DC Dielectric Strength Test

The EDC system features various stator configurations, enabling different DC Dielectric Strength Tests. Notably, the AST320/W Stator Tester, ATT320/W Transformer Tester, AAT320/W Armature Tester, ATS320/W Generator Tester, and AMT320/W - PAE320/W Motor Tester can conduct the following tests when integrated with appropriate options:

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1. Winding(s) versus Ground
2. Winding versus Winding (for distinct phases that are not internally connected and have accessible terminals)
3. Thermal Protector versus Ground
4. Thermal Protector(s) versus Windings
5. Other elements (like terminals of a possible brake or fan) versus Ground
6. Other elements (such as terminals of a potential brake or fan) versus the Windings of the main Motor/Stator.

During testing, the EDC system performs crucial controls:

• The total current is measured and compared to the maximum acceptable current value set in the program parameters. A test is considered successful only if the current during the test remains less than the maximum acceptable current defined in the program parameters.
• Verification that the set testing voltage is applied to the stator under examination. This is vital to ensure the stator stack connects properly to the high-voltage generator. This verification can be performed by directly measuring the testing voltage or indirectly checking the capacity value present between the testing points.

For additional information, please refer to dc dielectric test sets.

The Dielectric Strength Test using DC Voltage is mainly utilized for high-power stators, where the capacitive current is typically high and often exceeds the maximum current that a typical AC High Voltage source can generate (30mA, 100mA, 500mA, etc.). Additionally, the testing voltage must be 1.41 times greater than the testing voltage used with an AC Voltage source.

The diagram illustrates the fundamental scheme of the Dielectric Strength Test in DC voltage measurement.

IC Capacitive

Capacitive (or geometric capacitive) current, commonly referred to as inrush current, is an essential factor as the windings possess capacitance. Current is needed to elevate the voltage potential. Typically, the capacitive current diminishes to zero within seconds once the test voltage provided by the motor tester stabilizes.

IA Absorption

Absorption current manifests during atomic and molecular polarization of the insulation and is the significant current during a PI test. This current may drop to zero or near zero over a variable timeframe, depending on the motor. The decline can occur within seconds or may extend to 10 minutes or more.

IG Volume Conduction

Volume conduction current represents the flow of current through the entire volume of insulation between ground and conductors. In good windings, this current is usually minimal or nearly zero, affected by the composition and condition of the insulation system. This current is often confused with leakage current; while it indeed leaks through the insulation, surface conduction current (IL) is typically the principal leakage in used motors.

IL Surface Conduction

Surface conduction current is often called surface leakage current, which flows over the end winding surfaces of the insulation.

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• Surface conduction arises from surface contamination, dirt, and moisture present on the windings connected to ground.
• As contamination escalates, the resistance of the contaminants declines, resulting in an increase in current.
• With increasing voltage, the current also increases, roughly in proportion to the voltage applied by the motor tester.
• In good used motors, this current typically surpasses absorption and volume conduction currents due to the relatively lower resistance in surface contamination.
• In new, entirely clean, and dry motors, this current should ideally be zero or very close to zero.