By Jeff Jowet

Electrical testing is essential to the evaluation and maintenance of motor performance and life. There are a variety of properties that can be tested, with a broad range from the simple and general tests, like voltage measurement, to the highly complex and specific, like surge testing. (This latter is a technique that pinpoints turn-to-turn shorts.) They are too diverse to all be covered comprehensively as a single topic, but a general method that is universal and indispensable is that of insulation testing.

In its basic terms, an insulation tester applies a high dc voltage across an insulation barrier, measures the amount of current (commonly called “leakage”) that flows through the insulation, and applies Ohm’s Law to calculate the resistance of the insulation. The basic theory is quite simple, but the testers themselves are complex, primarily because they must generate a high voltage and be able to accurately measure a miniscule current.

There is no perfect insulator. Apply enough voltage and any material can break down (think of a lightning strike!). But by applying the test voltage across the insulation, the tester is establishing a highly atypical “circuit” in which an insulating material is being forced to act as a “conductor.” Obviously, it is not going to conduct very much, so the tester must be able to sense currents on the nano-amp level and below. These are the principle characteristics of an insulation tester. Because resistance is high, measurements are made in units of meg-(millions of)ohms.

A caution about voltage supply is in order. Many “economy-level” testers provide only nominal test voltage selection. That is to say, a given voltage selection may only provide that voltage over a limited portion of the resistance range. To avoid this problem, the tester should be provided with a load curve of output voltage versus load resistance. This curve should indicate a fast rise up to full selected voltage at a level of 1 to 5 megohms, then maintain that voltage throughout the remaining range. While dc insulation testing is non-destructive (as opposed to “high-pot” testing), there are limits where severely deteriorated insulation is concerned. To avoid any further damage to insulation that is already near breakdown, test current is limited. Since IEEE Standard 43 recommends never less than two megohms for rotating machinery, a simple application of Ohm’s Law indicates that at a typical 1000 volt test, this represents no more than half a milliamp! Consequently, testers need output no more than a few milliamps to be able to cover the full range of anything fit to be considered as insulation. Once this limit is reached, there is nothing but for the voltage to drop, further protecting a test item that is on the unacceptable end of the scale. The voltage should rise sharply through the low end of the range, then maintain full selected voltage through “good” values. A poorly designed tester will exhibit a slow rise, not achieving full voltage until well up into the range. The critical range of values, where the motor is still operating but is approaching a need for maintenance, will be getting tested at deceptively low voltages and yielding potentially misleading results.

The benefit of this testing is that the readings can be used like the odometer on a car, to give an idea of the working age of the motor, how much can be reasonably expected of its performance, and where it is on its life cycle. From the time a motor is put into service, insulation picks up moisture, dirt, and other contaminants, suffers wear and tear, is carbonized from voids and other imperfections, or may be damaged by line disturbances. From initially high values (possibly tera-ohms), these factors will cause insulation resistance to decline, often at a fairly steady and predictable rate. An insulation test is a quick and simple maintenance check that captures the information available from the aging process, and by inference, reflects the overall condition of the motor itself. Of course, catastrophic failure can also occur, as in flooding, fire or lightning strike. An insulation test is typically performed here also, but such events exemplify the separation between the two broad areas of testing: pass/fail and preventive maintenance.

Pass/fail testing is the more commonly employed. A motor has failed, is performing erratically, or tripping breakers. An insulation test is performed to look for breakdown. It may appear as a dead short or unacceptably low reading (typically, less than 2 megohms). In such applications, a sophisticated tester is not required, and the operator may not be concerned about the actual readings. Experienced personnel may look for direction of pointer travel only, toward the low or high end of the scale. If low, an electrical problem exists, and more specific troubleshooting can commence. If high, time might be better spent looking for a mechanical problem such as bearing wear.

Pointer travel is significant in insulation testing as with no other type of test. It is also frequently confusing and frustrating to the uninitiated. Motor windings, by their structure, possess a good deal of capacitance. Each turn represents two conductors separated by insulation: a tiny capacitor. The larger the winding, the greater the capacitance. Secondly, insulating material itself will realign molecularly under the influence of an applied voltage field. This too represents a flow of charging current called absorption. Because of these charging effects, readings start low and gradually rise as charging goes to completion. What remains after full charge is the current that flows as a product of the level of imperfection of the material called “leakage”. With the largest windings this can take hours, an impractically long test time. Therefore, special procedures called test methods have been worked out to bring the testing into workable time frames and act as an aid in interpreting results.

The second broad area of testing, preventive maintenance, is more sophisticated. High quality testers are advised because these will have the range necessary to measure equipment that is in the earliest stages of its life cycle, preferably beginning with an installation test. In such applications, a simple pass/fail tester with limited range will yield only an over range test, typically called “infinity”. This is not a true measurement, but merely an indication that the motor has a resistance greater than the sensitivity of the tester. Such an assurance is satisfactory for pass/fail applications, but does nothing to establish a time line for life expectancy. It may be years before resistance drifts down to where it can be measured, with considerable loss in flextime for scheduling routine maintenance. There is no better way to evaluate test results than comparison to a previous test. Readings may be quite high, and if evaluated singly, appear “good”. But if the motor is operating in a hostile environment, possibly exposed to corrosive fumes or high moisture, readings may be dropping rapidly. Early detection would allow time for corrective action before significant damage could occur. Conversely, comparatively “low” readings holding steady probably indicate a widely dispersed low level of harmless leakage, so that critical maintenance time can be spent on more pressing problems.

Standardized test methods can be judiciously employed to help with test time and interpretation. A number of methods have been devised, and each “looks at” the insulation in a different way. Except for the most critical and high-capital equipment, it is not practical to perform a full regime of test methods. Rather, judicious selection of the most appropriate to a given set of circumstances can maximize results while conserving test time. Two broad categories of insulation failure are pervasive contamination and localized damage. Two of the most popular methods are Polarization Index and Step Voltage.

Polarization Index, more popularly known as “PI,”, takes advantage of the differential beteen “harmless” charging currents and damaging leakage. In healthy equipment, leakage is small by comparison, so the charging currents dominate pointer travel. There will be a fairly large difference between a reading at 1 minute and at 10. When converted to a ratio, numbers typically in the range of 2 to 4 are seen. But deteriorated insulation will be dominated by leakage, which is constant for a given test voltage and will mask the effects of charging. The 10- minute reading will be similar to the 1- minute, and a poor PI of less than 2 is the result. The PI is good for detecting pervasive contamination because the effect on leakage versus charging current will be readily apparent. It also solves a number of practical problems. Dependence on a knowledge of resistance values is eliminated because whether they are in the tens, hundreds or thousands of megohms, the result will be the same. Therefore, the PI is a good test where no prior records are available.

PI is also useful on very large motors for two reasons: time and voltage. Large windings can take a prohibitively long time to reach a stable reading, but that is not necessary with PI. The 1- and 10-minute readings can be relatively low on the time-resistance curve; the result is still valid. Caution must be taken with smaller windings, however, as they may reach full charge quickly and yield a low PI even though perfectly “good.” Similarly, the tester must have a good range so that it will not over range before 10 minutes. “Infinity” is not a number and cannot be used to calculate a PI. The second advantage with large motors is that they may have high operating voltages that put “as rated” tests beyond the test voltages of all but large, expensive testers. PI can be performed successfully at values below rated.

A Step Voltage Test is performed by increasing the test voltage in steady increments and observing the response of the insulation readings. Voltages and time intervals can be defined by the operator to meet the demands of test equipment and work schedule, but industry standard is five voltages (such as 1, 2, 3, 4 and 5 kV) in 1-minute intervals. Good insulation will stand up to this treatment, but deteriorated insulation will open more leakage paths with each increase, and readings will steadily drop. These two tests, and most other standard methods, are not mutually exclusive, but complementary. Each will reveal a variety of problems, but each has individual strengths. Step Voltage is a good test for recognizing elusive intermittent failures that can arise from localized damage in otherwise "good” insulation. A carbon track that has burned through or a pinhole caused by a voltage spike may be lying against an air gap instead of direct contact to ground. An insulation resistance test may still yield a high reading and overlook the critical flaw. But Step Voltage can arc the gap and expose the problem. Another specialized problem is dry and brittle insulation on old motors. Failure is imminent, but this type of insulation will often yield deceptively high readings. An unnaturally high PI will reveal the condition, but can be mistaken for a “good” reading by those not familiar with past history. A Step Voltage can be expected to draw increasing amounts of leakage through cracks in the dried insulation with each voltage increment and readily reveal the condition.

The accepted standard for electric motor testing is IEEE Std 43-2000: Recommended Practice for Testing Insulation Resistance of Rotating Machinery, published by The Institute of Electrical and Electronics Engineers. This standard was revised in 2000 in order to take into account the effects that newer types of insulation have on standard testing procedures and interpretation of results. Industry practices, in the interest of time, may gang all windings and test simultaneously to ground or case. But IEEE recommends phases be tested individually, with remaining phases and stator core or rotor body connected to a common ground. This will reveal leakage between phases, as well as phase-to-ground. Similarly, testing can be done through external equipment like motor controls, but it must be understood that these will be included in the measurement, and bring it down. IEEE prefers that external equipment be disconnected. Testers may also have guard terminals, which act as shunts for one or more parallel leakage paths. In a complex piece of equipment like a motor, there may be several leakage paths. These can be examined more specifically by elimination of one of more through guard connection. For instance, stator to rotor leakage can be measured with ground leakage guarded.

The revision includes a table of recommended test voltages based on equipment rating. In response to newer insulation types, the recommended test voltages have been increased up to 10 kV for motors rated at 12 kV and above. Both basic insulation resistance and polarization index tests are recommended, with one or the other performed on equipment rated less than 10,000 kVA, and both tests suggested for equipment rated higher. There are also some interesting observations for atypical windings. Large turbine generators may have unencapsulated field windings where little absorption can take place, so that a PI is not a recommended test. It is also advisable for the operator to have knowledge of the basic schematic in order to avoid testing errors. For instance, windings of some induction motors may not be insulated from the rotor body, so that a PI is not possible.

Traditional notions about PI testing are also undergoing an upgrade as the result of new insulation types. Absorption currents may decay to near zero in much shorter time frames, making the traditional 1 to 10 ratio dubious. Accordingly, newer definitions have incorporated 1-minute to 5-minute and 30-second to 1-minute ratios. An alternative suggestion is to record the reading every minute until three consecutive intervals yield the same result. The caveat for all of these newer methods is that they are not standardized, and there hasn’t been enough data collected to set new pass/fail values. IEEE requests such information be forwarded by field sources.

In summary, insulation testing offers a quick and convenient window into the electrical condition of a motor, and provides a reliable basis upon which further maintenance and troubleshooting decisions can be made.