© surge test2016, values and diagnostics for itig ii … · source: electrom instruments, itig ii...

© 2016,

SURGE TEST VALUES AND

DIAGNOSTICS FOR ITIG II

Surge Comparison Tests

Pass/Fail Recommendations Three Phase AC Stators, Assembled Motors and

Generators June 2016

Jacob Beck

Kristoffer Torvik

Electrom Instruments

www.electrominst.com

[email protected]

+1.720.491.3580

http://www.electrominst.com/

mailto:[email protected]

Surge test values and diagnostics for iTIG II Contents

© 2016, Electrom Instruments, Inc. All Rights Reserved. June 2016 │ 2

Contents

1. Introduction – The Surge Comparison Test 4

1.1. What is a Surge Test used for? 4

1.2. How Surge tests work 4

1.3. Faults Not Found with a Surge Test 6

2. Pass/Fail Guidelines 7

2.1. Lead to Lead (Phase to Phase) Surge Comparison Tests 7

2.2. Pulse to Pulse (P-P) Surge Tests 7

2.3. P-P Failure Pattern in the bar graph 8

2.4. P-P % between 10% and 30% 9

2.5. Faults not found by a P-P Test 10

2.6. Alternative Test to find welded shorts in an Assembled Motor 10

3. Case Study and conclusion 11

3.1. Surge Test is non-destructive 11

3.2. Partial Discharge in P-P surge test 11

4. Appendix – Pass/Fail Guideline Tables 12

Surge test values and diagnostics for iTIG II List of Figures and Tables


List of Figures and Tables

Figure 1.1 Wave Comparison Test. Balanced Phases and Near-Identical Surge

Waves ..................................................................................................................... 5

Figure 2.1 Wave Comparison Test. Phase Imbalances - Surge Wave Separation 7

Figure 2.2 P-P Patterns. Failed vs Good. .............................................................. 8

Figure 2.3 P-P/Wave Comparison. Assembled Motor with Dampened Waves. .. 9

Figure 3.1 P-P Pattern. Case study of 5HP, 460V Stator .................................... 11

Table 4.1 Recommendations for Stators ............................................................. 12

Table 4.2 Recommendations for Assembled Motors .......................................... 12

Table 4.3 Recommendations for Pulse to Pulse Test – All Windings/Motors .... 12

Surge test values and diagnostics for iTIG II Introduction – The Surge Comparison Test


1. Introduction – The Surge Comparison Test

1.1. What is a Surge Test used for?

The surge test is used to find shorts or insulation weaknesses in coils, windings,

electric motors, generators, alternators and transformers. These faults are

typically turn to turn, coil to coil, or phase to phase. They can also be shorts to

ground, but IR and Hipot tests will clearly identify these.

Other problems that can be found include wrong connections, wrong turn counts

and some other issues in DC motor armatures.

Weak turn to turn insulation is the source of most winding failures. Because it is

the only test that can find weak insulation at elevated voltages, the surge test is

important for motor reliability and maintenance programs, for diagnosing

problems and for quality control.

Surge tests are done on individual coils, stators, wound rotors, and for fully

assembled motors and generators without the need to turn the rotor. See Figure

2.3

1.2. How Surge tests work

A set of fast rising pulses are passed through the coil or motor (DUT). The

voltage of the pulses depends on what voltage the test operator sets, or the

standard that is used. The test voltage can range from the RMS operating voltage

of the DUT to around 3.5 times the operating voltage of the DUT.

2E+1000V where E is the operating RMS voltage of the DUT is common and

recommended by the EASA AR100 standard.

The pulses produce a decaying wave in the circuit that includes the tester and the

DUT. The wave is compared to the wave from another coil or to the waves from

the other motor phases. All are displayed on the screen. See Figure 1.1

The frequency, shape, and amount or ringing of the wave (number of wave

peaks) depend on the resistance, inductance and capacitance of the DUT. A



highly dampened load such as a small assembled motor may only have one peak.

Rotors dampen the ringing. See Figure 2.3

Figure 1.1 Wave Comparison Test. Balanced Phases and Near-Identical Surge

Waves

Source: Electrom Instruments, iTIG II Screenshot

The waves will be nearly identical if the coils or windings are identical (have the

same impedance). If the motor is assembled (the rotor is installed) the rotor may

influence the inductance in each phase differently due to transformer action. If so

the waves for each phase will differ depending on the angular position of the

rotor. See 2.1, P-P surge test.

If there is a fault or insulation weakness, the wave for the faulty winding will

have a different frequency from the others and the zero crossings will shift to the

left. See Figure 2.1.

The iTIG II testers calculate and display the difference between pairs of

waves/phases (%WD). See 1-2, 2-3 and 3-1 percentages, also referred to as L-L

difference (L=Lead).

Good test, all 3 phases overlap, and

the WD %’s are very low, 0.3-0.8%.



1.3. Faults Not Found with a Surge Test

There are faults that can only be found with a surge test such as turn to turn

insulation weaknesses, but there are also faults that a surge test will not see.

These are:

Resistive connections internally or externally

All strands (round wires) not connected at both ends with several in hand

(several in parallel)

Wrong gauge wire in a coil or feeder cable

Partial Blowout of multi-wire random wound coils where some of the parallel

wires are still connected

The reason for this is that the surge test results are independent of the resistance

in the circuit.

Always do winding resistance measurements and comparisons unless this

information is available from other sources.

Surge test values and diagnostics for iTIG II Pass/Fail Guidelines


2. Pass/Fail Guidelines

2.1. Lead to Lead (Phase to Phase) Surge Comparison Tests

The general guideline for a good 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. The wave difference is 22% in

Figure 2.3. The difference can be significantly higher.

2.2. Pulse to Pulse (P-P) Surge Tests

This is a test done on one coil, or one phase of a motor. 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/phase is

compared to itself. The %WD is calculated at each step. The maximum %WD seen

by the tester is displayed on the screen. A bar graph of all the WD percentages for

each voltage step can be displayed by tapping on the P-P % number for each phase.

Since the comparisons are to “itself”, the position of the rotor in an assembled

motor test does not matter. The tests are independent of other phases or windings.

Figure 2.1 Wave Comparison Test. Phase Imbalances - Surge Wave Separation

Failed test on L2 (3-1 is good), P-

P also failed on L2 at 34%.




2.3. 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 or two tall bars above 30% at the point where the arcing started.

The P-P test is very useful for

testing the following:

Assembled motors: (rotor

installed) where the rotor

position influences 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.

Single phase motors or other

coils/windings where there is

no other winding to compare

to.

Any time a phase to phase

comparison is questionable.

Figure 2.2 P-P Patterns. Failed vs Good.


Failed P-P test. The large bar

indicates a flashover.

Good P-P test – low P-P%. No

large spikes in graph.



2.4. P-P % between 10% 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. See Figure 3.1.

Flash-over in weak insulation (arcs) is represented by one or two bars above

30%.

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.

Figure 2.3 P-P/Wave Comparison. Assembled Motor with Dampened Waves.


Test Passed, L-L% is high,

but P-P% is below the limit.



2.5. Faults not found by a P-P Test

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

it since there will be no change as the surge 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.

High precision resistance measurements may tell if windings have shorted turns.

If the number of turns measured are more than say 50 or 100, this also becomes

harder to interpret since the welded adjacent turns change the resistance by an

amount that may be less than the failure limit.

An Inductance measurement can also be used, but not in an assembled motor

with rotor influence.

2.6. Alternative Test to find welded shorts in an Assembled Motor

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.

Surge test values and diagnostics for iTIG II Case Study and conclusion


3. Case Study and conclusion A 5HP 460V stator from 1980 was taken out of service around 2005. This stator

remained in storage for ten years. In 2015 it was cleaned and blown out with high

a pressure air and used in a case study. The stator first arced when the surge test

voltage reached 8000V. It has since been tested repeatedly and the arc voltage

remains in the 7500V to 8000V range.

3.1. Surge Test is non-destructive

This is a good example of why surge tests are not destructive when done

properly. A 460V motor like this is typically tested to 1920V, not 8kV. The stator

withstand strength remains unchanged even after arcing.

3.2. Partial Discharge in P-P surge test

Partial Discharge starts a little below the test voltage, and can be seen in this

picture as the group of taller bars 10% to 20% high towards the high end of the

graph. Note that the L-L %WD is still in the good range.

Figure 3.1 P-P Pattern. Case study of 5HP, 460V Stator

Source: Electrom Instruments

Surge test values and diagnostics for iTIG II Appendix – Pass/Fail Guideline Tables


4. Appendix – Pass/Fail Guideline Tables

Table 4.1 Recommendations for Stators

WD% RESULT

L-L less than 8% Winding is good - Pass

L-L from 8% to

30% Do P-P tests

L-L above 30% Winding is bad - Fail


Table 4.2 Recommendations for Assembled Motors

WD% RESULT

L-L less than 8% Pass

L-L above 8% Do P-P tests


Table 4.3 Recommendations for Pulse to Pulse Test – All Windings/Motors

WD% RESULT

P-P less than 10% Winding is good - Pass

P-P 10% to 30% Check P-P bar graph

pattern

P-P above 30% Winding is bad - Fail


NOTE: Always do visual inspections, and other tests when possible,

before making final PASS/FAIL decisions.

Surge test values and diagnostics for iTIG II Appendix – Pass/Fail Guideline Tables


For help with interpretation of test results, contact Electrom Instruments.

Pictures of the screens with the waves and the bar graphs are very helpful.

Alternatively, send us the raw data folder for the motor.

Electrom Instruments

www.electrominst.com

[email protected]

+1.720.491.3580

http://www.electrominst.com/

mailto:[email protected]

© surge test2016, values and diagnostics for itig ii … · source: electrom instruments, itig ii...

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