Insulation and dielectric
Power factor tip-up testing measures insulation loss at a low voltage and again at higher voltages, and reports how much the loss rises. A rise points to voids and partial discharge inside the winding insulation of large machines and dry-type transformers.
Power factor tip-up testing is an extension of the ordinary insulation power factor or dissipation factor measurement. Instead of a single reading at one voltage, the insulation loss is measured at a low voltage and then at a series of higher voltages, up to a level set by the winding rating and the procedure. The tip-up is the difference between the loss at the highest step and the loss at the lowest, and it shows how much the insulation behavior changes as the electric stress increases. The test is called for as an optional measurement on the stator windings of large induction and synchronous motors and generators and on large dry-type air-cooled transformers, where the winding insulation is solid and may contain internal voids. It is performed with an insulation analyzer or bridge capable of stepping to the higher voltages, and it is evaluated together with the ordinary power factor, the capacitance and other insulation tests. The question it answers is whether the insulation holds a steady loss as the stress rises, or whether something inside begins to discharge and dissipate extra energy.
Windings made of mica, resin and glass are strong in bulk but can hide voids, delaminations and poorly impregnated areas, particularly in older machines and in machines subjected to thermal cycling, vibration or repeated starts. A low-voltage measurement does not stress those voids enough for them to ionize, so the insulation can look sound while the defect waits for working voltage to activate it. Once discharge begins inside a void, it erodes the surrounding insulation and the process feeds itself, which can end in a groundwall failure, a damaged core, a lost machine and a long outage. The tip-up test finds this class of condition during a planned shutdown, early enough to plan a rewind, a cleaning and revarnish, or a closer look with partial discharge testing. At acceptance it confirms the quality of the manufacturing and impregnation process in new or rewound windings and sets a baseline for the future. In maintenance it follows how the voids develop. It complements resistance and polarization tests, which are insensitive to this kind of defect, and it gives a quick overall indication that partial discharge testing then explores in detail.
At low voltage, dielectric loss in sound insulation is small and nearly constant, and it comes from the material itself. If the insulation contains gas-filled voids, nothing special happens at low voltage because the gas still withstands the stress. As the applied voltage rises, the stress in the gas in a void rises with it, and at some point the gas ionizes and a small discharge takes place each cycle. Those discharges dissipate real energy, which appears as additional loss current in phase with the voltage, and the measured power factor goes up. The tip-up is therefore a measure of how many voids discharge and how intensely they do it between the low and the high voltage. A winding with few voids shows almost no change in loss with voltage, and a winding with many voids shows a clear rise. The measurement also reflects other voltage-dependent effects such as surface leakage on contaminated end windings, corona in the end-winding region and moisture, and a good procedure helps separate them. The result is a ratio of behaviors, not an absolute loss, so differences between like windings and between tests over time carry the meaning.
The machine or transformer is de-energized, isolated from every source, proven dead and grounded under the owner's lockout and permit procedures, and the connected leads, cables, surge protection and any electronic devices are removed from the winding under test. The winding surfaces and terminals are cleaned and dried, because surface leakage and end-winding corona would distort the voltage-dependent readings. Each phase winding is tested in turn with the others, and the core and frame, connected to ground, or the guard of the instrument is used to exclude paths that are not of interest. The analyzer is connected to the winding and the measurement is first taken at a low voltage as a base reading. The voltage is then raised in controlled steps, holding each long enough for the reading to settle, up to the highest level permitted by the equipment rating and the procedure, with readings of power factor and capacitance at every step. The voltage is returned to zero before leads are moved, and the winding is grounded afterward. Winding temperature, ambient temperature and humidity are recorded, since both the base reading and the rise depend on them, and the same sequence of steps is used for every phase and on every later test so that the results can be compared.
The record holds the power factor or dissipation factor and the capacitance measured at each voltage step for each winding or phase tested, together with the tip-up derived from the lowest and highest steps. It notes the test voltage at every step, the connection and guard arrangement, the temperature of the winding and of the surroundings, the relative humidity, and the condition of the end windings and terminals as found. The identity and calibration status of the instrument are noted too. Since the tip-up is a difference between readings, the way each step was set up must be documented so that a later test can be repeated step for step and compared fairly.
No single number decides whether insulation is acceptable, and this page does not give one. The tip-up result is judged in several ways that are used together. The first is the size of the rise itself, read against the guidance in ANSI/NETA ATS-2025 for acceptance testing and ANSI/NETA MTS-2023 for maintenance testing and against any limits in the manufacturer's data or the machine specification. The second is comparison between the phases of the same winding, where one phase that stands apart from the others suggests a local problem. The third is trending against earlier results on the same machine, where a growing tip-up is more telling than a single value. The fourth is reading the tip-up alongside the base power factor and the capacitance, since a high base value combined with a rise suggests a different cause from a low base value with a rise. The fifth is cross-checking with partial discharge, resistance and polarization results. Because the test is optional in the standards, the owner's criteria and the experience of the testing agency carry weight. Refer to your own licensed copies of the standards named above for the governing language.
Acceptance and maintenance tests differ in purpose. At acceptance, the tip-up checks the manufacture and impregnation of new or rewound windings and creates the baseline, so it is made carefully and with documentation. In maintenance it follows change, and the most useful comparison is with the earlier tests of the same winding at the same steps. The class of equipment shapes the work. On large motors and generators the voltage steps run up to a level set by the winding rating and the procedure, and the test is made phase by phase on the stator winding with the rotor in place or removed as the situation allows. On large dry-type transformers the same approach is used on the primary and secondary windings, but the geometry and the air spaces change what the readings mean. Windings that have been recently cleaned or dried give different results from those with surface contamination. The test is usually paired with insulation resistance, polarization, surge comparison and partial discharge measurements, since each catches faults that the others can miss, and it is not a substitute for them.
The equipment classes and standard sections that call for this test, and the UltraDb forms that carry it.
It tells how much the insulation loss increases as the test voltage is raised from a low level to a higher one. A small rise suggests insulation without active internal voids, while a larger rise suggests that voids or other voltage-dependent effects are starting to discharge and dissipate energy.
At a low voltage, voids do not discharge, so a single low-voltage reading can look healthy while a defect is waiting for working stress to activate it. Measuring at several steps reveals the behavior that a single point cannot show, and the difference between steps is the tip-up.
No. Tip-up measures the total extra loss that discharge activity produces across a whole winding, giving a quick overall indication. Partial discharge testing detects and characterizes individual discharges and their patterns. The two are often used together, with tip-up pointing to where a closer look is warranted.
Yes. Dirt, moisture and oil on end windings and terminals carry leakage current and can produce corona that changes with voltage, which raises the apparent tip-up. Cleaning and drying the winding before the test, and using the guard where appropriate, helps keep the result a measure of the groundwall insulation.
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