power fac­tor, capacitance, or by more severe tests such as "high-pot ." Unfortunately, none of these tests gives a true indication of the "heal th" of the dielectric, but rather indicate its present condition—a condition that may exist only temporarily and can be corrected by suitable action. I t is important, of course, to have this information; however, it is of equal importance in many applications to have an indication of the useful life that can be expected from the dielectric at any time during its use.

The user, as well as the designer, of electric apparatus can predict the minimum useful life of the insulation in such equipment when operated under an assumed set of condi­tions by referring to appropriate thermal aging curves for the insulation. However, after the equipment has been in operation for a period of time, the past operation will influ­ence the future life and these thermal aging curves are no longer useful. I t becomes necessary then to be able to measure some property of the dielectric that is related to this thermal aging process and which takes into considera­tion the deterioration. During early studies on various insulation measurements the electrical properties revealed a correlation with thermal aging. Further studies were then made on Formex insulated magnet wire to verify these observations on test specimens that reflect the con­figuration of an electric apparatus.

Measurement of changes in dielectric properties, during aging at a constant temperature, reveals a gradual reduc­tion in capacitance with time due to shrinkage of coil speci­men owing to the evaporation of plasticizers. Concur­rently, the loss of plasticizer increases the viscosity of the dielectric which reduces the magnitudes of dipolar oscilla­tion and of ion migration. The reduction of molecular and ionic motion reflects a decrease in dielectric loss and an in­crease in insulation resistance. These variations were found relatively independent of measuring frequency, within the spectrum of 400 to 3,000 cycles per second. However, these coeflficients were found to be markedly influenced by the state of humidification.

At the embrittlement point the viscosity of the insulation reaches a maximum and a minimum in dielectric loss is ob­tained. Using the criterion of minimum dissipation as an index of embrittlement a number of specimen coils were aged, each at a diff'erent temperature level ranging from 180 to 260 degrees centigrade. The embrittlement charac­teristics are presented in Figure 1 together with a life tem­perature characteristic obtained during an earlier investiga­tion^ on the same size and type of wire. The aging band represents the first and last failure of 20 specimen coils.

It may be concluded from this investigation that the di-

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^ 100

10 150 210 250

TEMPERATURE CO

Figure 1. Embrittlement characteristics of Formex

electric properties of Formex insulation undergo a change during thermal aging. This is particularly evident in the dielectric loss and in the insulation resistance. The varia­tions in these values during aging tests can be correlated with the mechanical properties of the materials making it pos­sible to establish the time at which the plasticizer has been removed and embrittlement exists. This point of embrit­tlement correlates with the minimum point on the normal life temperature curves for Formex insulation.

The usefulness of these changes in dielectric properties during thermal aging in determining the true picture of life remaining in insulation can be evaluated only through further studies of other insulations with dipolar as well as those with nonpolar groups. An investigation of the ef­fects of environment and conditions of contamination is also in order. It is important that studies be made of the change in properties of insulated structures which are com­posed of two or more dielectrics, as is found in machinery construction. I t is expected, too, that further work will reveal the eff'ects of temperature cycling on these dielectric properties; this is particularly desirable information at the higher temperatures.

REFERENCE 1. Aging Gharactcrbtics of Electrical Insulation, R. E. Whipple. Naval Research Laboratory Report Number 3708 (Washington, D. C) , July 24, 1950.

Digest of paper 53-125, "Nondestructive Testing of Insulation" recommended by the AIEE Committee on Basic Sciences and approved by the AIEE Committee on Technical Operations for presentation at the AIEE Winter General Meeting, New York, N. Υ., January 19-23, 1953. Scheduled for publication in AIEE Transactions, volume 72, 1953. E. L. Brancato is with the Naval Research Laboratory, Washington, D. G.

M A Y 1953 Brancato—Nondestructive Testing of Insulation 425