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Diagnostic of Rotating Machines Insulation
6 Diagnostic of Rotating Machines
Insulation
For motors and generators, the expected life of a stator winding depends on the
ability of the electrical insulation to prevent winding faults. High temperatures,
and high rates of temperature changes, can generate inner micro-voids
particularly at the interface between Mica and resin, and between
semiconductive layers and the resin. Partial discharges in these voids can cause
a growing of the voids by erosion and complete breakdowns are only a matter
of time.
6.1 Tan Delta Measurements
Capacitance (C) and Dissipation Factor (DF) - often also called "Tan delta" or
power factor (PF) - measurement is an established and important insulation
diagnosis method. It can detect:
• insulation failures
• aging of insulation
• contamination of insulation liquids with particles
• water in solid and liquid insulation
• partial discharges, e.g., in voids and cracks.
Definition
In an ideal capacitor without any dielectric losses, the insulation current is
exactly 90 ° leading according to the applied voltage. For a real insulation with
dielectric losses this angle is less than 90 °. The angle
δ = 90 °- ϕ is called loss
angle. In a simplified diagram of the insulation, C
p
represents the loss-free
capacitance and R
p
the losses (figure 6-1). The definition of the dissipation
factor and the vector diagram are shown in figure 6-2.
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