When reviewing the IEEE and EPRI large electric motor studies on which this ongoing article series has focused, and looking at what those studies considered as “excellent maintenance,” a number of concerns surfaced. Among them was the fact that many technologies we now use for trending and prognostics weren’t included or cited in those studies. That, in turn, brings up some questions.
Prior to the 1990s, high-voltage testing, such as high potential and surge comparison testing, were primarily performed by utilities and repair shops to evaluate existing and repaired electric machines. To this day, surge comparison and many of the other tests performed as part of modern prognostics and trending to detect defects are not required to be performed by manufacturers with the focus on insulation to ground. Once reliability programs insisted on field testing of electric machines for winding shorts and contamination, the need for winding tests increased, first with low-voltage testing methods, and then later surge-comparison and insulation-to-ground testing. Along with all this has come a lot of misunderstanding of standards and what different tests are used for.
In this article, we bypass discussion of insulation-to-ground and resistance tests with the intent on addressing them in a later article. Instead, we’re jumping into the realm of high-voltage testing and the process motor users would follow before applying potentially destructive high-potential and surge-comparison testing. For the purposes of this article, we will be using an Electrom ITIG II Model D 12kV instrument with offline PD and low voltage testing, which is similar to a variety of testers on the market.
NOTE: Up-front cautions here relate to the term “potentially destructive.” While we’ve heard most manufacturers say high-potential and surge-comparison test are not destructive in nature, a few things need to be clarified before motor owners hear repair personnel say, “We found your problem,” when a previously functioning motor no longer operates.
The purpose of high-voltage testing is to act in a similar manner to over-pressurizing a compressed air system: you will not only find existing leaks, but you will potentially finish off a few weaknesses. The objective is to find problems during testing instead of during operation, and a plan must be in place when testing is being performed. What happens if the equipment fails the test? Are we able to use the machine for a while longer? Do we have the resources to fix the issue before moving on? Are these problems real? If you are performing these tests in the field, you will most likely have had the experience.
Understanding high-voltage testing is important to reduce the number of potential problems that you will run in to. For instance, if performing testing on large synchronous motors, it becomes important to disconnect the exciter from the circuit, or raise brushes on slip rings, to avoid damaging rotor insulation systems or the electronic excitation package. We’ve run into issues where voltage has jumped as much as half an inch to damage diodes in an exciter to breaking down the insulation in a rotor shaft. While most of the instruments have built-in protection systems, when the arcing is occurring on the secondary (rotor winding) it is not easily detected through the stator winding.
Another area of concern relates to contamination. A winding that is covered in materials such as contaminated grease, oil, conductive dusts, etc. can create conditions where an arc can occur between conductors or conductors to ground. Carbon forms where the arc occurs resulting in a semi-conductive, and temperature dependent, path referred to as tracking. This will either cause the machine to trip immediately or once the area where tracking has occurred gets warm enough to become fully conductive. As a result, there are steps that must be taken to avoid this type of condition, such as insulation resistance testing. The tables and methods from IEEE Standard 43-2013 related to the applied voltage, temperature adjustment, dielectric absorption, polarization index and minimum insulation resistance values are not really there for testing machine condition. They exist to determine if you can advance to higher voltage testing with less risk of damaging the insulation system. For instance, after temperature correction, a form wound stator measures 101 MegOhms, then it is less risk to apply a high potential or surge comparison test.
The surge test uses the concept of a ‘tank circuit,’ where a capacitor discharges an impulse that travels to an inductor and a resulting RLC dampening occurs at the peak of the impulses. What this means is that as the fast-rise-time square-wave impulse travels through the test lead and hits the coils of an electric motor (which are inductors), a “ringing” occurs at the top of the wave. If like coils or inductive circuits, such as a winding, have the same inductance, then the dampening is the same. Conditions where there are shorts or weakness between conductors show as a difference between “like” inductive circuits. These can also be separated by air. Thus, a voltage significant enough to ionize the air between exposed conductors, per Paschen’s Law, must be applied.
One of the challenges with the surge test is that it doesn’t measure defects in parallel conductors. An open conductor, or conductors, amongst a group of parallel conductors will often not be detected by the surge test. This means that you must have a way to identify partially open circuits, such as resistance measurements. For a large three-phase machine, you can compare the resistance between phases and determine if there are defects. The unbalance depends on which standard that you are following, which may be as tight as 1% unbalance from the average resistance to as high as 5% unbalance in resistance. Resistance is also normally adjusted to 25 C.
RELATED TESTING
As a result of various concerns around high-voltage testing in the field, specific processes that build on previous testing now tend to be followed. These processes can include additional tests, which we identify as “options.” Upcoming articles in this series will address each of the following tests individually.
1. Resistance Check: This is done to verify there is continuity and that the circuit is balanced. Usually corrected for temperature, which can be trended.
2. Low-Voltage Tests (Option): These would include inductance, impedance, capacitance and related low voltage tests to see if any obvious faults exist. Some faults detected can be corrected or the higher voltages can be stopped while options are reviewed.
3. Insulation Resistance Tests: Such testing is done to a specific voltage and insulation-resistance value, depending on the voltage of the machine. This is used to determine if the insulation system is in poor shape or heavily contaminated. Usually performed along with dielectric absorption or polarization index. For random wound machines, after temperature correction, the minimum value is 5 MegOhms and for form wound machines the minimum value is 100 MegOhms.
4. High-Potential Tests: With the testing instrument used for purposed of this article, this type of testing is usually a DC high-potential test. There are a variety of methods of performing a DC high potential test with the least potential harm coming from a ‘step test,’ where the voltage is increased and then held for one minute at each level. The instrument and operator can watch to see if there is a sharp increase in leakage current to ground, usually in pico or micro-Amps.
5. Surge-Comparison Tests: This type of testing was normally performed on stators with the rotors removed. Newer methods have been employed that provide peak to peak and line to line tests that provide support for fully assembled motor tests. An additional test includes the PD Surge test, which is a measurement of partial discharge that occurs along with the fast rise-time impulses from the instrument. This increasingly popular test can be used on motors that will be applied to inverters to see where partial discharge may start and extinguish, in addition to the earlier detection of insulation breakdown.
ADVANCEMENTS IN TECHNOLOGY
With regard to surge-comparison testers, “portable” units typically have a limit of about 12kV, although some higher voltage testers are appearing on the market, and they usually have some type of booster pack. They will normally be plugged in to a power supply, such as a wall outlet, to produce the power necessary to test. The quality of the power supply, if there are no built-in filters, can produce errors, so many of modern surge testers will alert if there is a bad ground.
One of the most important advancements in high-voltage-testing technology has been the introduction of the auto-stop features. With older analog surge and high-potential testing instruments, the operator could deliberately or accidentally force a fault. In the case of modern instruments, this would require the change of specific settings or performing the test manually. There are times when an operator would want to force the fault, such as with root-cause-analysis or fault finding.
COMING UP
In the next installment of this series, we will discuss resistance and resistance-balance testing, as well as identify some of the standards that outline resistance tests.TRR
Click the Following Links for Previous Articles In this Reliability of Large Electric Motors Series
March 28, 2020: “What The Studies Really Said”
April 12, 2020: “Comparing What The Studies Said”
April 18, 2020: “How The Studies Applied to Larger Motors”
May 2, 2020: “A Data Mind-Bender”
May 29, 2020: Site and Study Findings Compared
June 13, 2020: Developing Testing Programs
ABOUT THE AUTHOR
Howard Penrose, Ph.D., CMRP, is Founder and President of Motor Doc LLC, Lombard, IL and, among other things, a Past Chair of the Society for Reliability and Maintenance Professionals, Atlanta (smrp.org). Email him at howard@motordoc.com, or info@motordoc.com, and/or visit motordoc.com.
Tags: motors, drives, electrical systems, motor testing, reliability, availability, maintenance, RAM, EPRI, IEEE
