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INSULATION INTEGRITY If the insulation in an electric motor never failed, neither would the motor winding. The only thing preventing an energized electric motor from shorting directly to ground is the winding insulation system. The only way to determine the strength of this interturn and ground wall insulation before it shorts to ground is with controlled over-potential (dielectric/hipot) testing. With the wide range of meters and test instruments on the market, it is important to select the proper device for performing the necessary diagnostics. TEST VS. MEASUREMENT Although the term “test” is commonly used, there is a distinct difference between a test and a measurement. An example of this is a simple continuity test with a multimeter to determine if winding is “open” or the phases are “shorted” together or to ground. This is actually a measurement to confirm the winding resistance rather than a test of the winding integrity and provides useful information only on the present "static" condition of the motor winding. This is also true of a megohmmeter measurement on the ground insulation system.
Another way to describe the difference between a test and measurement is to use the example of a pvc water pipe with a working pressure of 70 psi. We can measure the applied pressure and measure any losses of water presure (or air pressure). However, a true test would be to pressurize the pipe above the working pressure - but not exceeding its rated 310 psi. A measurement taken on the pipe at the working pressure of 70 psi does not give us any indication of its stability during variations in temperature and pressure. We apply the same principals to motor winding insulation to test above the working voltage, but not exceeding the rated dielectric strength. That's why we can test at voltages higher than operating voltage and still not damage the insulation. DIELECTRIC TESTS The Surge Comparison Tester and DC Hipot Tester are the two most common instruments used for dielectric testing of motor winding insulation. The "Surge" test is a unique impulse test used to test the integrity of the turn-to-turn and phase-to-phase insulation. Also known as an "Impedance Balance Test" the surge tester is often misused or misinterpreted because of the waveforms displayed on an oscilloscope. Interpretation of the waveforms is actually secondary to effects observed by the applied voltage - hence, the dielectric strength. Even very small (incipient) faults in the interturn insulation can lead to a complete motor failure. Manufacturers' operation manuals usually elaborate on the use of the instrument itself, rather than analysis of the results. The DC High Potential Test (DC Hipot) is designed to safely test the ground wall insulation. Not to be confused with an AC test, the DC Hipot test is safe, reliable, and the best choice for determining the strength of the ground wall insulation. The "Step-Voltage" test is used to safely test the ground wall insulation with minimal damage to unstable or faulty insulation. INSULATION STANDARDS An applied voltage over that of the nameplate voltage is an over-voltage or hipot test. Although the insulation in any electric motor must be rated substantially higher than the operating voltage for a margin of safety, the precise level of over-voltage has been debated and researched for many years. Several industry organizations have standardized the applied test voltages as to prevent unnecessarily high or destructive levels on weak or damaged insulation. These standards are outlined in the High Potential Seminars material. Please contact us for details on how to arrange for on on-site training workshop. |
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