Hipot Step Voltage
The Hipot Step Voltage Test is recommended for DUTs with an operating RMS voltage of 2300V and higher. It is done on motors with any operating voltage when more information than what a 1-minute Hipot test provides is needed.
In a Hipot Step Voltage Test the voltage is usually raised in equal steps. At each step the voltage and measured current are recorded after 1 minute per IEEE 95. The number of steps is determined by the test operator, and can be anything from 5 to 30 or more voltage steps. 10 steps are common for medium voltage motors; more steps may be used for high voltage motors. Profiles for step voltage tests are programmable in the iTIG II D, including the number of steps, the dwell time at each step, and the voltage ramp rate.
Step Tests provide more information
Hipot Step voltage tests provide much more information than a 1-minute Hipot test because the data is recorded as the voltage is raised. By charting a current vs voltage curve, one can usually tell if the leakage current is mainly due to contaminated dirty windings, or due to a breakdown of the insulation as shown in the pictures below.
With a Step Voltage test the motor tester may be able to shut down the test before an arc happens using the ROC limit (the current acceleration limit). The test operator can also manually abort the test if the leakage current appears to accelerate.
Modern Hipot and motor testers have arc detection that will shut off the test immediately when an arc is detected instead of continuing to ramp up the voltage with more and more severe arcing.
Unfortunately, an arc may happen shortly after the leakage current starts to accelerate. Per IEEE 95, the acceleration may start 5% or less below the arc voltage. In these cases of abrupt insulation breakdown, the current acceleration event may not be detectable.
A clear Pass: Straight line, low current
Good result; 3,300V motor, test voltage 7600V:
Good windings will have a curve that is more or less a straight line as shown in the graph. The current depends on how contaminated the windings are. In this case the current and contamination level is low with a total leakage current less than 1.4µA.
OK result; 4000V motor, test voltage 9000V:
Four 10-point Step Voltage Tests were done over time. The two bottom curves from the first two tests are very good.
The red and blue curves from the next two tests show minor acceleration in the current. But, the max current is still relatively low at 11µA, and the acceleration starts above 7,000V, significantly above peak operating voltage.
The elevated total leakage is mainly due to increasing winding surface contamination (including moisture) judging from the previous tests.
If the acceleration in the current was significant and the blue line moved more towards vertical, it would indicate a breakdown of the insulation.
Concern; 6000V motor, test voltage 11,700V:
The test represented by the blue curve did not shut the test down early because the max current at about 25µA did not reach the total leakage current limit, and the acceleration of the current was not high enough to exceed the chosen ROC limit of 2 (current Rate of Change between voltage steps).
However, the accelerating leakage current and the current level at the end of the test indicates weak insulation that is starting to break down. The current acceleration starts slowly at about 6,000V, so the motor condition is concerning, but still OK.
No arc was detected during the test since that would have shut off the test early. With a lower ROC limit, the test would have been shut off before reaching full test voltage.
Problem: 4160V motor, test voltage 9320V:
In this test, there is a sudden breakdown of the insulation between about 5.6 and 6.5kV, and the test is shut down at the end of step 7 at about 6500V because the acceleration in the measured current exceeded the ROC limit from step 6 to step 7.
With rapid acceleration of the current, the chance of an arc increases significantly, but that did not occur here. The breakdown is happening around the peak operating voltage of 5,882V. With this result the motor may be able to operate for a while, but is on its way to a complete breakdown.
In general, any deviation from a smooth curve should be viewed as a potential warning. A very abrupt drop in conduction current is rarely found, but when it occurs above the peak operating voltage for the winding, it may indicate approaching insulation failure. Mechanical abrasion and cracking may cause abrupt and unexpected insulation breakdown.