Tom Bishop, P.E. 
EASA Technical Support Specialist 

A frequent question that arises during repair or maintenance is how much voltage to apply when performing a high potential test. The test voltage for a new winding will be higher than that of a winding that has been in service, but what is the appropriate test voltage? Seeking answers to this question can sometimes lead to confusion. Our goal in this article is to clarify high potential test­ing and test voltages. 

What is meant by high potential (hipot) testing is not always clear. The term “high potential test­ing” is defined by NEMA (MG1-1.57) and IEEE (IEEE Standard 858) as a test that “consists of the application of a voltage higher than the rated volt­age for a specified time for the purpose of determining the adequacy against breakdown of insulating materials and spacings under normal conditions.” For example, a 5000-volt hipot test on a motor rated 4000 volts would be considered a high potential test. 

Before proceeding into the selection of a test voltage we should consider a critical step in hipot testing. That is the determination of the suitability of the winding to be hipot tested. An insulation re­sistance (“megohmmeter”) test is the first step prior to the hipot process. EASA AR100 and IEEE 43 recommend the test voltages in Table 1 for determining the insulation resistance of motor and generator windings. 

Most random and form coil windings rated below 1000 volts should have a minimum insula­tion resistance of 5 megohms; and for most form coil windings rated 1000 volts or more the minimum is 100 megohms. All insulation resistance values must be corrected to 40º C, and the reading taken after applying the voltage for 1 minute. In addition to measuring the winding tem­perature to determine the correction factor, the humidity should also be measured. 

Appy voltage gradually 
Hipot test voltage should be applied gradually until the final voltage is attained. The final voltage should be held for 1 minute before reducing the volt­age gradually to zero. An alternative to gradually increasing the test voltage would be to apply the test voltage in steps until the final voltage is attained. The number of steps and length of time to hold the voltage at each step will vary with machine rating and is seldom used on machines rated less than 600 volts. A 4000-volt machine might be tested in 10 steps whereas a 13 kV machine might be tested in 30 steps; a 200 hp motor might have the voltage steps held for 30 seconds while an 8000 hp motor might have voltage steps held for 2 minutes each. If there is a current indicator on the test set, the steps should be held until the current stabilizes. 

A pass-fail test 
A winding that withstands the final voltage, and return to zero voltage, is considered to have passed the hipot test. There are no standards for minimum insulation resistance for a hipot test. It is a pass-fail test. An AC hipot does not indicate insulation resis­tance, and although a DC hipot can indicate insulation resistance, it is not an acceptance param­eter for the test. After performing any hipot test, for safety purposes, make certain that the winding is discharged to ground through a resistance circuit (e.g., the hipot) for a long enough period that the winding does not recharge. IEEE 43 recommends a minimum discharge time of 4 times the length of time the test voltage was applied. 

According to NEMA MG1 and EASA AR100, a new winding should be AC hipot tested at twice rated voltage plus 1000 volts. If a DC hipot is used, the AC test voltage is multiplied by 1.7.  A new winding rated 4000 volts would be AC hipot tested at 9000 volts (2 x 4000 + 1000), or 15300 volts DC (9000 x 1.7). It is usually more practical to read the display of a hipot in 250 volt increments, making 15250 a more practical value than 15300. This ulti­mate test voltage must only be applied once to a winding. Therefore, the winding should be impreg­nated and fully cured for this ultimate test. 

If a subsequent hipot test is desired or re­quired, the test level should not exceed 85% of the new winding test. This test is sometimes termed an acceptance test and is also a one-time test. It is often applied to the new winding after the motor or generator is assembled. Using the 4000-volt rating example, the 85% AC hipot test level would be 7650 volts, and the DC equivalent would be 13005 volts. Note that some calculated values are not practical, and should be modified to a value that can be read on the hipot. Thus, the 15300 value may be modified to 15250, 7650 to 7750, and 13005 to 13000. 

Conducting subsequent tests 
Subsequent hipot tests on a new winding, per NEMA MG1, should be at 75% of the initial new winding test value. The 4000-volt machine in the ex­ample above would subsequently be AC hipot tested at 6750 volts, or DC hipot tested at 11475 volts or a more practical 11500 volts. Note that this test (75% level) applies to all tests after the initial (100% level) test and the one subsequent (85% level) test. Also note that all of the above hipot tests apply to new windings that have been impregnated and fully cured. Untreated random or form (“green”) coils must be tested at reduced levels. For form coil wind­ings make certain that the coil supplier agrees to the test voltages that will be applied to untreated “green” coils. There are no industry standards for hipot test­ing of untreated coils. At most, a level of 60% of the equivalent treated winding test value is suggested. 

What test levels do we use for windings that are no longer new? EASA AR100 states that a hipot test on a reconditioned winding should be at 65% of the new winding test value. The 4000-volt machine would be hipot tested at 5850 volts AC, or 9945 volts DC; or 5750 volts AC and 10000 volts DC in more practical values. It is suggested in AR100 that windings that have not been recon­ditioned be insulation resistance tested, and not hipot tested. That suggestion applies to service center work, where the subject winding would be reconditioned prior to a hipot test. 

Maintenance hipot test 
Machines installed at the end user location can be hipot tested, if desired or required, at 125­150% of rated voltage in accordance with IEEE 
432. This test is often referred to as a maintenance hipot test. The 125-150% level applies to an AC hipot test. The DC hipot equivalent would be 1.7 times the 125-150% levels, or 215-255% of rated voltage. For the 4000-volt machine used in the earlier examples, the AC hipot level would be 5000-6000 volts, and the DC hipot level would be 8600-10200 volts (more practical values would be 8500-10250 volts). 

As all of the above stated test values suggest, the appropriate test level determination takes some thought and understanding of the type of hipot test that will be applied. For simplicity and consistency, we chose a 4000-volt machine rating and used it in each hipot example. Table 3 further illustrates this point by indicating the test voltages for each type of test. Whenever you are consider­ing a specific hipot test, calculate the test voltage to be applied for the rated voltage of the motor or generator to be tested. Then calculate the test level using a 4000-volt rating as a check to verify that it matches the example given here. That will con­firm that your hipot test is at the correct level, before the test is actually applied. If the planned test level is too low, the test will have to be re­peated, and if the test level is too high, the winding may fail due to the overpotential. 

What test levels do we use for DC motors and generators? The nameplate rated voltages, whether AC or DC, for the windings to be tested are used in calculating the hipot test level. For ex­ample, an armature rated 500 volts DC would be AC hipot tested at an ultimate test level of 2000 volts (2 x 500 plus 1000). 

A note of caution 
There is a precaution to keep in mind about hipot testing. The main purpose of a hipot test is to detect weaknesses in the insulation structure that could lead to an in-service failure. The application of overpotential carries the inherent risk that the winding may fail the hipot test, and would prob­ably need to be rewound. The owner of the motor or generator to be tested should be willing to accept that risk; otherwise, hipot testing is not advisable. As one hipot testing expert commented after a winding failed while under test: “The winding was faulty and we proved it.” Table 2 indicates that some hipot tests can be repeated; however, the fre­quency of repetition should be extended as far as possible. Hipot testing is a circumstance where more is not better.

Overpotential testing can leave residual dielectric stresses in the winding that could lead to premature failure. Hipot test as necessary to verify winding suitability for continued service, but don’t “overtest.” 

Categories: Testing, Motor testing

Tags: Motor testing High potential (hi-pot) testing

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