A winding properly protected from slot discharge should not produce hash at voltages below approximately 125 percent of normal.
This is the point where slot discharge is considered to begin. The voltage is gradually raised until there is distinct change in the hash on the oscilloscope screen. Voltage of about 50 percent, normal or below which no corona occurs, is first applied as a calibration point or base. With the oscilloscope sweep set for one-half the applied frequency, the characteristic hash indication appears at two or four places along the sweep since the discharge occurs at particular parts of the voltage cycle.īecause the detector cannot distinguish between hash originating in the winding and that originating in the power source, it is essential that the stator winding be energized from a good high-potential test set or a transformer operated well below its rated voltage. The detecting apparatus is coupled through a condenser to the terminal of the winding being tested or to a probe which is placed ill contact with the stator coil shielding. The detecting apparatus is essentially a band pass filter passing a band of frequencies between 1,000 and 2,000 Hz to an oscilloscope.
CORONA DEFINITION GENERATOR
Test apparatus for corona detection requires much higher sensitivity than detection of slot discharge.Ī method used for detecting slot discharge in AC generator windings consists of energizing the winding from a high-potential AC test set at voltages of approximately 50 to 125 percent of normal rated machine voltage or running the unit self excited and isolated from the system and observing on an oscilloscope the resulting induced potentials from slot dis-charges. Coils which fit loosely in the slots may be subjected to this trouble. Deterioration is from the destructive effect of this relatively concentrated discharge. Slot discharge is a capacitance discharge and occurs across poor contact points between the coil surface shielding and the stator iron. Destruction is often aggravated finally by mechanical vibration of the conductors where the insulation has been softened by the other effects.
CORONA DEFINITION HOW TO
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