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Fault Zone Analysis “INSULATION” Presented by: David Almand at the 2004 Motor Reliability Technical Conference Company: PdMA Corporation Department: Training & Technical Support INTRODUCTION The importance of sound electrical insulation systems has been acknowledged from the early days of electricity. As the years passed and the electrical industry expanded, the need for improved electrical insulation system testing became even more significant. The designs and applications of electrical equipment are almost infinite in their variety, but all units have one common characteristic. For electrical equipment to operate properly, one of the most important characteristics is that the flow of electricity takes place along well-defined paths or circuits. These paths are normally limited to conductors, either internal or external to the electrical component. It is important that the flow of current be confined; not leaking from one path to another through material not intended to be a conducting path. Deterioration of insulation systems can result in an unsafe situation for personnel exposed to the leakage current. Ensuring that the insulation system is confining the flow of electrical current to the intended conducting path insures that personnel coming into contact with the insulation are not at risk of becoming a lower resistance conductor path for current-to-ground than the intended path. Just as the walls of a pipe contain the flow of fluid, the insulation surrounding a conductor confines the flow of electric current. The walls of a pipe may have impurities, cracks, or other defects that limit its ability to withstand the pressure of the fluid. The insulation may develop impurities, cracks, or other defects that limit its ability to withstand electrical potential, which is the force or voltage that drives the flow of electrons, we call current, through the electrical circuit. Despite great strides in electrical equipment design in recent years, the weak link in the chain is still the insulation system. When electrical equipment fails, more often than not the fault can be traced to defective insulation. Even though an electric motor is properly designed and tested prior to installation, there can be no guarantee that a fault in the insulation will not occur at some time in the future. Many outside influences affect the life of electrical insulation systems. Outside influences include contamination of the insulation surfaces with chemicals from the surrounding atmosphere that attack and destroy the molecular structure, physical damage due to improper handling or accidental shock, vibration, and excessive heat from nearby industrial processes. Voltage transients in the conductors inside the insulation, such as surges or spikes caused by variable frequency drives, can lower the dielectric strength to the point of failure. The deterioration occurs in many ways and in many places at the same time. For example, as chemicals and/or heat change the molecular structure of the insulating materials, they become conductive, allowing more current to be forced through them by voltage resulting in leakage current. Properly conducted insulation system testing, analysis of the data collected, and appropriate corrective action can minimize the possibility of failures. Therefore, the significance of understanding insulation system testing has never been more important. OBJECTIVES To describe the recommended practices for measuring insulation resistance in rotating machines. The leading industry governing insulation testing is the IEEE 43-2000 standard (The Institute of Electrical and Engineers, Inc.). Explain how the IEEE 43-2000 standard correlates to the MCE tester. SAFETY IEEE 43-2000 claims that before any insulation testing can take place, safety must be addressed. It is not safe to begin testing until the discharge current is negligible and the there is no discernable return 1

Transcript of Fault Zone Analysis “INSULATION” - · PDF fileFault Zone Analysis...

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Fault Zone Analysis “INSULATION” Presented by: David Almand at the 2004 Motor Reliability Technical Conference Company: PdMA Corporation Department: Training & Technical Support INTRODUCTION The importance of sound electrical insulation systems has been acknowledged from the early days of electricity. As the years passed and the electrical industry expanded, the need for improved electrical insulation system testing became even more significant. The designs and applications of electrical equipment are almost infinite in their variety, but all units have one common characteristic. For electrical equipment to operate properly, one of the most important characteristics is that the flow of electricity takes place along well-defined paths or circuits. These paths are normally limited to conductors, either internal or external to the electrical component. It is important that the flow of current be confined; not leaking from one path to another through material not intended to be a conducting path. Deterioration of insulation systems can result in an unsafe situation for personnel exposed to the leakage current. Ensuring that the insulation system is confining the flow of electrical current to the intended conducting path insures that personnel coming into contact with the insulation are not at risk of becoming a lower resistance conductor path for current-to-ground than the intended path. Just as the walls of a pipe contain the flow of fluid, the insulation surrounding a conductor confines the flow of electric current. The walls of a pipe may have impurities, cracks, or other defects that limit its ability to withstand the pressure of the fluid. The insulation may develop impurities, cracks, or other defects that limit its ability to withstand electrical potential, which is the force or voltage that drives the flow of electrons, we call current, through the electrical circuit. Despite great strides in electrical equipment design in recent years, the weak link in the chain is still the insulation system. When electrical equipment fails, more often than not the fault can be traced to defective insulation. Even though an electric motor is properly designed and tested prior to installation, there can be no guarantee that a fault in the insulation will not occur at some time in the future. Many outside influences affect the life of electrical insulation systems. Outside influences include contamination of the insulation surfaces with chemicals from the surrounding atmosphere that attack and destroy the molecular structure, physical damage due to improper handling or accidental shock, vibration, and excessive heat from nearby industrial processes. Voltage transients in the conductors inside the insulation, such as surges or spikes caused by variable frequency drives, can lower the dielectric strength to the point of failure. The deterioration occurs in many ways and in many places at the same time. For example, as chemicals and/or heat change the molecular structure of the insulating materials, they become conductive, allowing more current to be forced through them by voltage resulting in leakage current. Properly conducted insulation system testing, analysis of the data collected, and appropriate corrective action can minimize the possibility of failures. Therefore, the significance of understanding insulation system testing has never been more important. OBJECTIVES

• To describe the recommended practices for measuring insulation resistance in rotating machines.

• The leading industry governing insulation testing is the IEEE 43-2000 standard (The Institute of Electrical and Engineers, Inc.). Explain how the IEEE 43-2000 standard correlates to the MCE tester.

SAFETY IEEE 43-2000 claims that before any insulation testing can take place, safety must be addressed. It is not safe to begin testing until the discharge current is negligible and the there is no discernable return

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voltage. The return voltage should be less than 20V after the ground is removed. The MCE (Motor Circuit Evaluation) tester checks for this voltage before allowing testing. In order to start testing, there needs to be less than 15V line-to-neutral voltage before the test will begin. See Figure 1.

Figure 1

Also for testing at 5000V, the lead between the tester and the winding must be appropriately insulated and spaced from ground, otherwise, surface leakage currents and corona loss may introduce errors in the test data. DEFINITIONS By definition, the insulation resistance is made up of the applied direct voltage across the insulation divided by the total resultant current. The total current is the sum of four different currents: surface leakage, geometric capacitance, conductance, and absorption. The geometric capacitance current is a reversible component of the measured current on charge or discharge that is due to the geometric capacitance. That is the capacitance as measured with alternating current of power or higher frequencies. With direct voltage this current has a very short time constant and does not effect affect the usual measurement. The conduction current in well-bonded polyester and epoxy-mica insulation systems is essentially zero unless the insulation has become saturated with moisture. Older insulation systems, such as asphaltic-mica or shellac mica-folium may have a natural and higher conduction due to the conductivity of the tapes used back of the mica. The surface leakage current is constant over time. Moisture or some other type of partially conductive contamination present in the machine causes a high surface leakage current, i.e., low insulation resistance. The absorption current is made of two components the polarization of the insulation material and the second is due to the gradual drift of electrons and ions through the insulating material. The polarization current is cased by the reorientation of the insulating material. This material, usually epoxy, polyester, or asphalt tends to change the orientation of their molecules when in the presence of direct electric field. It normally takes a few minutes of applied voltage for the molecules to have been reoriented, and thus for the current-supplied polarizing energy to be reduced to almost zero. The absorption current, which is the second component, is the gradual drift of electrons and ions through the insulating material. These electron and ions drift until they become trapped at the mica surfaces usually found in rotating insulation systems. See Figure 2.

Figure 2

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Section A of Figure 2 shows the random orientation of the insulation’s molecules. As a direct voltage is applied via the MCE tester, the molecules start to polarize and align, see Section B of Figure 2. The energy required to align the molecules, and subsequently reduce the amount of escaping molecules, is known as absorption current. Since absorption current is a property of the insulation material and the winding temperature, a specific absorption current is neither good nor bad. The absorption currents will vary from different insulating material. Prior to 1970, older thermoplastic materials used were typically asphalt or shellac has a higher absorption current. After 1970, the shift was made to thermalsetting polyester or epoxy bonded insulating material, whish significantly decreased the absorption current. Nonetheless, this doesn’t mean that the more modern insulating materials are better because they have less absorption current. The MCE tester measures the sum of all the currents, also called total current and calculates the resistance-to-ground values. This is represented in Figure 3.

Figure 3

THEORY Contamination and moisture affect the insulation resistance or total current. Total current is the sum of all of the currents added together. If the windings are wet or contaminated, the total current will be much larger than the absorption current. See Figure 4.

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Figure 4

Legend: Total current Leakage current Absorption current Conductance current Capacitance current If the windings are cleaned or dried, the total current will decrease with time, since the total current is dominated by the absorption current. This will result in less total current loss and higher resistance-to-ground values. See Figure 5.

Figure 5

The amount of applied voltage must be appropriate to the nameplate voltage and the basic insulation condition. This is particularly important in small, low-voltage machines where there is only a single layer of insulation. If test voltages are too high, the applied voltage may over stress the insulation. The MCEMAX tester has a low current output. The maximum current output is 1 milliamp, or 1000 microamps,

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but normally the tester only outputs in the microamp range. This limits the likelihood of insulation damage. See Table 1 for recommended voltage application.

Table 1

Guidelines for DC Voltages to be Applied During Insulation Resistance Test Winding rated voltage (v) Insulation resistance test direct voltage

< 1000 500 1000 – 2500 500 – 1000 2501 – 5000 1000 – 2500

5001 – 12,000 2500 – 5000 > 12,000 5000 – 10,000

Ref. IEEE Std. 43-2000 A Rated line-to-line voltage for 3 phase ac machines, line-to-ground voltage for 1-phase machines, and rated direct voltage for dc machines or field windings. Even after the voltage has been removed from the motor, there can still be a discharge current. This discharge current is made up of two components:

1) A capacitive discharge current component, which decays nearly instantaneously, depending upon discharge resistance of the motor and test set.

2) The absorption discharge current, which will decay from a high initial value to nearly

zero with same characteristics as the initial charging current but with the opposite polarity. This decay may take more than 30 minutes depending on the insulation type and machine size of the tester. This is also when the polarization of the insulation molecules slowly become scrambled again. A random alignment of the molecules is the natural state for the insulation.

IEEE has a thumb rule to protect the technician as well as not to adversely effect subsequent testing. The grounding time should be a minimum of four times the charge time. Therefore, if a DC voltage is applied to the insulation for one minute, and minimal of four minutes should elapse before the next test is started. EFFECTS FROM CONTAMINATION There are many factors that can affect insulation resistance. The surface leakage current is dependant upon foreign matter, such as oil and carbon dust on the winding surfaces outside the stator slot. The surface leakage current may be significantly higher on large turbine generator rotors and DC machines, which have relatively large exposed creppage surfaces. Dust and salts on insulation surfaces, which are ordinarily nonconductive when dry, may become partially conductive when exposed to moisture or oil, and this will cause increased surface leakage current and lower insulation resistance. The reason a motor’s capacitance increases with contamination is because of how a capacitor works. Any two conducting materials, called plates, separated from each other by a dielectric material, form a capacitor. A dielectric material is anything that is unable to conduct direct electric current. A cable or motor winding surrounded by insulation provides one conductor and the dielectric material. The second plate is formed by the stator core and motor casing iron. It is this second plate that is increased in plate size as contamination builds up. See Figures 6 and 7.

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Figure 6 Figure 7

EFFECTS OF TEMPERATURE A higher temperature affects the resistance of both the insulation and conductor. There is a term called temperature coefficient (KT). A material has either a positive or negative KT. If the material has a positive KT, then with added heat the resistance readings will increase. Inversely, if a material has a negative KT, then the resistance readings will decrease with higher temperature. In metals, i.e., the magnetic wire of the stator, higher temperature introduces greater thermal agitation and reduces the movements of free electrons. Because of this reduction in free movement, the resistance readings will increase with added heat and therefore the conductor has a positive KT. However, in insulation, the added heat supplies thermal energy, which frees additional charge carriers and reduces the resistance reading. Therefore, an increase in temperature on insulation reduces the resistance and it is said to have a negative KT. This higher temperature affects every current except the geometric capacitive current. The recommended method of obtaining data for an insulation resistance versus winding temperature curve is by making measurements at several winding temperatures, all above the dew point, and plotting the results on a semi-logarithmic scale. A semi-logarithmic scale is the same type of scale used to track earthquake activity, called the Richter scale. Earthquake behavior needs to be graphed so that small tremors, as well as a magnitude 9, are still visible on a chart. Nonetheless, the results should plot out to be a straight line. Since this type of temperature coefficient plotting is usually not feasible, IEEE has developed a corrective rule of thumb. This standard states that to avoid the effects of temperature in trend analysis, subsequent tests should be conducted when the winding is near the same temperature as the previous test. Otherwise the insulation test values are corrected to a common base temperature of 40°C. Therefore, all RTG, resistance-to-ground, readings must be temperature corrected for trending and comparison purposes. Temperature correction of the reading is required because the temperature of the insulation system under test may vary depending on operating conditions prior to testing, atmospheric conditions, or ambient temperature. Insulation material has a negative temperature coefficient which means that the resistance characteristics vary inversely with temperature. In the test setup screen of a Standard test, the temperature of the windings is imputed. The Measured Mohm value is then adjusted to a temperature correction to 40°C. The result is the Corrected Mohm. See Figure 8.

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Figure 8 EFFECTS FROM MOISTURE No matter how clean the windings are, if the winding temperature is at or below dew point, a film of moisture may form on the insulation surface, which will lower the insulation resistance. The effects are more pronounced if the windings have contamination present. This lower than normal RTG reading is even truer when dealing with older types of insulations. The older types of insulation, i.e., asphaltic-mica and shellac mica-folium, have great tendencies to draw moisture into the body of the insulation. The absorbed moisture increases the conduction current (IG) and significantly reduces insulation resistance. Motors out of service that don’t have space heaters, and are tested with windings below the dew point will experience lower resistance readings. It may be necessary to dry out the machines before returning them to service. One method for drying out the windings is to surround the machine in plastic and place a light bulb in the enclosure. An even more involved method uses a dehumidifier to suck moisture out of the air. When the temperature falls below the dew point level, it is hard to determine the amount of moisture buildup on the windings. Therefore, using the 40°C temperature correction still may deliver errors in the Corrected Mohm readings. In such cases, it is recommended that the weather conditions be part of the insulation testing tracking. The ambient temperature, as well as the dew point, can be added in the notes for future trending. Dew point indicates the amount of moisture in the air. The higher the dew point, the higher the moisture content of the air at a given temperature. Dew point temperature is defined as the temperature to which the air would have to cool (at constant pressure and constant water vapor content) in order to reach

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saturation. A state of saturation exists when the air is holding the maximum amount of water vapor possible at the existing temperature and pressure. Dew point temperature for a region/state can be found at http://www.wunderground.com. MCE TECHNOLOGIES The MCE tester has technologies used to analyze an insulation system. The MCE is a predictive maintenance technology, which provides comprehensive, portable motor testing. The MCE tester allows you to test deenergized electric motors, large or small, AC or DC, and determine the condition of the motor. There are four MCE tests that will evaluate the condition of an insulation system. The Standard test, with the RTG and CTG values, Dielectric Absorption (DA), Polarization Index (PI), and the Step-Voltage tests. Standard Test As mentioned earlier, the two values from the Standard test used to evaluate insulation integrity are the RTG and CTG readings. RTG Reading To accurately trend RTG for one motor over time, keep the test voltage and duration of applied voltage constant. The only other factor that will affect the RTG reading is temperature, and therefore it is necessary to correct the temperature in the test setup screen. IEEE recommended applied voltages can be seen in Table 2.

Table 2

Guidelines for DC Voltages to be Applied During Insulation Resistance Test Winding rated voltage (v) Insulation resistance test direct voltage

< 1000 500 1000 – 2500 500 – 1000 2501 – 5000 1000 – 2500

5001 – 12,000 2500 – 5000 > 12,000 5000 – 10,000

Ref. IEEE Std. 43-2000 A Rated line-to-line voltage for 3 phase ac machines, line-to-ground voltage for 1-phase machines, and rated direct voltage for dc machines or field windings.

The temperature corrected megohm readings should be recorded and graphed for comparison over time. If a downward trend is observed, look for dirt or moisture. A single reading will not have much meaning in regards to the overall health of the insulation system; a reading as low as 5 megohms may be acceptable if related to a low voltage application. See Table 3 for recommended minimum insulation resistance.

Table 3

Recommended Minimum Insulation Resistance Values at 40°C (All values in Mohm)

IR1 min = kV + 1 For most windings made before about 1970, all field windings, and others not described below

IR1 min = 100 For most DC armatures and AC windings built after 1970 (form wound coils)

IR 1 min = 5 For most machines with random wound stator coils and form wound coils rated below 1kV

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Recommended action based on trending and RTG results can be found on Table 4.

Table 4

RTG Motor Condition Recommended Actions Stable trend, comparatively* high value

Good; maximum margin against leakage current

Monitor motor on current schedule.

Downward trend Observe; condition should be trended more often.

Monitor motor more frequently.

Downward trend, reading in caution condition, comparatively* low value.

Caution; path for current leakage to ground is developing

A problem may be developing with moisture or dirt buildup in the motor’s insulation system. Also, check the cable for moisture. Schedule cleaning and inspection. Isolate fault to motor or power circuit. Monitor more often.

Downward trend, reading in alarm condition, comparatively* low value.

Severe; path for current leakage to ground exists.

Isolate ground fault. Correlate with CTG readings. Troubleshoot and repair / replace prior to returning to service.

* Comparatively means compared to an identical motor in a similar environment. CTG Reading During the CTG measurement, the MCE tester applies an AC potential between phase 1 and the ground lead. Circuit capacitance of the insulation system is measured. This capacitance value reflects the cleanliness of the windings and cables. A buildup of contamination on the surface of the windings and cables results in higher capacitance readings. With a build up of contamination on the insulation surface, dirty windings and cables produce higher capacitance values than clean ones do. Over time, CTG values steadily increased indicating an accumulation of dirt and that cleaning is necessary. This can be correlated with a lower RTG and higher CTG values. See Figure 9 for an example.

Figure 9

This is important because dirt and contamination reduce the motor’s ability to dissipate heat generated by its own operation, resulting in premature aging of the insulation system. A general rule of thumb is that a motor’s life decreases by 50% for every 10°C increase in operating temperature above the design temperature of the insulation system. Heat raises the resistance of conductor materials and breaks down

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the insulation. These factors accelerate the development of cracks in the insulation, providing paths for unwanted current to flow to ground. The effects from temperature to insulation resistance can be seen on Figure 10. Temperature Conversion

oC oF 40 104 35 95 30 86 25 77 20 68 15 59 10 50 5 41 0 32

oC = 5/9 (oF – 32)

oF = (9/5 X oC) + 32 Figure 10

Normally, when the outside of the insulation is clean and dry, it is a good insulator. When dirt, moisture, and other contaminates begin to cover the stator windings, they cause the outer insulation surface areas to become conductive. Since this surface is in contact with the ground, it allows an AC current path to ground. Cables in the power circuit are also subjected to the same effect, when moisture penetrates the outer casing. The cleanliness of the windings and cables can be determined by looking at the CTG value within the Standard test. CTG is a function of many factors. For example, CTG is influenced by the design of each individual motor, the length of the cable between the MCE and the motor, the type of insulation on the cables and motor windings, and the number and type of connectors in the circuit. Therefore, a comparison of several CTG values is more revealing of a motor’s condition than the analysis of a single CTG value is. CTG must be analyzed by trending readings on the same motor or by comparing values taken on similar motors, with similar histories, operating under the same conditions. Polarization Index and Dielectric Absorption The Polarization Index (PI) and Dielectric Absorption (DA) are tests performed by the MCE on a deenergized motor. During the PI and DA tests the MCE tester applies a DC potential between the phase 1 and the ground leads for a pre-determined amount of time, ten minutes for the PI test and 30 seconds for the DA test. During the test, RTG readings are taken every second. Every five seconds the average of the previous five readings is plotted on the RTG (megohms) versus time (seconds) display. It is not necessary to perform a DA test if you are performing a PI test. When you perform a PI test, WinVis automatically saves the first minute as a DA test and the entire ten minutes as a PI test. When the test is complete, ratios for PI and DA are calculated. Taking the RTG reading at ten minutes and dividing it by the one minute RTG reading calculates the PI ratio. Taking the RTG reading at one minute and dividing it by the 30 second reading calculates the DA ratio.

PI ratio = 10 min Β 1 min DA ratio = 1 min Β 30 sec

The purpose of the PI test is to determine whether or not a motor’s insulation system is suitable for operation. The PI test is not limited to AC Induction motors only. It also applies to wound rotor motors,

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salient pole machines, and certain DC fields. The DC field would have to have conductors that are fully encapsulated in insulation. Therefore, the PI test can be a worthwhile test for multiple type machines. Total current is what is measured and is the sum off all four components; capacitive current, absorption current, conduction current, and surface leakage current. See Figure 3. Absorption (polarization) Current (IA) is a current resulting from molecular polarization and electron drift, which decays from time of voltage application at a decreasing rate from a comparatively high initial value to nearly zero, and depends on the type and condition of a bonding material used in the insulation system. Conduction current (IG) is a current that is constant in time that passes through the bulk insulation from the grounded surface to the high-voltage conductor, and depends on the type of bonding material used in the insulation system. Surface leakage current (IL) is a current that is constant with time and which usually exists over the surface of the end-turns of the stator or between exposed conductors and the rotor body in insulated rotor windings. The magnitude of the surface leakage current is dependant upon temperature and the amount of conductive material, i.e., moisture or contamination on the surface of the insulation. Geometric capacitive current (IC) is a reversible current of comparatively high magnitude and short duration, which decays exponentially from time of voltage application and which depends on the internal resistance of the measuring instrument and the geometric capacitance of the winding. Polarization Index Correction When performing a PI test, it is not necessary to temperature correct. Since the machine temperature doesn’t change appreciably between the one-minute and the ten-minute readings, the effect of temperature on the PI index is usually small. However, if the motor recently shut down and a PI test is performed, the results may be a substantial increase in insulation resistance. This would result in an unusually high PI, at which point additional testing should be performed once the windings have cooled to 40°C or lower. Measured resistance is determined by the voltage applied and the resulting current (R=E/I). A recommended minimal value for a PI ratio for most insulation classes is 2.0. Lower readings may indicate insulation damage. Table 5 displays the minimal PI ratio values per IEEE 43.

Table 5 Thermal Class Minimum PI ratio

Class A 1.5 Class B, F and H 2.0

Per IEEE, if the one-minute RTG reading is > than 5000 Mohms, the calculated PI ratio may not be meaningful. With the MCEMAX ability to read such high Mohm values, the total current measured can be in the submicroamp range. At this level of tester sensitivity, small changes in power supply output, ambient humidity, test connections, and other non-related components can greatly affect the total current measured during the ten-minute PI test. This sensitivity of the results justifies why it’s best to start a PI test and step away from the tester and the insulation system being tested. Since the PI ratio may not reflect the insulation system health if the one-minute reading is > 5000 Mohm in the first minute, a DA test may be the best choice. These high Mohm readings may be found on the newer insulation systems for low-voltage motors. These low-voltage motors are typical random wound motors with a single layer of insulation. The results from a single layer PI can be seen in Figure 11 and a picture of the insulation in Figure 13. With only one layer of insulation, it takes less time for the absorption current to align the insulation molecules resulting in higher Mohm reading in a shorter time. Therefore, it is sufficient to only perform a DA test on insulation systems like there. See Figure 12.

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Figure 11

Figure 12

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Figure 13

The PI test may still be the correct test on medium and high voltage motors that contain multiple layers of insulation, which takes longer for the Absorption current to polarize molecules through multiple layers of insulation. See Figure 14.

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Figure 14

Data Interpretation for PI and DA In Managing Motors Richard Nailen, P.E., offers the following guidelines shown in Table 6, for interpreting PI and DA ratios.

Table 6

Test Unacceptable Acceptable PI 1 to 1.5 2 to 4 DA < 1.25 > 1.50

Excellent results should contain a PI ratio of 2–4, achieve higher than minimal RTG, and should be a non-speratic rise in the megohm reading. See Figure 15.

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Figure 15

Erratic RTG values occurring at any time during the test is indicative of short-term current transients. These may be due to contamination or moisture. An important aspect is this situation is what level did the RTG values fall to. The IEEE minimum RTG value is 100 Megohm for form wound coils. The RTG values dip below the suggested minimum in Figure 16.

Figure 16

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According the EASA’s, Principles of Large AC Motors, it states that PI ratios of > 5 should be considered the result of dry or imbrittled insulation. This may be because of age of the insulation or operating the motor at higher than designed temperatures. See Figure 17.

Figure 17

PI and DA testing can be used for both a single go-no-go, based on the minimum RTG readings established by IEEE 43-2000. See Table 7.

Table 7

Recommended Minimum Insulation Resistance Values at 40°C (all values in Megohm)

IR1 min = kV + 1 For most windings made before about 1970, all field windings, and others not described below

IR1 min = 100 For most DC armatures and AC windings built after 1970 (form wound coils)

IR 1 min = 5

For most machines with random wound stator coils and form wound coils rated below 1kV

Values less than the recommended minimum can result in a degregated insulation system. Figure 18 shows a grounded motor.

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Figure 18

If the PI or DA ratio is low, the problem is in the circuit or the motor. Assuming that the first test was made at the MCC, run another test after disconnecting the “T” leads from the motor. If the low values are gone, the problem is in the cables between the motor and the MCC. If the low value still exists, the problem is in the motor. If the RTG value is low, isolate the problem to either the power circuit or the motor. Assuming the first test was made from the MCC, perform another test at the motor leads after disconnecting it from the power cables. If the RTG value is higher testing the motor, the fault is in the cables between the MCC and the motor. Check the connections in the motor connection box, look for moisture in the conduit and examine the cables. The cables may require cleaning, drying, or replacement. See Figure 19. If the RTG value at the motor connection box is still low, the fault is in the motor. If the value is in caution, the motor may need to be dried, cleaned in place, or removed for a clean, dip, and bake. If the value is in alarm, the motor may need to be rewound. If the RTG value is less than the IEEE minimum, look for a ground fault and clear this condition before starting the motor.

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Figure 19

Table 8 is the recommended actions from PI and DA values.

Table 8

DA Value PI Value Motor Condition Recommended Actions

> 1.5 > 2.0 Good Monitor motor on current schedule

1.25 – 1.5 1.5 – 2.0 Observe Monitor motor more frequently

1.0 – 1.25

1.0 – 1.5

Moderate

Warning levels are tentative and relative. Isolate to motor or cables. Increase monitoring frequency.

< 1

< 1

Severe

Warning levels are tentative and relative. Isolate to motor or cables. Clear ground fault prior to starting.

Step Voltage Test Step Voltage is a controlled overvoltage test in which the DC test voltage is increased in a series of uniform or graded steps at regular time intervals. The subsequent leakage current, in microamps, is recorded and graphed. This graph is examined for increases or other variations in leakage current versus applied voltage that are possible indications of insulation weakness. The Step Voltage test is a process of applying a DC test voltage for 60 seconds at a time, and recording the leakage current for a series of voltage step ups to a predetermined level of voltage. The level and steps of voltage applied and the amount of allowable leakage current are set prior to beginning the test. Maximum voltage applied during the test is normally well above the AC peak voltage. Moisture and dirt in

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the insulation are usually revealed at voltages far below those expected in service. The effects of aging or mechanical damage in fairly clean and dry insulation may not be revealed at such low voltage levels. When the voltage is increased in steps to produce electrical stresses, which approach or exceed those in service, local weak spots in the insulation will be observed in the insulation resistance. The use of controlled stepped or ramped voltage, offers certain advantages over proof-type acceptance testing. By observing measured current during the controlled application of voltage, variations in current versus applied voltage may be useful in diagnosing certain application defects and modes of deterioration. Controlled overvoltage tests may also afford the possibility of detecting impending insulation problems by recognizing abnormalities in the measured current response, thereby allowing the test to be discontinued prior to insulation failure. Data Interpretation for Step Voltage Test To minimize the effects of the changing current on the current measurement, the test voltage is held at each step allowing it to decay. It is not practical to hold it until the voltage completely decays. The curve of the plot of current versus voltage recorded by the MCE displayed in the left panel, should be nearly linear for a motor in good condition. The right panel reflects the voltage at each time interval. Notice in Figure 20 how there is great current decay and how the current vs. voltage graph is linear. This is an acceptable reading.

Figure 20

Figure 21 shows an insulation system breaking down with excessive leakage current once the tester increases the voltage to 3500V.

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Figure 21

CONCLUSION Compare the different MCE test results with each other and with other technologies. The more information you have to make a maintenance decision, the more confident you will be about your decision. Use all the technologies to maximize troubleshooting efforts. As other technologies indicate faults, use the MCE to track the changes in motor condition. Is there any single insulation system integrity test that will always reject a bad winding, but accept a good one? Unfortunately not, but this is not a reason to omit insulation system testing. More importantly, it is an argument for doing more testing, to compare the results of various procedures, and to make a reasonable judgment based on all of the various test data obtained. Keep in mind that insulation system testing has three purposes: (1) assure that personnel coming in contact with the insulation are not at risk of becoming a lower resistance conductor path to ground than the intended path, (2) determine if the insulation is healthy and in proper condition for service under stresses for which it was designed, and (3) to give the technician some basis for predicting whether or not the healthy condition will continue or if deterioration is under way which may result in an abnormally shortened life.

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