A Comprehensive Guide to Electrical Apparatus Testing: Ensuring Safety and Reliability

A deep dive into the essential testing procedures for electrical machine windings and components, as outlined in Section 4 of the EASA AR100 standard, reveals a meticulous process designed to ensure the operational integrity and safety of new and repaired electrical apparatus. This section of the standard provides a detailed framework for a variety of tests, from assessing insulation condition to verifying performance under no-load conditions.

A cornerstone of the testing protocol is a strong emphasis on safety. Before any tests are conducted, it is imperative to consult the safety considerations outlined in the appendix of the standard. This ensures that all personnel are protected from potential electrical hazards.

In-Depth Insulation Condition Inspection and Tests

The longevity and reliable operation of an electrical machine are heavily dependent on the condition of its insulation. A series of tests are performed to evaluate the suitability of the insulation for continued service. It is recommended that insulation resistance (IR) tests are carried out with satisfactory results before proceeding to high-potential tests. The consistent recording of all test results is crucial, as trends in these results can be more indicative of the insulation's condition than absolute values.

Key insulation tests include:

• Insulation Resistance (IR) Test: This one-minute test is performed at DC voltage levels specified in Table 4-1 of the standard, which vary based on the winding's rated voltage. The recommended minimum insulation resistance values, detailed in Table 4-2, are crucial for determining if the winding is fit for further testing and operation.

• Polarization Index (P.I.) Test: This ten-minute test, conducted at the same voltage as the IR test, provides a more detailed assessment of the insulation's condition. A minimum P.I. value of 2.0 is recommended for windings rated Class B and higher. However, if the one-minute insulation resistance is above 5000 megohms, the P.I. value may not be meaningful and can be disregarded.

• Surge Tests: Surge tests are critical for verifying the winding's ability to withstand voltage surges. For form-wound stator coils, test levels are adapted from IEEE 522 and IEC 60034-15, as shown in Table 4-3. For all other windings, the surge test is typically applied at twice the rated voltage plus 1000 volts. A single, stable waveform indicates a healthy winding, while a multiple or unstable waveform suggests a fault that requires further investigation.

• Other Specialized Tests: The standard also outlines several other important tests, including insulation power factor tests for large machines, interlaminar insulation tests for AC cores, and bearing insulation tests to prevent bearing currents, particularly in variable frequency drive applications.

Recommended Tests for Various Winding Types

To ensure comprehensive quality control, specific tests are recommended for different types of windings to detect potential issues like grounds, short circuits, open circuits, and high-resistance connections.

• Stator and Wound-Rotor Windings: A combination of IR, winding surge, and winding resistance tests should be performed. Additional tests like phase-balance, polarity, and dummy rotor tests may also be conducted.

• Squirrel Cage Windings: To detect defects in the squirrel cage, methods such as the growler test, single-phase test, or magnetic field analysis are employed to induce currents in the bars and end rings.

• Armature Windings: An insulation resistance test is mandatory, supplemented by one or more of the following: a growler test, a winding surge test, or a bar-to-bar test.

• Field Windings (Shunt, Series, Interpole, Compensating, and Synchronous Rotor): These windings should undergo an insulation resistance test, along with other tests like winding resistance, surge, AC voltage drop, or impedance tests to ensure their integrity.

High-Potential (Hipot) Tests: A Critical Verification

High-potential tests are performed to ensure that the insulation can withstand voltages higher than its rated voltage. These tests should only be conducted after the machine is clean and dry, and has passed an insulation resistance test. It is important to note that repeated application of high-potential test voltage is not recommended to avoid excessively stressing the insulation.

The tests are applied between each winding and the grounded frame of the machine. The standard specifies test voltages for both AC and DC high-potential tests in Table 4-4 and Table 4-5, respectively. For DC high-potential tests, the test voltage is 1.7 times the specified AC voltage.

The test voltages vary for new and reconditioned windings:

• New Windings: Tested at the full voltage specified in the tables. Immediately after a rewind, if a high-potential test is required, the test voltage should not exceed 80% of the original test voltage.

• Reconditioned Windings: If approved by the customer, these are tested at 65% of the new winding test value.

• Windings Not Reconditioned: For these, an insulation resistance test is performed instead of a high-potential test.

Similar principles apply to the accessories of the electrical machine.

Final Checks: No-Load and Load Tests

Once all winding and insulation tests are successfully completed, the assembled motor undergoes a no-load run test to ensure satisfactory operation. This includes checking the speed at the rated voltage and frequency, monitoring no-load currents, and verifying the proper functioning of the cooling system. Bearing temperature and vibration levels are also monitored until they stabilize.

Tests with a load may also be performed as agreed upon with the customer to check the machine's operating characteristics under real-world conditions.

Finally, to ensure the accuracy of all measurements, all instruments and transducers used for testing must be calibrated at least annually against standards traceable to a national standards laboratory. This comprehensive testing regimen ensures that repaired and new electrical apparatus will perform reliably and safely throughout their operational life.