The Dielectric Strength Test allows to identify:
- Windings in contact with the stator stack (ground)
- Windings in short-circuit with thermal protectors
- Windings in short-circuit among them (when the different phases are not internally connected and the related terminals are accessible).
The EDC System, having different configurations, performs separate dielectric strength tests with AC testing voltage. For example, in case of Stator Tester AST320/W or Transformer Tester AAT320/W or Armature Tester AAT320/W or generator Tester ATS320/W or Motor Tester AMT320/W - PAE320/W with the special functions, it’s possible set tests between:
1. Winding(s) versus Ground
2. Winding versus Winding
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 Windings of main Motor/Stator.
For some particular types of product, in order to completely verify the product and not only the initial part of the terminals, it may happen that the product must be powered. In this particular test version, therefore, the test voltage set is applied to the product being fed (specifically defined).
During the test, the system performs the following controls:
- Measurement of the total current and separation of the capacitive component from the resistive component. The capacitive component is always present also in good pieces and depends on the size of the stator. Typical values range start from 0.5 mA for stators which motor power is about 100/200W to 10/20 mA for stators which motor power is up to 10/30 KW. Instead the resistive component is only due to the presence of faults (discharges or lack of insulation). In fact, even in the stators of large motors, if they have no defects, the resistive component is always almost equal to 0mA.
- Using the Resistive Current Measure the ability to identify faults improves a lot. In fact, for example, for a stator with a total current of 10mA it is possible to set as Maximum Resistive Current a value of 1mA improving about 10 times the capability to identify defects comparing to testing equipment which use only the Total Current as control parameter.
- 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 (especially for stators with varnished stack). This control is normally performed verifying that the Total Current is higher than a minimum value. In special systems, in addition to this control, it is present a 4 wires system that enables a direct reading of the voltage applied on the testing leads. If the voltage result is lower than the Nominal Value less a settable Tolerance, a message informs that the test is not acceptable and will be cancelled.
- At last a verifying procedure is made during the test to make sure the total current is lower than the maximum set in the test parameter. In case the current is higher than the maximum acceptable current the test will Failed (NOGO).
The Picture shows the principle scheme of the dielectric strength test in AC voltage measurement.
IMPORTANT NOTE: this test is able to detect the defect only if this generates a detectable current. To maximize this test and therefore identify all the defective products, both with obvious defects and with latent defects, E.D.C. has specifically developed a measurement that integrates the Dielectric Strength test and greatly increases its effectiveness: this is the Measurement of Partial Discharges performed during the Dielectric Strength test (320 / PDH).