Slot Partial Discharge
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Depending on the size of the asset, partial discharge can be detected using different sensors and monitors. Epoxy-Mica Capacitor (EMC) sensors are typically used on small to medium-sized assets and for larger assets Stator Slot Couplers (SSCs) are used for testing. In electrical engineering, partial discharge (PD) is a localized dielectric breakdown (DB) (which does not completely bridge the space between the two conductors) of a small portion of a solid or fluid electrical insulation (EI) system under high voltage (HV) stress. DISCHARGE OF.MORTGAGE OR CHARGE. MORTGAGE OR CHARGE. Mortgage/Charge/Deed Vol. CT / SSCT / SCT Vol Fol. MORTGAGEE OR CHARGEE. ID / Co regn no Name. Address (within Singapore for service of notice). MORTGAGOR OR CHARGOR. ID / Co regn no Name. Address (within Singapore for service of notice) (D) CONSIDERATION. Slot discharge refers to the observation that partial discharges (PD) may occur on the surface of the bar (half-coil), either within the stator core slot, or just outside of the slot. Figures 1–2 show photos of bars experiencing slot discharge considered as fairly serious and corrective action would be considered necessary. There are three. Slot discharges are known to have a distinct phase resolved partial discharge pattern for power frequency (50/60 Hz) excitation. These distinct patterns are useful in identifying the presence of slot discharge activity in rotating machines.
IEEE Transactions on Dielectrics and Electrical Insulation>2012>19>1>61–75
Abstract
The impact of humidity on the slot partial discharge (PD) process was evaluated in a laboratory experiment conducted in a climatic chamber under constant humidity and temperature. Two stator bars, subjected to slot PDs at two temperatures (28°C and 85°C), were aged at twice their nominal phase-to-ground voltage (16 kV) under three absolute humidity levels (5 g/m3, 9 g/3 and 13 g/3). The phase resolved partial discharge (PRPD) patterns were monitored during short aging sequences and visual observations of the surfaces were made at the beginning and end of each aging sequence. Observation confirms that humidity has a significant impact on the discharge phenomenon but other parameters, such as surface condition and temperature, affected the discharge process just as much. This paper presents the way slot PD activity evolves and affects the related PRPD pattern under different conditions of humidity, temperature and surface degradation.
Identifiers
journal ISSN : 1070–9878 DOI 10.1109/TDEI.2012.6148503
Authors
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Slot Partial Discharge Meaning
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Levesque, M.
- Ecole de Technologie Superieure
David, E.
- Ecole de Technologie Superieure
Hudon, C.
- Institut de Recherche dHydro-Quebec IREQ
Belec, M.
- Institut de Recherche d’Hydro-Quebec IREQ
Keywords
Partial dischargesHumidityAgingInsulationSteelSurface treatmentSurface discharges
Partial dischargesHumidityAgingInsulationSteelSurface treatmentSurface discharges
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Fields of science
After a motor is rewound, the windings must be further insulated with resin or varnish. This insulation protects the windings from contamination, shorting electrically, and also make the windings more mechanically rigid. There are four types of varnishing methods that we will describe below: Dip and Bake Epoxy Varnishing, Trickle Varnishing, Vacuum Pressure Impregnation, and Ultra-Sealed Winding.
Dip and Bake is a standard varnishing technique where the motor windings are dipped into a varnish tank and then set to cure in an oven. Typically, a new motor winding should be dipped twice (double dip and bake) to ensure the varnish fully covers the windings.
Many repair shops do not allow the windings to cool once they are cured in the oven after the first dip and before the motor is dipped a second time. Because the windings are still hot during the second dip, the varnish becomes more viscous and runs off the motor easily. This results in a less effective second dip of varnish.
Because we want to ensure adequate varnish coverage, Dreisilker’s quality standards denote that the windings must be cool before the second dip. Our varnish is Class N and Hermetic rated, meaning the varnished used can withstand higher temperatures and is fit for compressors (respectively).
Motor before being dipped in varnish.
Motor after being dipped in varnish.
Slot Partial Discharge Symptoms
Trickle Varnishing The winding is connected to a rotating table and electrical wires. Using electrical resistance, the winding gets heated while spinning. Once at temperature, a tiny stream of varnish is trickled onto the winding head. The varnish follows the wire into the entirety of the slot, eliminating chances for partial discharge in random windings. Once fully saturated, the current is increased in the windings, resulting in the varnish curing while on the machine. This process is faster and better than the traditional dip and bake process because it cures on the machine — making it ideal for emergency repairs.
Slot Partial Discharge Procedure
Dreisilker’s Trickle Varnish Machine.
Trickle Varnished Winding.
Slot Partial Discharge Definition
Vacuum Pressure Impregnation (VPI) utilizes a vacuum pressure tank filled with varnish to fully impregnate motor windings and insulation with resin or varnish. At Dreisilker, we preheat our windings, place the windings in our 10 ft. diameter tank, draw a vacuum, fill the tank until the resin or varnish covers all of the winding, then pressurize the tank. These cycles are all set to various times and capacitance is monitored to determine the resin or varnish fill is acceptable. This process is typically used on medium voltage motors and form coil systems because traditional varnishing methods will not fully saturate the motor coils and their insulation tapes with varnish. The VPI method is the most time consuming process.
Ultra-Seal Winding is an alternative method of insulating a motor’s windings. Ultra-Sealed windings are completely impregnated and seals the coils with high molecular weight thermoset polymer resin. This allows for total protection against moisture, contaminants, and more efficient cooling. We recommend Ultra-Seal Windings for motors found in extreme environments, where contamination is likely. Want to learn more about Ultra-Seal Windings? Read our blog: How to Extend the Life of Your Servo and Spindle Motors in Extreme Environments
