Before we talk about generator rotor testing, let’s make sure we are all on the same page in terms of understanding what a rotor does.  First, let’s start with a quick discussion of terminology.  Some OEMs such as GE refer to the rotating portion of the generator as the field.  Others, such as Siemens refer to it as the rotor.  Both are correct, but both have their limitations.  “Rotor” is convenient because it tells us that it is a rotating component.  “Field” tells us that it is a DC electromagnet with multiples of two poles.  “Rotor” and “Field” get confusing when we talk about a brushless exciter where the field (the DC part) is stationary and the armature (the AC part) is the rotor.  The correct term from an electrical engineering standpoint is “field”, whereas the term “armature” always refers to the alternating current component whether it is stationary or rotating.  The rotor transmits its torque to the stator by means of locking or synchronizing its magnetic field rotation to that of the stator.  The strength of this magnetic locking is proportional to how much current we force to flow through the rotor.  We can also influence the voltage of the grid, if the grid is small compared to the capacity of the generator, OR, in cases where the generator is connected to an infinite grid we can not raise the grid voltage appreciably but we can help the grid by exporting VARs.  The subject of VARs is a subject matter deserving its own separate discussion—for another time.

On a 60 Hertz unit, the rotor’s magnetic field makes 60 rotations per second, and the stator’s magnetic field also rotates 60 times per second.  If the generator breaker is open, and excitation is on, the generator will produce full rated voltage, and zero current.

With the breaker open, or if the unit is on a small island grid where it is the main or only generator (isochronous mode), raising excitation will result in an immediate raise in grid voltage.

Unlike the stator, where virtually all of the windings are buried under many layers of mica and epoxy insulation, the rotor windings are open to the environment, and have minimal insulation.  For this reason, rotors are particularly vulnerable to ground faults and inter-turn shorts (“shorted turns”).

As with all generator testing, the objectives are fairly clear:

  • Verify that all insulators insulate properly, and;
  • Verify that all conductors conduct properly

Not much more to it than that!  We are talking about copper, steel, and insulation, after all.

There are a multitude of tests that the OEMs call for on rotors during troubleshooting or re-winding, but the most common maintenance tests are as follows:

  • Insulation resistance and polarization test (“Megger & P.I.”)
  • DC resistance test
  • AC impedance test
  • RSO test

Sidewinders follows IEEE 56 – §8.2 and OEM guidance in performing and evaluating these tests.

The following provides a summary of each of the above-listed tests and how Sidewinders evaluates the data.


Insulation resistance and polarization test

GENERATOR ROTOR TESTINGMost commonly referred to as “megger & PI”, this test is a very short, simple, and safe test which gives us a lot of information about the insulation system in a relatively short amount of time.  The “megger” part of the test consists of applying a steady DC voltage to the winding under test.  For most stators 13,800 and over, most OEMs call for 5,000 to be held for 10 minutes.  For rotor windings, 500 VDC is the standard voltage.

When explaining electrical concepts, it is helpful to make an analogy to a water piping system that everyone understands. A common garden hose with zero pressure, and the valve closed at the other end will swell up when the water is first turned on, and you can hear and feel the water rushing into the hose, even though nothing is coming out of the other end due to the valve being shut.  Comparing voltage to pressure, an electrical winding has a similar charging-up current when we first apply the megger voltage.  Even though the system is an open circuit, somehow there is still current flowing in!  The is because the insulation molecules are re-orienting themselves so that the dipole aligns with the electric field that is stressing the insulation.  As the insulation electrically “stretches” just as the garden hose swells up a bit, the back pressure in the hose pushes back to resist the flow of additional water from coming in as the pressure equalizes with the source (faucet / megger) pressure (60 PSI / 5,000 VDC).  This will cause the resistance readings on the test set to rise, giving higher resistance values with time.  By the end of the 10 minute “soak” period, the winding charging current will have essentially bled off to zero, and any leakage current that remains will be assumed to be due to imperfections in the winding insulation.

The polarization index (PI) is computed as a ratio of the 10 minute resistance divided by 1-minute resistance.  On stators you want to see at least a 100% improvement over a ten-minute period, or a PI=2.0 or higher.  On rotors, owing to the open insulation system, a lower PI is expected, while we like to see 2.0 or higher, it is much more common to see PI’s in the 1.2 – 1.5 range.  These readings are acceptable as long as the actual meg-Ohm value is sufficiently high.  Sidewinders has additional OEM-specific criteria for interpreting low PI values.


DC resistance test

GENERATOR ROTOR TESTINGThis test is very straightforward.  Using a digital low resistance ohmmeter (DLRO), we apply a 10-amp current through the rotor winding circuit and measure the voltage drop.  The instrument takes this data, and by using Ohm’s law, calculates the resistance.  Due to the thermal properties of copper, the resistance varies greatly with temperature, so it is not enough to just record the resistance value—we must also record the temperature of the winding.  Today, you may be testing a unit outdoors at 75 degrees F and get a certain test value, and the next person may test the unit in the middle of January and get a much lower value.  Sidewinders always converts the actual resistance to what it would have been if it were taken at 25 C.  This standardization allows an “apples-to-apples” comparison between all test data.

This test is important as it allows us to see if there is any change in resistance with time, since the previous test, or since the manufacture date.  Winding resistance rarely goes down—if things go bad, they usually go up.  Resistances go up when braze joints start to fail, or when silver plated surfaces degrade.  In cases where resistance goes down, we would suspect shorted turns.


AC impedance test

GENERATOR ROTOR TESTINGThe AC impedance test is used to find indications of shorted rotor turns.  The test is performed by means of applying an AC voltage across the field winding and raising it in 10-volt steps up to 100-120 volts, or till the current supply gets maxed out.  We measure the current at each step.  Using Ohm’s law, we calculate the impedance Z=V/I where Z is the magnitude of the complex impedance (Z=R + jwL) the resistive component and the inductive component), V = voltage applied, and I=resultant current.

As we raise the voltage, the voltage differences across each turn of the winding also increases.  At low voltages, we often don’t see the inter-turn short conducting until the voltage raises to a point where the shirt begins to conduct.  At this transition point, we would see a step change in the impedance.  We would see a step in the graph.

Another way we analyze the data is to run it up in 10-volt steps, then run it back down in 10-volt steps.  Normally, the data will show some hysteresis, which is normal and expected.  The important part is that the data start and end at the same point.  A closed path is good—a path that starts and ends at two different points is suggestive of shorted turns. Due to hysteresis, it is important that as we raise voltage between steps, we don’t back-track if we overshoot a test value.  In other words if we are trying to dial in 20 volts, but we overshoot to 21.05 volts, we don’t go backwards and try to tune in exactly 20 volts.  The hysteresis will cause a different current to flow than if we had not back-tracked!

In cases where a shorted turn is suspected, we typically call for an RSO test to confirm it.  It is Sidewinder’s philosophy not to condemn a unit to be rewound without performing additional testing to confirm the fault.  This is where the RSO test comes in handy!


RSO test

GENERATOR ROTOR TESTINGRSO stands for Recurrent Surge Oscillography.  RSO is a low-voltage test which applies a high-frequency (RF) train of pulses into one end of the rotor and detects the energy wave form coming out the other end.  It is similar to the RADAR concept in that it uses a time-of-flight concept to detect electrical obstructions such as shorted turns within the winding.  The test alternately injects energy in one direction and measures the energy from the other side,  then switches directions.  This provides two oscilloscope traces.  In an ideal rotor winding with no shorted turns, the two waveforms can be superimposed perfectly over each other.  If they cannot be perfectly matched up, this is an indication of shorted turns.  We use the “math” function on the oscilloscope to subtract channel 1 from channel 2 to give a “difference” trace.  If traces on channels 1 & 2 are identical, then CH1 – CH2 will be equal to zero.


Plotting the difference trace will show a flat line in a perfect unit, and will show a flat line with a “blip” on a unit with shorts.

RSO by itself can confirm shorted turns, but it does not tell us WHERE the short is located.  It is just a “go-no-go” test.  The only true test for shorted turns which tells us which coil has a short, and how many, is the flux probe.  We will discuss the merits of the flux probe in a future edition of this blog.



Just as every bell has a unique tone when struck by a hammer, so do all generator winding components.  Every structure, in fact has its own characteristic modes of mechanical response when deformed after being struck.  While not very musical in nature, the response waveform gives the trained Technician valuable information on the integrity of the windings under test.

Bump testing can be done discretely (giving specific results for each specimen struck), or as a composite modal wave shape function where some of the specific information about individual specimens gets lost in favor of getting system-level resonant response information.  The two major design philosophies are split between GE & Westinghouse, which dates back as far as the companies respective founders, Edison and Westinghouse.

End winding resonance testing determines the mechanical resonant frequency of each of the series and phase connections.  Each of the series and phase connections are struck with an instrumented mallet equipped with an accelerometer, and the responding oscillation is detected by an accelerometer mounted on the specimen winding.  The data are sent to a Fast Fourier Transform processor where the stimulus and response are analyzed in the frequency domain.  Y(s) = H(s)*X(s) is a standard transfer function relationship where Y(s) is the output in the frequency domain (response), X(s) is the forcing function (how much energy you put into the system and at what frequencies), and H(s) is the transfer function that is the “fingerprint” function of the system under test.  This transfer function is displayed graphically as amplitude versus frequency, so that the resonant peak frequencies can easily be identified from a graph.

The resonant frequency of an object is the frequency where the system most efficiently accepts energy from an outside stimulus.  In the case of generator windings, the “system” is composed of the end windings, and the stimulus the twice-per-revolution mechanical force that acts upon the windings.  In the 60 Hz grid the forcing function will be 120 Hz; similarly for 50 Hz the forcing frequency will be 100 Hz.

In general, the current-carrying components of the generator must be mechanically designed in such a way that their resonant modes are not near this forcing frequency.  The word “near” means avoiding the band of -5% to +10% of the forcing frequency, or (115 – 135 Hz, for a 60-Hz unit).

Each OEM has certain parameters for determining acceptable levels of resonance.  Sidewinders applies the most sensitive criterion of all the OEMs, which is 0.20 g/Lb-f (read as: g’s per pound force).  Working through Newton’s Second Law of motion, F=ma, if you solve for units of [acceleration/force] you arrive with units of reciprocal mass (1/m).  The higher the mass, the lower this fraction; the lower this goes, the lower the amount of acceleration you will see.  This is a perfect moment to segue into the topic of tuning.

If you’ve ever played with a guitar, you quickly saw that the thicker, longer strings vibrated with the lowest frequency.  You probably also saw that if you tightened any string, the frequency went up.  You also noticed that if you put your finger down somewhere on one of the frets, the frequency went up.  We apply these ideas to bump testing as follows:

If a unit displays resonance, we first must characterize it as being above, or below the desired frequency band.  If above, we may apply “low tuning”, and as you might expect, if the results are below the desired range we would apply high tuning.

A way to low-tune a generator would be to either add mass, or increase the spacing between unsupported spans of the winding.  Sidewinders generally prefers to “high tune” machines, as this generally involves adding additional means of structural rigidity.  Ways to high-tune a winding system include:  adding additional blocking, applying wicking & flooding resins, replacing failed ties, adding nose rings, and in the case of certain Siemens-Westinghouse and MHI units, we can re-tension radial banding or radial studs.   All of these measures tend to drive frequency response upward.


Screenshots before (L) and after (R) Sidewinders made repairs due to bump test findings

Sidewinders has attended to many cases where a serious in-service failure occurred due to an unmitigated resonance issue that could have been detected and corrected by means of this testing modality.

Bump testing is a fast, economical way to gather valuable information about the condition of your generator and help predict and avoid costly failures!  Contact Sidewinders for a bump test quotation for your next planned outage.

Insulation Resistance and Polarization Index Testing of Generators

Megger & P.I. as it’s commonly referred to, is one of the quickest, safest, and simplest tests in the arena of generator electrical testing, but it is also one of the most useful.  It is this simplicity that often makes the test somehow seem less important than some of the “big ticket” tests such as Hi-Pot or ELCID.  Megger and PI is a very quick way to get an overall assessment of the health and cleanliness of an insulation system in just a few minutes’ time.


Sidewinders makes engineering evaluations based on the overall analysis of many tests and inspections considered as a whole, but the Megger & PI is considered the front line test when evaluating the condition of an insulation system.  The main goal of a test & inspection job is to verify that all insulating systems properly confines the flow of electricity within the conductors by means of Megger & P.I.; and that all conductors allow the unimpeded flow of current by means of DC resistance testing using a digital low resistance ohmmeter (DLRO).  Copper should pass current freely, and insulation should block the flow of current.


The megger test, as with most electrical systems, is best understood when an analogy is made to a piping system.  Take the common 50-foot long garden hose, for example.  Initially, the hose is empty and has zero pressure, and is closed at the far end.  The instant the faucet is turned on, the hose swells up, and you can hear and feel the water current rushing in as it charges the hose to the same pressure as the supply faucet, say 50 PSI.  If the hose doesn’t have any leaks, the current will stop once the hose has fully charged to 50 PSI.  If the hose has some microscopic leaks, there will be a small trickle current which could be measured as the leakage current.  Substituting voltage for pressure, and current for flow, we can make a direct analogy to an electric winding insulation system.  During the megger test, the winding will initially accept large current flows as the unstressed insulation becomes stressed up to the test voltage of say, 5000 volts.  Once the insulation reaches the test voltage, the insulation molecules continue to rotate and orient themselves to be parallel to the electric field lines of force.  After a period of ten minutes, the system is considered to have reached steady state, and any remaining current going into the winding is assumed to be purely due to leakage through imperfections in the insulation system.

Insulation Resistance and Polarization Index Testing of GeneratorsThe polarization index is a measure of how much the insulation system resistance improves with time.  Let’s return to the garden hose.  A perfect garden hose with the other end closed off would exhibit zero current 10 minutes after the pressure was turned on, and the hose could not expand any further.  If the other end was wide open, the flow at 10 minutes would be exactly the same as it was the moment the hose was turned on.  Taking the 10 minute flow divided by the 1-minute flow would give a ratio of 1.0.  In an electrical winding system, if there is a large amount of contamination, or a major breach in the insulation system, the leakage current would be large compared to the inrush charging current, and the ratio of the 10-minute resistance divided by the 1-minute resistance would be close to 1.0.

In a winding system, most OEM’s recommend a PI value of 1.25 or higher on a rotor winding, and 2.0 or better on a stator winding.  The reason for the difference lies in the fact that most generator rotors are an “open” insulation system, with naked conductors in the end winding potion and thus it is expected to tolerate lower insulation and P.I. values.  In stators, the voltages are far higher, and the insulation system is “closed”, that is, the entire length of the copper windings are entirely enclosed in insulation and thus we would expect higher insulation values.

 The polarization index is calculated as 

    \[PI  = \frac{R_{10}}{R_1}\]

where R10=the ten minute resistance reading, and R1 is the 1-minute reading.

In a perfect world, all winding systems would exhibit both high PI & high resistance values.  But we don’t live in a perfect world—we encounter units that are very old, facing demanding operating conditions, heavy contamination, and environmental effects such as humidity and intrusion of salt mist for seaside units, or industrial contamination such as sulfides or heavy metals for units in caustic environments.  Surface contamination and humidity are the most common causes of low resistance and/or PI values.  If we encounter low test values, the first step is to inspect the system and clean up any areas that appear to have contamination.  Low PI values are most often corrected by cleaning, followed by the application of dry heat for 12-36 hours.  Based on our experience, 95% of units will drastically improve after cleaning and dry out.  Units with electrical defects in the ground wall insulation will usually not improve after these efforts.  In these cases, further investigation is required.

As is often the case, marginal readings where the PI is good, and the Insulation Resistance is lower than we would like to see, we have OEM guidance which helps us make a tradeoff between the two factors.  For example, a unit has 2 gig-ohm resistance, but a low PI of 1.05, we could make the determination that the unit was suitable for service due to the excellent IR values and attribute the low PI to humidity.  Conversely, a resistance value of 25 meg-ohms and a PI of 2.6 would also constitute a unit that is safe to return to service.  In either of these cases, we would recommend performing additional testing to determine the nature of the defect and provide a plan for improving these readings.  Virtually anyone can perform the test; but it takes a highly trained and experienced operator to interpret the data and make the right call on what to do if the readings are less-than-perfect!

Sidewinders takes many factors into consideration, including but not limited to:

  • The OEM of the unit
  • The cooling technology (air-, hydrogen-, or water cooled)
  • The unit vintage
  • Plant-specific factors including environment, temperature, humidity, and altitude

The megger and PI test is one example of how Sidewinders’ experience and technical acumen can be instrumental in helping you ensure the maximum reliability of your generating assets.


Insulation Resistance and Polarization Index Testing of Generators




Case Study:
Repair of a Generator Stator with signs of
extensive end winding vibration and degradation.


During a major inspection of a 55375 KVA Generator, signs of extreme winding vibration and resonance issues were apparent.

After completing a visual inspection of the end winding
support systems, the following issues were identified and documented:

  1. Extensive greasing and dusting on coil-to-coil ties, circuit ring to gunstock ties and circuit ring blocking
  2. Extensive greasing and dusting of stator bars where they exited out of the slot
  3. Broken and loose coil-to-coils ties
  4. Cracked and, in some cases, broken gunstocks
  5. Indications that circuit ring ties had cut into the insulation on the circuit rings

Read more

Case Study: A Complex Repair on an Aged 6.6kV Hydro Generator 1


Sidewinders’ case studies help you understand the performance, quality and value of our services.


During recent annual maintenance of a vertical hydro generator, a stator coil was damaged as the rotor was being removed by the client.   Damage to the coil was extensive, with bare copper exposed where it had been hit.

Due to the utility company going through a large transition period, Sidewinders’ was unable to visually check the damage and complete electrical testing until the unit was completely reassembled—only four days away from its scheduled startup date.

In cases like this, the utility company generally has several options for a repair based on the outcome of the visual inspections, electrical testing and whether or not spare coils are available. The available options are:

  1. Localized repair of the damaged area
  2. Cutting and removing the coil from the circuit
  3. Half coil splice
  4. Rewind

Read more