Electrical Apparatus magazine, August 2005 Power factor: Indicator of Insulation Integrity

Evaluating the integrity of a motor or generator Insulation system requires several different tests. The effects of moisture or chemical contamination; the quality of an impregnation process; and the presence of air voids within the insulation wall must all be accounted for.

The test most indicative of voids involves measurement of the insulation power factor -- the ratio between capacitance and resistance within the material. Names for the procedure include "power factor," "tangent delta" (abbreviated "tan delta"), "tip-up," or "dielectric loss" test.

A Schering bridge is the instrument used. Because the a-c test voltage used need not exceed the insulation system rating, the test is considered non-destructive. The test voltage causes leakage current to flow through the insulation. As that voltage is increased from an initially low value, electrical discharges occur within the voids, causing the power factor (indicative of the phase separation between resistance and capacitance components of the insulation) to increase (the "tip-up"). Quality of the insulation is judged by the amount of that change.

The true power factor angle "theta" is large, approaching 90 degrees, because for insulation in good condition the resistive component will be quite small. Most easily measured is the complement of that angle, or 90 degrees minus theta -- a small angle delta. The bridge circuit responds to the tangent of delta. For such a small angle, the tangent very nearly equals the sine, which is the same as the cosine of theta (the power factor). Hence, the "tan delta" becomes the test criterion, and its increase with voltage is called the "delta" (increase of) tan delta.

A power factor test is particularly valuable in determining continuity of the semi-conducting surface coating on coils at the higher voltage ratings (above 6.6 kV). For that reason, and because at higher voltages the insulation is more likely to contain internal voids, the power factor test is most often used for rated voltages of 11 kV and higher.

An IEEE standard (No. 286) and the IEC standard (60894) each describe the test method and offer some guidance for interpreting results.

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Featured Technical Article
August 2005

On this page is a summary of the Electrical Apparatus August 2005 featured technical article, by Richard L. Nailen, P.E. , "Power factor: Indicator of Insulation Integrity" Leistungsfaktor: Gradmesser der Isolierungsintegrität ... Facteur de puissance: une indication de l'intégrité de l'isolation ... El Factor de Potencia - Indicador de la Integridad del Aislamiento

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