DC Hipot Test Pass/Fail Analysis
A DC Hipot test is done at a voltage higher than the peak operating voltage of the device under test (DUT), where Peak Voltage = RMS voltage x 1.41. Therefore, if a test of an installed motor in a plant fails above peak operating voltage, it does not mean that the DUT itself has failed and cannot continue to operate, or that it needs to be condemned.
The DUT may have weak insulation, but can potentially continue to run for a long time. How long depends on what voltage it failed at, operating conditions for the DUT, frequency of starts, how
contaminated and moist the DUT is, if the Hipot test failed at a higher or lower voltage before, and other factors.
For critical motors this information is important. If tracked over time, which the iTIG III motor analyzer can do, planning for when to take the motor out of service is made much easier.
Failure Below Peak Operating Voltage
If the Hipot test failed at or below the peak operating voltage, the chance of imminent failure during operation or at start-up is high. However, even in a case like this, a motor can continue to run under the right circumstances, especially if the test was done under moist conditions.
The Owner Decides
No standard provides specifics on when a motor should be taken out of service because of a failed Hipot test. Nor does it state what conditions are acceptable when tests are done in a motor shop or motor manufacturing facility. Standards only give recommendations for how to perform the tests.
Keep in mind that if a test fails, it is the test that failed, not necessarily the motor, because failure limits are set by the test operator or other “humans”. The purpose of a limit can be to set a “red flag” as well as provide information for planning purposes in a plant.
When issues are found above peak operating voltage, and in advance of catastrophic failures below peak operating voltage, the test information provides important inputs for planning purposes.
Different methods are used to determine what should be done with the DUT if the Hipot test fails. Should one continue to run the DUT, or take it out of service for reconditioning, repair or rewind. Here are a few examples:
- The test operator or DUT owner has set total leakage current levels, above which they have determined that the DUT is too weak for continued operation, or needs to be replaced or repaired soon.
- The user has set arc voltage levels below which the DUT is considered too weak for continued operation or longer-term operation.
- The user tracks leakage current or arc voltage levels over time and determines when to take the DUT out of service based on the trend.
- Hipot Step Voltage graphs or Ramp Test graphs are compared over time, and an evaluation of the actual or potential insulation breakdown is assessed based on the step voltage/leakage current graph(s). More on this under Step Voltage Tests.
- Assessments including other test types as well as DUT operational information such as voltage and current levels during operation, operating temperature, and so forth.
PASS/FAIL LIMITS TYPES THAT CAN BE SET FOR MOTOR ANALYZERS
Various limits and automatic test shutoff mechanisms are used in modern motor testers and DC Hipot testers like the iTIG III. They include:
- Leakage current limit: An over-current trip-out level that can be set in µA up to a couple of mA by the user. If the limit is exceeded, the test is
immediately shut off. This limit must be set high enough so the capacitive inrush current does not trip the limit. If the inrush current is high and the limit is to be set low, the test voltage may have to be raised manually and slowly which creates a lower inrush current. Or, the automatic voltage ramp controlled by the motor tester must be set to a lower voltage ramp rate if such an option is available.
- Arc detection: If an arc is detected, the test is immediately shut off regardless of what the leakage current and the voltage are.
- Leakage current acceleration limit: This is called a rate of change (ROC) limit in the iTIG III. If the leakage current accelerates faster than the ROC limit in a Hipot step voltage test, the test is shut off before the next voltage step. The ROC factor is adjustable.