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Pass/Fail Guidelines

Pass/Fail Guidelines

This page describes motor test result pass and fail guidelines. To learn more about how the iTIG is used for surge comparison tests, see Surge Test Summary and Surge Test Methods.

Using Electrom Instruments proprietary technology, manufacturers can track and trend surge wave signature properties in each unique stator/coil/winding-based product. Leading industry MROs rely on Electrom diagnostics for simple onsite assessment and use the surge test report as a quality stamp for overhauls performed at the service center. How are Electrom’s wave difference (WD) test results interpreted in the various applications and designs?

Lead-To-Lead (L-L) or Phase-To-Phase Surge Comparison Tests

The general guideline for a good, passing test result is that the L-L %WD should be less than 10%. However, there is a “gray area” around 10% for lap wound stators. Concentric wound stators may have differences up to 20%, sometimes even more. Many assembled motors, especially fast running small motors, may have big differences because of influences by the rotor.

Pulse-To-Pulse (P-P) Surge Tests

This is a test done on one coil, or one phase of a motor at a time. The surge voltage is automatically raised in small voltage steps, and the surge wave at each voltage step is compared to the wave from the previous step. This means that the coil or phase is compared to itself. The %WD is calculated at each step. The maximum %WD detected by the tester is displayed on the screen. A bar graph of all the WD percentages for each voltage step can be displayed on the tester and included in test reports. Since the comparisons are from the device under test to itself, there is no comparison to another phase so the position of the rotor in an assembled motor surge test does not matter. The tests are independent of rotor position, other phases, and other coils.

Note: Always do visual inspections and, when possible, other tests before making final pass or fail decisions.

Recommendations for Stators
%WDResult
L-L < 8%Winding is good—Pass
L-L = 8%-30%Do P-P test
L-L > 30%Winding is bad—Fail
Although L-L WD% < 10% is normally used as “pass,” 8% is a more conservative limit that is better for motors with a high turn-count.
Source: Electrom Instruments Inc.
Recommendations for Assembled Motors
%WDResult
L-L < 8%Pass
L-L > 8%Do P-P test
Source: Electrom Instruments Inc.
Recommendations for Pulse-to-Pulse Test—All Windings and Motors
%WDResult
P-P <15%Winding is good—Pass
P-P = 15%-30%Check P-P bar graph pattern
P-P > 30%Winding is bad—Fail
Source: Electrom Instruments Inc.
Figure 1 Failed surge test L L and P P 1536x1152 1
Surge test failed because pulse-to-pulse wave differences exceed limit set as threshold in iTIG.

P-P Failure Pattern in the Bar Graph

When there is an arc turn to turn in a winding, the bar graph will usually have one to three tall bars above 30% at the point where the arcing started and above.

The P-P Test Is Used and Useful in Many Service Situations

The P-P test is often used as a tie-breaker when the L-L %WD is elevated, but not conclusive. Here are some examples of when the P-P test is useful:

  • Assembled motors: (rotor installed) where the rotor position influences the inductance in the stator coils, and therefore the surge waves.
  • Concentric wound motors: They sometimes have a large separation of the surge waves indicating that there might be a turn to turn fault (arc). The P-P test will determine if there is an arc or not. The difference in inductance, and therefore the surge waves, is due to a lower level of symmetry in the construction of the stator compared to lap wound stators.
  • Single phase motors or other coils/windings where there is no other -coil to compare to.
  • Any time a phase to phase comparison is questionable.

Note: Electrom’s iTIG software algorithm automatically replaces the L-L pass/fail result with the P-P test pass/fail result.

P-P Percentage Values Between 15% and 30%

Pass/fail interpretation in two common cases of elevated levels of P-P:

  • If there are a few bars at the beginning of the graph (left side – low voltages) up to 30%, the result is a pass.
  • If there are many bars on the right side of the graph (high voltages) that are up to 30%, Partial Discharges (PD) have been detected.
  • Flash-over in weak insulation (arcs) is typically represented by 1-3 bars above 30%.
Figure 2a Failed pulse to pulse surge test 1024x7682 1
Failed P-P test: the large bar indicates a flashover.
iTIG Displaying Good Pulse-to-pulse Surge Test Results
Surge test passed: %WD in L3 is large, but the pulse-to-pulse result is good. Notice there are no large spikes in the bar graph.

Note: The P-P test is not designed to be a PD measurement and does not accurately detect when PD starts. But with enough PD, several higher bars will usually be present in the P-P bar graph.

The presence of PD does not necessarily mean there is a failure or problem, but can mean that the insulation is starting to weaken, especially in low voltage motors. Around 30% and above, the PD typically turns into an arc. Click here for more information on PD.

iTIG Assembled Motor Surge Test Results
Test passed: Line-to-line %WD is high, but pulse-to-pulse is below the failure limit.

The P-P test looks for a change in the waves as the surge voltage rises. Therefore, if there is a hard short, or welded short turn-to-turn in a winding, and at the same time the insulation around this short is stable and strong, the P-P test will not find the fault since there will be no change as the surge test voltage rises.

This fault can be seen with an L-L surge test. But, if the motor is assembled, one cannot usually tell if it is rotor influence or a fault unless the fault is severe, or one knows that there is no rotor influence in this motor.

High precision resistance measurements may tell if windings have shorted turns. If the number of turns measured is more than about 50 or 100, measurement also becomes harder to interpret since a welded adjacent turn changes the resistance by an amount that may be less than the failure limit. Sometimes shorted turns in random wound motors are not adjacent but a few turns away from each other. The resistance difference then is often easy to see with a high precision microohm measurement, such as what is available in some Electrom iTIG models. Resistance measurements tracked over time can make the determination of good or bad much easier.

An inductance measurement can also be used, but not in an assembled motor with rotor influence.

If it is possible to turn the rotor between surge tests of the 3 phases, this can help find a welded short. The rotor must be turned between tests so that the waves line up. If they do line up there is no problem. If the minimum difference attained is significant, there is a problem. Beware of potential rotor problems when doing this test.

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