DC Dielectric Strength Test

The EDC system, having different stators configurations, performs different Dielectric Strength Test in DC Voltage. For example, in the case of AST320/W Stator Tester or ATT320/W Transformer Tester or AAT320/W Armature Tester or ATS320/W Generator Tester or AMT320/W - PAE320/W Motor Tester that have integrated the appropriate options, it is possible to perform the following tests:

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1. Winding(s) versus Ground

2. Winding versus Winding (when the distinct phases are not internally connected, and the related terminals are accessible)

3. Thermal Protector versus Ground

4. Thermal Protector(s) versus Windings

5. Other elements (for example, terminals of a possible brake or fan or other mounted on the motor) versus Ground

6. Other elements (for example, terminals of a possible brake or fan or other mounted on the motor) versus the Windings of main Motor/Stator.

During the test, the EDC system performs the following controls:

• Measurement of the total current and comparison between the current measured value and the maximum acceptable current value set in the program parameters. The test result is passed only if the current during the test is lower than the maximum acceptable current set in the program parameters. 
• Check that set testing voltage is applied to the stator under test. This is very important to ensure that the stator stack is connected to the high-voltage generator. This control can be realized reading directly the testing voltage or checking indirectly the capacity value present between the testing points.

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  The Dielectric Strength Test with DC Voltage is used mainly in case of high-power stators, where the capacitive current results to be high and in whichever case higher than the maximum current that the eventual available AC High Voltage source could generate (30mA, 100mA, 500mA, mA). The testing voltage must be 1,41 times the testing voltage used with an AC Voltage source. 

The Picture shows the principle scheme of the dielectric strength test in DC voltage measurement.

The Picture shows the principle scheme of the dielectric strength test in DC voltage measurement.

IC&#;Capacitive

Capacitive (or geometric capacitive) current is also called inrush current. The windings have capacitance. Current is required to elevate its voltage potential. Capacitive current typically drops to zero within seconds after the test voltage provided by the motor tester is stable.

IA&#;Absorption

Absorption current is present during atomic and any molecular polarization of the insulation, and is the current of interest during a PI test. This current will drop to zero, or near zero, over a period of time that varies by motor. The drop can happen in seconds or may take 10 minutes or more. 

IG&#;Volume Conduction

Volume conduction current is the current that flows through the entire volume of the insulation between ground and the conductors. In good windings, this current is usually zero or near zero, and depends on the composition and condition of the insulation system. People sometimes think of this current as &#;leakage&#; current. The volume conduction current certainly leaks through the insulation, but the surface conduction current (IL) is usually the main leakage in a used motor.

IL&#;Surface Conduction

Surface conduction current is often referred to as surface leakage current. The surface conduction current runs over the end winding surfaces of the insulation.

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• Surface conduction is a result of surface contamination, dirt and moisture on the windings that are connected to ground.
• As the contamination level increases, the resistance of the contamination drops, and the current increases.
• As the voltage increases, the current increases more or less proportionally with the voltage applied by the motor tester.
• For used, good motors, this current will dwarf the absorption and volume conduction currents because of the relatively lower resistance in the surface contamination.
• For new, totally clean, and dry motors this current should be zero or near zero.