Testing insulation resistance is an important practice that ensures the reliable operation of electrical wires and motors. Megohmmeters serve as a valuable tool to quickly assess the condition of insulation on various components, including wires, generators, and motor windings. This electric meter effectively measures extremely high resistance values by sending a high voltage signal through the object under test. However, it’s essential to understand that this is the primary function of a megohmmeter.

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While many people may casually use the term insulation tester to refer to megohmmeters, this is somewhat misleading. You might wonder why there is a distinction between the two. The main difference is that an insulation tester performs the primary function of a megohmmeter—measuring high resistance values using a high voltage signal—but it also offers a broader range of capabilities, such as more sophisticated testing and data recording features.

A comprehensive insulation tester can conduct high-voltage insulation resistance tests along with additional functions that improve testing accuracy and effectiveness.

What Makes Insulation Testers Different

One significant distinction is that insulation testers, unlike megohmmeters, can also provide measurements of voltage and current. For instance, the Fluke FC insulation multimeter can conduct insulation tests up to volts and functions as a full-featured digital multimeter, capable of generating up to volts for these tests.

Additionally, insulation testers can execute complex evaluations by compensating for changing environmental conditions, such as humidity and temperature, during tests. This compensation is crucial as it provides insight into motor performance under various conditions. Since ambient conditions or chemical pollutants can expedite insulation degradation, it’s vital to compare test outcomes that account for differing conditions.

Tools like the Fluke FC and Fluke C FC also offer memory storage through Fluke Connect®, enabling users to save measurements to their phone or the cloud. This not only saves time but also reduces errors and helps in tracking data more effectively over time.

The decision to choose between an insulation tester and a megohmmeter should be aligned with your business requirements. If basic insulation testing suffices for your needs, a megger may be adequate. However, for those requiring enhanced functionalities, safety, and efficiency, an insulation tester is likely the preferred option.

Fluke Insulation Resistance Meters and Testers

Hello everyone, as indicated in the title of this piece, I will address this topic in an understandable manner. I've received various questions regarding this subject from colleagues on LinkedIn and at work, prompting me to create this document. My insights are based on valuable resources that I've discovered online, and I hope you'll find this post informative as well.

So, what is the IR & PI test of generators and motors?

The insulation resistance and polarization index tests for electric motors or generators are essential tools for assessing the condition of their windings—particularly concerning dirt accumulation and moisture, as well as insulation deterioration.

IEEE Standard No. 43 covers the procedures for determining insulation resistance and polarization index in detail. This paper aims to provide a concise guide for conducting these tests.

The insulation system is vital for preventing winding faults. The stator winding is typically designed for a service life of around 20 to 40 years, contingent upon its operating conditions. In practice, insulation is made from organic compounds that include water in their chemical composition. Excessive temperature can lead to dehydration and oxidation, making insulation brittle and susceptible to damage from vibration.

Given that the lifespan of a generator or motor heavily relies on insulation integrity, it is crucial to assess this condition regularly. Insulation Resistance (IR) and Polarization Index (PI) are two widely accepted diagnostic tests for insulation, both of which have been in use for over 75 years.

The IR test measures the resistance of electrical insulation between copper conductors and the core of the stator or rotor. Ideally, this resistance should be infinite, as insulation's purpose is to block current flow between the copper and the core. In practical scenarios, though, achieving an infinite resistance value is impossible. Nonetheless, high resistance values are necessary to prevent significant leakage current. A lower IR value typically indicates insulation deterioration.

The Polarization Index (PI) elaborates on the IR test. It represents the ratio of the IR measured after voltage has been applied for 10 minutes (R10) to the IR measured after one minute (R1), calculated as follows:

PI = R10/R1

A low PI value suggests potential contamination from oil, dirt, or moisture. In testing, a relatively high DC voltage is applied between the copper conductor and stator or rotor core, usually between the winding and ground (as the machine core and body are grounded). Following Ohm's law, the IR (Rt) at time t is:

R1 = V/It

Where V is the applied DC voltage and It is the current flowing in the circuit.

The behavior of the insulation current varies over time. The purpose of measuring PI can be understood through the flow of different currents, namely:

1. Capacitive Current (Ic)
2. Conduction Current (IR)
3. Surface Leakage Current (IL)
4. Polarization Current (IP)

1. Capacitive Current

When a DC voltage is applied, insulators behave like capacitors, initially allowing high charging current to flow before it exponentially decays. This capacitance and the internal resistance of the voltage supply, typically several hundred kilo ohms, impact the decay of current. In generator or motor windings, the capacitive current usually decays to zero in less than 10 seconds. Therefore, the initial IR measurement is taken once the capacitive current has diminished, ensuring accurate calculations.

2. Conduction Current

This current results from the movement of electrons between the copper and the core, specifically through ground insulation that can absorb moisture, particularly in older thermoplastic systems. A constant conduction current arises when there are cracks, cuts, or pinholes in the insulation. With modern insulation, conduction current remains negligible unless there are visible defects.

3. Surface Leakage Current

This DC current flows over the insulation's surface, typically due to conductive contaminants (such as a mixture of oil or moisture with dust and dirt) on the windings' surface.

4. Polarization Current

Electrical insulation has hygroscopic properties, meaning moisture absorption varies. Water molecules are highly polar. When an electric field is applied across the insulation, they align with the field, allowing current to flow. The energy for this polarization comes from the current in the applied DC test voltage. Eventually, when the polarization process is complete, the current will stop—typically taking around 10 minutes, which is why IR is measured after that duration.

The total current is the sum of the aforementioned currents, and the calculations show:

It = Ic + IR + IL + IP

Initially, after one minute, Ic is zero, leading to:

It(1 minute) = IR + IL + IP

After ten minutes, IP is also zero, thus:

It(10 minutes) = IR + IL

PI = (IR + IL + IP)/(IR + IL) = R10/R1

Effect of Temperature on IR

Some may believe that measuring IR after one minute is sufficient to assess insulation condition and determine if it has deteriorated. However, measuring IR alone has proven unreliable over time because IR values are significantly influenced by temperature. An increase in temperature of 100°C can lower IR by a factor of 5 to 10. When plotting temperature against insulation resistance, a curved characteristic line emerges. However, using logarithmic scales, this can appear linear. Temperature affects insulation differently across materials and contaminants. Although IEEE-43 offers some temperature correction curves and formulas, they are deemed unreliable when extrapolated over 100°C. The result is a variance in reported IR with temperature changes, making it impractical to establish a scientifically accepted IR value at varying temperatures.

Importance of PI

The PI calculation was developed to lessen the impact of temperature on test interpretation. Since PI represents the ratio of two IR measurements taken at different times, temperature changes are negligible throughout the 10-minute testing period. Consequently, both R10 and R1 are usually measured at a consistent winding temperature, meaning the temperature correction factor remains constant and can be disregarded in PI calculations. Additionally, PI utilizes polarization current as a benchmark to determine any excessive leakage and conduction currents. If the latter currents greatly exceed the polarization current, PI will approximate one, indicating potential electrical tracking issues. Conversely, a PI greater than 2 indicates low leakage and conduction currents, suggesting minimal risk of electrical tracking problems.

Test Method

1. Instrument

IR is measured using a "mega-ohmmeter," often referred to by the brand name "Megger," which was the pioneering instrument for these tests. This device generates a controlled DC supply and measures current flow, allowing the operating resistance value to be read directly from its dial. Various types of mega-ohmmeters (motorized and hand-driven) are available. For constant and stable DC voltage outputs, a motorized meter is preferred. Modern mega-ohmmeters can apply voltages over 10 KV and measure resistance exceeding 100 GΩ.

2. Test Voltage

The test voltage should remain well below the rated peak line-to-ground voltage of the winding, as this isn't a "high-potential" test. The voltage should be sufficient to identify defects like cuts in the insulation. Recommended DC test voltages are as follows:

Rated voltage (line-to-line) of the winding in volts / Test voltage (DC) in volt

<600 / 500

600- / 500-

Link to Hengfeng

3. Test Procedure

IR and PI tests should be conducted simultaneously. If the winding temperature exceeds the dew point, heat can be applied to eliminate any moisture condensation. However, if the temperature is lower than the dew point, adjustments cannot be made. The test procedure is straightforward, as follows:

• Remove all external connections to the machine and fully discharge the windings to the grounded machine frame.
• Apply the required DC voltage across the winding and ground, utilizing a direct-indicating, motor-driven megohmmeter.
• Maintain the voltage application for 10 minutes.
• Measure insulation resistance at the 1-minute and 10-minute marks. Disconnect the supply to the meter and fully discharge the windings to the grounded machine frame.
• Calculate the polarization index by dividing the insulation resistance at the 10-minute mark by that at the 1-minute mark.
• Document the winding temperature.
• If testing is performed on only one winding of three-phase equipment, ensure the other windings are grounded during testing.

Interpreting Results

1. There are no recognized guidelines for determining the minimum acceptable IR value at which a machine can operate safely. However, standard IR values have been suggested; the minimum IR measured after one minute at 40°C is among the recommended thresholds for various winding types.
2. If the IR falls below these recommended values, do not expose the winding to high-potential tests or service, as this may lead to failure. Yet, if historical data indicates a consistently low IR for a specific winding, the machine may be safely returned to service with a lowered risk profile.

PI interpretation follows a similar logic. If either IR or PI falls below the set threshold for modern stator windings, this may indicate the winding has absorbed contaminants or moisture.

As a point of reference, let’s consider General Electric Power (GE), a significant manufacturer of generators and motors. For their 324H Gas cooler GEN, rated at 15.75 KV and 360 MVA for Gas Turbine applications, GE recommends three mandatory preservation procedures based on particular documents.

AVAILABLE TESTS

Multiple methods exist for testing the insulation system’s dielectric strength to confirm its service suitability. Each method entails applying voltage across the insulation ground wall, differing primarily in voltage levels and interpreting results. The available test categories are:

A. Proof Tests

B. Analytical Tests (Recommended)

Insulation resistance and dielectric absorption (Megger) DC leakage.

C. Analytical Tests (Customer Option)

Dissipation factor
Radio noise, slot discharge
Corona start voltage

The B category applies to our current discussion.

Analytical Tests (Recommended)

A. Insulation Resistance
This test is the simplest and most accessible way to assess insulation integrity. It utilizes a Megger, available in handheld, motorized, or electronic versions. One version of these tools is typically found in testing labs. The device generates a constant internal voltage with a high degree of internal resistance, yielding a reading calibrated in megohms that indicates the presence or absence of conduction paths through or around the insulation.

Typically, epoxy-mica insulation systems effectively prevent conduction through the insulation, though water-cooled windings can introduce a path to ground that greatly impacts resistance readings. After performing continuity checks in the factory, water is drained and windings dried under vacuum to ensure accurate insulation resistance readings. Typically, a 5,000 VDC Megger is recommended for these assessments, especially for new machines where drying is seldom required.

B. DC Leakage
This measurement, performed with a variable voltage test set, applies and measures leakage current at series voltage increments. Although this requires more complex gear than the Megger, it offers deeper insight into insulation conditions. Such testing has seen extensive use in older electrical systems, especially those prone to moisture absorption and humidity fluctuations.

The components commonly tested include the generator stator, with attention less frequently directed toward the rotating field. However, as modern generators grow in size, the economic risk points to greater necessity for thorough field testing, potentially enhancing reliability for both field windings and excitation equipment.

Insulation conditions are sensitive to temperature changes, reflected in readings taken by a Megger or equivalent equipment. Therefore, for dependable results, data should be compared at consistent temperatures.

Finally, appreciate that any voltage application intended for measuring winding conditions relies on clean, dry windings to yield the best data. Debris or moisture can skew results significantly.

Thank you for reading this article.

If you're interested in exploring more about insulation resistance tester for generators, feel free to reach out to us for expert consultation!