Insulation resistance test on the equipment is simple but important preventive measures which can significantly reduce maintenance cost.
You probably already familiar with the insulation resistance test (more commonly known as a megger). This test plays an important role in the maintenance of the cable, but do we realize that it can also help us in preventing unexpected motor failure? Not only is the motor insulation is susceptible to excessive temperatures, moisture, dirt, corrosive vapors, vibration, oil, and aging, but also be able to withstand the normal surge and inrush current VFD. An insulation resistance test program that can consistently deliver substantial savings and increased service life of / uptime motors.
Testing also reduces the waste of resources for maintenance. For example, stamping and assembly plant to replace the motor 250hp Plant Air System with motor rewinding purchased from a local store. After struggling to put the motor into place, termination or cable connection, and setting the position of the clutch, the maintenance crew tried to start the motor, but it turns out the motor does not respond at all. Service electric motor repair shop turned out to have been wrong to send a similar model which is actually still need to be re rewindning. Finally, the motor must be disassembled and transported back to the electric motor service shop. Maintenance staff of these cases could prevent delays in the replacement of the motor is happening there as well as loss of production time by just doing simple insulation resistance test as a condition of receiving an electric motor before it is installed.Testing Automated vs Testing Manual
Insulation resistance tests automatically (online) can save maintenance time = saving money, but the automated tests can not replace the need for manual testing. For example, you have to perform insulation resistance tests when we receive new or rewinding motors. Each time take the motor out of the garage service, we should always test the motor windings before the motor cable termination. If you do maintenance to the power outage, manual testing is the best choice.
Test results are automatically or manually, the data of test results should be stored in order to produce an electric motor performance trend graphs. In this process the manual method can be very long to find a graph of test results and test methods can be used automatically, can use the "CMMS" or similar software. Always use only the results of tests performed on the same test voltage for the same period, and under conditions of temperature and humidity are similar to results obtained graph is always valid, comprehensive and easy to analyze the existing discrepancies.
Voltage Test
In today's world, megohmmeters mostly been using DC voltage or simply by batteries. The main advantage of the DC assay test kits one of which is smaller and lighter, test equipment does not damage the object being tested (nondestructive testing), and be able to compile historical data / records a history of the results of the tests that have been made.
Testing with AC voltage test voltage value is usually two times the voltage on the nameplate plus 1.000V. When using a DC voltage, megohmmeters that we use today, just a test voltage at twice the nameplate voltage. The table (right) gives the test voltage is recommended. However, you should still contact the manufacturer to find out the recommended test voltage value.
Test Connections
Before starting the test, kebumikan / ground the starter terminal, frame, and the motor shaft. If you test DC motors, lifting the brush out. Discharge / charge field waste on rolls with membumikannya / grounding. Then remove the grounding of the motor windings and connect to the line (-) on megohmmeter. Connect the terminal (+) to the grounding. You also have to measure the stator in the same way.
Spot Reading Test
Perform this test only when the winding temperature above the dew point. Megohmmeter connect to each one of transverse roll insulation. Apply test voltage for a certain period (usually 60 seconds). Then note the reading of test equipment. Use the same duration for all tests performed as a comparison.
Spot measurements can be meaningful only if we compare the test results have been deposited with trend graphs developed from previous tests. A declining trend charts typically show a loss of insulation resistance due to unfavorable conditions such as humidity, dust accumulation, and oil penetration. Showed a sharp decline in insulation failure.
Dielectric Absorption Test
This test serves to compare the absorption characteristics of the insulation is still good until the parts are wet with moisture. During the test, applying the test voltage for an extended period, usually 10 minutes. Take the measurements every 10 seconds for the first minute and a per-minute for the next nine minutes. Then you can create a trend graph of insulation resistance value over time.
The slope of the curve indicates that the isolation conditions tested. Good insulation will show continuous improvement in the resistance, as shown in curve D in Figure (right). Contaminated, damp insulation, or the crack will produce a curve similar to curve E.
Polarization Index (PI) can we find by dividing the value of the 10-minute readings by 1-minute readings. This index shows the slope of the curve. A low PI usually indicates excessive moisture and contamination. For large motors or generators, are generally found values as high as 10.
Step Voltage Test
Apply a test voltage of two or more in a few steps. The recommended ratio is 1:5 for each stepnya. At each step, test voltage is given for the same period of time, usually 60 seconds. This will create a pressure on the electrical insulation area of the cracked / brittle. The test voltage greater is intended to reveal the onset of aging and damage to internal insulation which though looks relatively dry and clean insulation where damage can not be detected by low voltage or nominal.
Many people choose not to test insulation resistance testing because they fear it will actually damage the insulation, but in fact this is not true. By testing the motor on a regular basis, we will be able to analyze and correct the failure of insulation that will come before the turmoil on the production system is more severe.
The research was conducted by: John Kolibri, AEMC :: Hummingbird is a product development manager AEMC Instruments, Foxborough, Mass.
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