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Motor Run Capacitors
C27, Cylindrical Plastic Case, 275 – 470 VAC
Insulation Resistance
When the capacitor temperature increases, the insulation resistance decreases. This is due to the increased electron activity.
Low insulation resistance can also be the result of moisture trapped in the windings, caused by a prolonged exposure to
excessive humidity.
Dissipation Factor
Dissipation factor is a complex function involved with the ineciency of the capacitor. The tgδ may change up and down
with increased temperature. For more infomration, please refer to Performance Characteristics.
Sealing
Hermetically Sealed Capacitors
When the temperature increases, the pressure inside the capacitor increases. If the internal pressure is high enough, it can
cause a breach in the capacitor, which can result in leakage, impregnation, lling uid or moisture susceptibility.
Resin Encased/Wrap & Fill Capacitors
The resin seals on resin encased and wrap and ll capacitors will withstand short-term exposure to high humidity
environments without degradation. Resins and plastic tapes will form a pseudo-impervious barrier to humidity and
chemicals. These case materials are somewhat porous and through osmosis can cause contaminants to enter the capacitor.
The second area of contaminated absorption is the lead-wire/resin interface. Since resins cannot bond 100% to tinned wires,
there can be a path formed up to the lead wire into the capacitor section. Aqueous cleaning of circuit boards can aggravate
this condition.
Barometric Pressure
The altitude at which hermetically sealed capacitors are operated, controls the voltage rating of the capacitor. As the
barometric pressure decreases, the susceptibility to terminal arc-over increases. Non-hermetic capacitors can be affected by
internal stresses due to pressure changes. This can be in the form of capacitance changes, or dielectric arc-over, as well as
low insulation resistance. Heat transfer can also be affected by altitude operation. Heat, generated in an operation, cannot
be dissipated properly and can result in high RI2 losses and eventual failure.
Radiation
Radiation capabilities of capacitors must be taken into consideration. Electrical degradation in the form of dielectric
embitterment can take place causing shorts or opens.
