Insulation resistance testing (aka Meggering') is a specialized procedure where the integrity of the electricity-producing portion of your generator set (the alternator) is tested in a safe and isolated condition. It's a crucial part of maintaining a reliable emergency power system and is recommended annually by the National Fire Protection Association's 110 standards.
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The test identifies weaknesses in internal components that may not be detectable under normal conditions until a total failure occurs. Identifying and preventing these impending failures is a relatively simple process and typically will not impact any supported building systems. Depending on engine configuration, the unit may need to be partially disassembled to access the alternator.
During the test, we apply voltage to the alternator windings to determine if the insulation is damaged and may cause a short circuit under normal operation. To perform this work safely, we'll need to disconnect your generator from your building for a relatively short period of time, typically one to three hours.
You'll receive a printout of the test results along with your work order and any recommendations. If we find any problems, we'll help you develop a remediation plan that avoids costly damage to your entire system.
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.
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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:
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:
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.
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