The NETA ATS-2025 synchronous motor and generator acceptance test is a suite of electrical measurements performed on a de-energized unit before it is first placed in service. The test captures the equipment's identification (manufacturer, model, catalog number, horsepower rating, insulation class, and equipment designation), then performs a battery of electrical checks: insulation resistance measured phase-to-ground and phase-to-phase; polarization index or absorption ratio derived from timed insulation-resistance readings; insulation power factor and dissipation factor at line frequency (optional on some installations); surge comparison between windings if the generator or motor has multiple identical units in the same location (optional); and, for units rated 2300 volts and above, a dielectric withstand voltage test applied at a prescribed voltage and duration to confirm the insulation system will sustain normal operating transients without flashover. The test applies equally to new and reconditioned units.
A synchronous motor or generator only runs reliably if its windings are intact, properly insulated from ground and from each other, and free of moisture and contamination that would degrade the dielectric strength. The acceptance test, performed before energization, is a direct baseline snapshot of the insulation system's condition at the moment the unit enters service. Insulation resistance and power factor indicate whether the windings are dry and clean; absorption ratio trends hint at aging or moisture; surge comparison flags winding imbalances that might cause unequal voltage stress across parallel units; and the withstand test confirms the insulation can handle the operational voltage stresses it will see. Documenting this baseline before any load-on-state transients occur or in-service aging begins allows future maintenance tests to be compared against a known-good starting point, and gives the site a concrete record that the unit was sound when accepted.
The test is performed once, before the unit is first energized, as part of the standard pre-commissioning sequence on any new or reconditioned synchronous motor or generator, at any voltage rating, at any horsepower rating, and regardless of insulation class. It is not a periodic maintenance test; it is a one-time acceptance gate, typically scheduled during the same shop or site window as transformer TTR tests, breaker timing, relay settings, and other pre-energization checks on the rest of the electrical system.
With the synchronous motor or generator de-energized, isolated from the power system per the site's lockout-tagout procedures, and all field and armature circuits safely grounded, the test team connects insulation-resistance test leads between phase conductors and ground, between phase pairs, and between field and ground as applicable. Resistance is measured at the specified DC test voltage (typically 500 V, 1000 V, or 2500 V depending on the unit's rated voltage), and at least two readings separated by a fixed time interval (often 1 minute and 10 minutes) are taken to compute an absorption ratio or polarization index. Power factor and dissipation factor, if included in the scope, are measured at line frequency (60 Hz in North America) using a dedicated power-factor meter or insulation-analyzer function. For units at 2300 V and above, a dielectric withstand test applies a step voltage ramp followed by a hold at a specified AC or DC voltage for a specified duration (typically 1 minute or 3 minutes depending on voltage class and methodology), with any arcing, breakdown, or leakage current exceeding a defined threshold triggering a test failure. Surge comparison, if called for by the site, applies synchronized voltage pulses to two or more identical windings and observes their reflected-pulse waveforms on an oscilloscope or surge-analyzer screen, looking for differences that might indicate an internal defect in one of the units.
Equipment nameplate data (manufacturer, model, catalog number, horsepower, insulation class, equipment designation); the DC test voltage applied; insulation-resistance values in megohms phase-to-ground and phase-to-phase at each time interval, with the computed absorption ratio or polarization index expressed as a ratio or percentage; if power factor and dissipation factor are measured, the values in percent at line frequency for each phase; the test instrument make, model, serial number, and calibration due date; ambient temperature; and, for units 2300 V and above, the withstand voltage applied, the duration of the hold, the test method (AC or DC ramp, step-and-hold profile), and a pass or fail result with any leakage current or thermal signature observations. If surge comparison is performed, the oscilloscope or analyzer output showing the reflected waveforms for each unit and a notation of whether the waveforms matched within the site's defined tolerance.
NETA ATS-2025 section 7.15.2 governs acceptance-test interpretation for synchronous motors and generators. Insulation resistance must exceed a minimum value that depends on the unit's insulation class and rated voltage; the minimum is not a single number but steps upward with equipment voltage class, and the standard's tables spell out the thresholds. An absorption ratio or polarization index above a defined value is expected, indicating the windings are reasonably dry; ratios below the minimum suggest moisture ingress that may warrant further investigation or reconditioning before the unit is energized. If power factor and dissipation factor are measured, values within the insulation class's specification are required, and a result markedly above the expectation can indicate winding contamination or degradation. Withstand tests that occur without breakdown or arcing are a pass; any flashover, tracking, or sustained leakage current above the method's threshold results in failure and triggers analysis of whether the insulation system is sound or the unit requires rework. Surge-comparison waveforms for identical units must match within a margin agreed on by the site and equipment owner. See the purchased NETA ATS-2025 standard for the complete tables, voltage-class breakpoints, and any age- or insulation-class-specific adjustments.
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