|
Summary:
"Power factor: Indicator of Insulation Integrity" With
trilingual summary
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.
From "Power
factor: Indicator of Insulation Integrity" ...by
Richard L. Nailen, EA Engineering Editor -published in
Electrical Apparatus
August 2005
Visit our online
webstore to
order a
copy. © 2005
Barks Publications, Inc. All Rights Reserved.
|
