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Surge Test Methods

Test and Measurement Methods

Surge Test

This page describes surge testing details, including types of surge testing, iTIG surge measurement techniques, test conditions, and causes of test results. For a general description of surge testing using the iTIG, see the Surge Test Summary.

Why Surge Tests Are Critical

Surge tests are critical because they are the only tests that finds turn-to-turn insulation weaknesses. These weaknesses start at voltages above the operating voltage of the motor and are precursors to serious failures and shutdown of a motor. Surge tests are also used to find hard shorts and a number of other mistakes in windings and coils.

Most winding failures, including shorts to ground, start with weak turn-to-turn insulation. Once the weakness causes turn-to-turn arcs, an electrical closed loop is created. Due to transformer action, current starts flowing in the loop. This current is dissipated as heat and creates a hotspot. More turns short out due to the hotspot and subsequently more heat is created. Eventually the winding shorts to ground.

Surge tests are also known as surge comparison tests when the result from a coil or phase is compared to the result from another coil or phase. Since coils are designed to be identical, the surge test results should be nearly identical. When windings or phases are not identical, or there is nothing to which to compare, operators use the pulse-to-pulse surge test.

Surge Test Uses

Capabilities

surgetest overview
Three nearly identical waves from a 3-phase motor.

What Devices Can Be Tested by a Surge Tester?

Any type of coil is testable – from tiny sensors, antennas, and actuating coils in relays or solenoids, to the biggest electric motors and generators. The surge test is a load dependent test so operators must consider test voltage standards.

Which Issues Are Found Only with a Surge Test?

A surge test is the only test that finds weak turn-to-turn insulation. This is due to higher voltages used in a surge test. Low voltage tests do not stress the insulation and consequently dielectric weaknesses are not found.

A surge test is the only test that finds weak coil-to-coil and phase-to-phase insulation.  A hipot test is sometimes used if coils and phases are hipot tested individually against the other coils and phases but doing so is not practical.

Lastly, some connection mistakes are only found with a surge test. An inductance test is sometimes used but only when the resistance is correct.

Why Industrial Motor Users Need a Surge and PD Tester

Failure Type

  • Turn-to-turn weaknesses and shorts
  • Coil-to-coil weaknesses and shorts
  • Phase-to-phase weaknesses and shorts
  • Wrong turn count
  • Wrong coil or group connections
  • Short to ground*
  • High Partial Discharge
  • Weaknesses to ground††
  • Resistive connections internally or externally
  • All wires not connected with several in hand
  • Partial blowout of random wound coils when there is no arc-to-ground or turn-to-turn
  • Wrong gauge wire in a coil or feeder cable
  • Found with a surge test
  • Sometimes found with a surge test
  • Not found with a surge test

Failure Type Notes

  • A short to ground can be found with a surge test, but should be found first with an IR or hipot test.
  • Weaknesses to a high partial discharge can sometimes be found with a surge test using P-P, but it is better to do a PD test.
  • Weaknesses to ground may be found with a surge test, but using a Hipot test is recommended.

How a Surge Tester Works

The Electrom iTIG winding analyzer, or surge tester, is an advanced high voltage source and measurement unit constituted by a programmable pulse train and advanced pattern analysis software. The instrument takes normal 115 or 220V AC power and transforms it to the higher voltage required for the surge test. The higher voltage is rectified into a DC voltage which charges up a large discharge capacitor.

The other side of the discharge capacitor is connected to the test load, the device under test (DUT), through the surge tester’s output leads. The DUT is also connected to ground in order to have a complete electrical circuit. In the circuit there are switches that are open during the charging phase of the discharge capacitor. These switches are insulated-gate bipolar transistors (IGBTs) or silicon controlled rectifiers (SCRs). The switches close some time after the discharge capacitor is at the desired voltage. The closing speed is very high resulting in a pulse with a rise time of approximately 100nsec.

The easiest way to think about what happens next is to envision the switches opening again, trapping the pulse energy in a tank circuit between the tester’s capacitors and the inductive device under test (DUT). An oscilloscope is connected to the tank circuit to capture the wave created by the surge pulse. As the voltage oscillates in the tank circuit it decays to zero volts because of impedance in the circuit. The scope sends the wave to a computer for in-the-loop processing and the wave is immediately displayed on a monitor.

iTIG Surge Tester in Use on Floor
iTIG Surge Tester in Use

Above: Electrom iTIG Winding Analyzer and Surge Tester in use.

How a Surge Tester Works

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The whole process of circuitry response, wave decay and processing is complete within milliseconds, upon which, successive pulses may be generated and analyzed in quick succession.
The description presented here is a very simplified explanation of the how the complex hardware and software of the Electrom surge tester works.

Lead Switching Matrix

A lead switching matrix in the surge tester connects the output leads to provide a complete electrical circuit. For a 3-phase stator this is usually between two phases. One phase is energized, the other phase, or other two phases are grounded. The result of the test is displayed on the screen. Next, one of the other phases is energized. Finally, the third phase is energized, which results in three unique waves, or signatures, on the surge test screen. Each step in the sequence can be done manually one at a time, or by a push of a button activating the matrix, all in an automatic sequence.

Each step in the sequence can be done manually one at a time, or by a push of a button activating the matrix, all in an automatic sequence.

Wave Difference (Error Area Ratio) Percentage Calculation

The three surge waves are compared and a percentage of wave difference (%WD), which is also referred to as error area ratio (%EAR), is calculated for each wave pair by the surge tester. The WD or EAR is calculated somewhat differently by different surge tester brands, but the result is similar.

The general approach is to calculate the difference between many points along a wave pair (points on the two waves with the same x-axis position), add up all the differences and divide by an average. Electrom Instruments uses a root mean square (RMS) equation to calculate the %WD. If the %WD is too high, the surge test failed.

For information on failure limits see Pass/fail Guidelines.

What Causes Differences in Surge Test Waves?

Using the Electrom iTIG, manufacturers can track and trend surge wave signature properties for any unacceptable changes or differences from the golden, known-good, master surge wave signature in each unique stator/coil/winding-based product. Relying on the tester for simple onsite assessment and to manage quality and heavy work loads in the service center, the leading industry MROs can more easily measure, maintain, recondition, overhaul, rewind and rebuild high value machines and critical components to like-new standard.

Why is the wave difference (WD) inspection test so important to sustain high quality, and what are the defects or degradation failure mechanisms that causes unacceptable changes and differences in surge test waves? Using a three phase application as an example, the three waves in the surge test should be close to identical if the three phases are designed to have the same impedance.

iTIG Screen Displaying Failed Line-to-Line Surge Test Results

In three phase machines designed with symmetric impedance, the wave frequency and zero crossings will be different in the phase(s) affected by a hard short, or if arcing is occurring due to weak insulation. The amplitude of the wave may also change.

However, not all machines are designed and manufactured with perfect balance between the phases. See Pulse-To-Pulse Surge Test for further information on how to avoid false positive results in concentric stators, assembled power transformers and other designs with asymmetric inductive coupling etc.

What Causes Differences in Surge Test Waves?

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The formula for the surge wave frequency is:

\mathrm{f}=\frac{1}{2\pi}\sqrt {\bigg  [ \frac{1}{LC} - \frac{R^2}{4L^2} \bigg ]}
\frac{R^2}{4L^2}

is small compared to

\frac{1}{LC}

so the equation reduces to:

\mathrm{f}=\frac{1}{2\pi \sqrt{LC}}

Inductance L

When the insulation is weak, a change in inductance will only show up at elevated voltages. Consequently, it will not be detected in low voltage tests and measurements. The surge test reduces false negative results, which is why the surge test is so valuable. If the surge test is done above the peak voltage the motor operates at, early warning of problems to come may be detected. The motor may still run fine for a while if the weakness shows up at a voltage above the peak sinusoidal operating voltage.

The inductance, L, will go down when there is a turn-to-turn short in the windings. L is proportional to the square of the number of turns in the winding. As L goes down the frequency, f, goes up. An increase in f shifts the wave’s zero crossings to the left on the surge test screen.  Likewise, if there are other problems in the windings that can be found with a surge test, the inductance will change. Connection problems, for example, may add or subtract inductance to the circuit. Or, they may cause the electric fields generated in the coils during the surge test to oppose each other, effectively reducing the overall inductance.

Capacitance C

The capacitance does not change much with most turn-to-turn shorts. However it can contribute to the change with more massive failures.

Resistance R

Note that resistance is not part of this formula, which is why surge tests do not detect differences in resistance or resistive connections. This is why accurate winding resistance measurements are important to do in combination with surge tests, as well as in combination with megohm and hipot tests which also do not detect winding resistance differences.

Analysis tools other than the %WD or EAR (error area ratio) exist. These are all mathematical tools. The %WD/EAR is the most common and best tool, along with the good old visual inspection, which should always be done when practical.

The Importance of Surge Pulse Repetition Rate

Most high voltage winding analyzers made by Electrom Instruments have 50Hz surge pulse generators. The surge pulse repetition rate typically varies from one brand of tester to another. The repetition rate for other testers can be as low as 1Hz or less. Surge tests with a high repetition rate are an advantage because they are superior at detecting weaknesses in insulation, as explained here.

The science behind why surge testers with higher repetition rates detect more insulation weaknesses than those with lower repetition rates at a given voltage has to do with ionization of gases in voids in the insulation. Microscopic voids exist in all insulation systems. A higher repetition rate creates a higher level of ionization in these gaseous voids.

The gas or air is ionized because of the electrical fields created by operating motors. Likewise, when a coil or motor is surge tested, electrical fields are generated, and they ionize the gas or air.

The Importance of Surge Pulse Repetition Rate

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When rotating machinery is running, the dielectric strength of the insulation surrounding the void must be strong enough to prevent an arc.

For example, normal air is a very good insulator. Ionized air on the other hand, is not, it is a good conductor. Consequently, when the gas in a void is ionized, it takes a lower voltage drop across the void to cause an arc between turns adjacent to the void. When rotating machinery is running, the dielectric strength of the insulation surrounding the void must be strong enough to prevent an arc.

Ionization of a gas dissipates very rapidly when the fast-changing electrical field creating the ionization is removed, i.e. after the surge pulse has passed. Higher surge pulse rates therefore maintain a higher level of ionization. As insulation is weakened, or is damaged somehow, a high frequency surge tester will find more insulation weaknesses, or find them at a lower voltage. This is confirmed in lab tests, and also reported by many customers who have testers operating at different surge pules rates.

Electrom’s proprietary technology uses high frequency surge pulses. It not only finds more insulation weaknesses, but automatically gets to the surge test target voltage quickly. This may seem like an easy task, but requires complex algorithms since every load or DUT reacts differently to a surge pulse. A different internal discharge voltage is required to reach the target voltage in each DUT. The result of Electrom’s proprietary technology is a very quick set of 3 surge tests for a 3-phase motor.

Compromises must sometimes be made. The higher the surge test voltage, the longer it takes to charge the discharge capacitor in the surge pulse generator, or the bigger and heavier the power supply has to be. Portability, size and weight are obviously important. Therefore, lower surge test pulse rates are sometimes used in exchange for a tester with lower weight, a smaller physical size and consequently lower cost.

Pulse-To-Pulse (P-P) Surge Tests When Windings Are Different

Assembled Motors, Concentric Windings, etc.

Motor windings and coils are not always physically identical. An example is concentric wound coils where the size of the coils are different. Another example is when coils are connected such that the stator is not symmetrical from an inductance point of view.

Assembled motors (with the rotor installed) may have stators that are lap wound and electrically symmetrical. However, the rotor may cause the inductance in the stator phases to be different because of the transformer action between the stator and rotor.

To address these situations the Pulse to Pulse Surge Test was designed.

When Should the Pulse-To-Pulse Test Be Used?

  1. Concentric wound stators with a %WD above expectations
  2. Assembled motors (rotor installed) with rotor influence on the stator inductance that creates different surge test waves in the 3 phases
  3. Single phase motors
  4. Any test where there is nothing to which to compare the standard surge test
  5. Any time the surge test results are questionable or in the “gray zone”
  6. To find the inception voltage in Partial Discharge (PD) Measurements 

Pulse-To-Pulse (P-P) Surge Tests When Windings Are Different

Assembled Motors, Concentric Windings, etc.

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The pulse-to-pulse surge test should be used in combination with accurate winding resistance measurements any time it is uncertain whether the standard surge test result is a pass or fail.

iTIG Displaying Good Pulse-to-pulse Surge Test Results
iTIG displaying good pulse-to-pulse surge test results.
iTIG Displaying Failed pulse-to-pulse Surge Test Results
iTIG displaying failed pulse-to-pulse surge test results.

How the P-P Surge Test Works

Instead of rapidly going to the surge test design voltage and recording the surge wave after the voltage is achieved, the P-P surge test raises the surge voltage in small steps up to the design test voltage. At each step a surge wave is recorded, and the difference between this wave and the wave from the previous step is calculated. This means that the winding under test is compared to itself as the surge test voltage is raised. Since the comparison is to itself, it does not matter that the phases are different, or that there is rotor influence in an assembled motor. Hence for an assembled motor the rotor does not have to be turned during the test.

When the pulse-to-pulse surge test is completed the P-P %WD numbers for all voltage steps are stored, and the highest P-P %WD displayed. With the iTIG surge tester the results from all the voltage steps are available in a bar graph.

If there is an arc or flash over at some voltage step, the P-P %WD will be significant for that step because the wave frequency is different from the previous step. For information on failure limits see Pass/Fail Guidelines.

Since the P-P surge test is looking for a change during the voltage ramp, it may not see hard shorts in the windings if the insulation around the hard or welded short is stable and strong. In such a case there is no change or arc during the voltage ramp. Accurate micro ohm winding resistance measurements or inductance measurements may be used to find such faults.

The other alternative for assembled 3-phase motors is to turn the rotor between each surge test so that all 3 waves line up. If they cannot be made to line up properly, there is a failure. Turning the rotor may not always be possible or practical making the P-P surge test a valuable tool.

What Can Be Done When the Recommended Surge Test Voltage Cannot Be Reached?

The surge tester is rated based on the maximum voltage it can generate internally (12kV for example). This rated voltage is not the highest voltage generated in every device under test (DUT). The voltage that results in the DUT from any surge pulse voltage applied by the surge tester depends on the characteristics of the DUT. For example: 12kV may be generated by the surge tester, but 9kV may be the voltage reached in a large motor. This means that if a motor is “too large” for the tester, the desired test voltage will not be reached. It also means that if the inductance in the DUT is “too low”, such as in a Surge Test of Large Motorsingle low inductance form coil, the desired surge test voltage will not be reached. Click here for information on low inductance surge tests of coils and DC motor armatures.

The energy required to reach a given surge test voltage in a DUT increases with several factors including the line to line operating voltage, HP or kW, frame size and number of poles. The capacitance of a motor plays a significant part in DUT peak voltage as motor capacitance must be overcome by the energy available from the surge tester. This is one reason why frame size matters. Likewise, when motors are tested from a motor control center, the capacitance in the power cables is added and the voltage reached in the motor will be less.

What Can Be Done When the Recommended Surge Test Voltage Cannot Be Reached?

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The energy available from the surge tester is:

\mathrm{E}=\frac{1}{2}{CV^2}

For guidelines on the motor sizes Electrom Instruments surge testers can handle, please contact us.

Surge Test of Large Vertical Motor

If a surge tester cannot reach the desired surge voltage for the DUT, options for getting to a higher voltage are to use a tester with:

  1. Higher charge capacitance
  2. Higher max output voltage
  3. Both 1 and 2

A Power Pack can be added to any iTIG surge tester taking the max voltage to 18kV, 24kV, or 30kV. Power Packs have their own output leads but are controlled by the surge tester. Electrom Power Packs can be added at any time. They are calibrated independently and can be used with multiple iTIG surge testers.

Another alternative for getting to a higher voltage in a large DUT is to break the DUT down into individual phases, or, if possible, individual groups. The smaller the section that is energized by the surge tester, the higher the voltage will be. When a DUT is broken down for testing, portions of the DUT that are not being energized should be shorted to ground.

The final alternative is to test to whatever voltage the surge tester can reach. You may be able to record results obtained at a voltage higher than the peak voltage normally seen by the motor while operating.

(Peak = RMS voltage x 1.41)

In this case, while you cannot know the condition of the motor at the voltage recommended in the standard, you will still get data important for finding weak insulation before it degrades to the point where turn to turn failures occur at the operating voltage of the motor.

Surge Test of Large Vertical Motor

Low Inductance Surge Tests

Generally, low inductance coil/winding applications are those with a low turn-count and an inductance below a value between 20µH and 40µH. Some examples of typical low inductance Devices Under Test (DUTs) include DC motor armatures, interpoles, form coils with one or a few turns.  The lowest applicable inductance depends on how accurately the voltage in the DUT is to be measured by the surge tester.

The challenge with these coils is that there is almost no impedance in the test circuit, i.e. the instrument output is nearly shorted. A very high amount of current is required to generate a voltage across such a low impedance. This is by design since these types of coils are made for high current. The surge tester may not have sufficient current available to reach the desired test voltage if the impedance is too low.

The other challenge with low inductance is that the output lead inductance of the surge tester may compete with the inductance in the DUT and therefore create a significant voltage divider. This means that the voltage generated and displayed by the surge tester will be partly dropped in the output leads, and partly across the DUT.

Low Inductance Surge Tests

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iTIG Armature Bar-to-Bar Surge Test Accessory

To solve these challenges when voltages up to 1400V are required, Electrom uses a “booster box.” The Electrom Instruments ABT connects to the iTIG surge tester. The output voltage from the surge tester is stepped down in the ABT, and its output current increased. The ABT is a 4-wire system, which eliminates the effect of the lead inductance. The iTIG surge tester measures and displays the real voltage across the DUT. Learn more about the ABT.

When higher voltages are required, a tester with higher output voltage and energy may have to be used. It is also possible to connect two or more coils in series to increase the inductance to make it easier to reach the desired voltage.

For more information on specific applications, contact Electrom.

Old, brittle, or otherwise damaged winding insulation is more likely to carbon track. Because cracks in the damaged insulation may become contaminated, they can provide “food” for carbon tracking.

Are Surge Tests Destructive?

Since surge testers find weak windings by detecting an arc from one turn in the winding to another turn, people sometimes ask what such an arc does to the insulation. Even if there is no arc, they may wonder whether surge test pulses weaken the insulation.

The question also comes up due to misconceptions, some of which are addressed below.

  • Most winding insulation failures start as a turn-to-turn weakness, which can be found before it is too late only with an over-voltage surge test. Because the insulation still has some dielectric strength, the flaw cannot be found with low voltage measurements.
  • Over-voltage surge tests are not destructive when done correctly. The voltages used for testing are far below what a motor is designed to handle both when new and old. See results of 8000V surge tests on an old 460V motor below.
  • Motors typically see voltage spikes routinely during normal operation. These routine spikes are significantly higher than the peak operating voltage and often higher than the surge test voltage.

With Electrom iTIG motor testers the surge test process is automatic. The voltage and number of surge test pulses are controlled by the tester rather than the operator.

Are Surge Tests Destructive?

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Winding Failures

The root cause list for weak insulation is long and includes age, over-heating, vibration, large frequent voltage spikes, voltage spikes from variable speed drives, humidity, dirt and grease or oil where it should not be, and manufacturing defects to mention some.

It is estimated that about 80% of winding insulation failures in motors are the result of turn-to-turn weaknesses or shorts.  A turn-to-turn failure typically starts with a turn-to-turn insulation weakness that progresses to a short.  The turn-to-turn short then progresses to multiple shorted turns, and eventually to a ground fault that is listed as the reason for the failure. This progression happens because a shorted turn creates a loop in which current is circulating, and the energy is transferred to heat. The heat weakens the insulation further and eventually excessive heat will cause a ground fault.

Finding Insulation Weaknesses

Windings and coils have to be tested to a voltage higher than the operating voltage of the equipment to find weaknesses before it is too late and the motor has a catastrophic failure.  This is what a surge tester does. A weakness is a condition where the motor runs for the time being, but has a “defect” that will lead to electrical failure.

A turn to turn arc caused by the higher test voltage is the result of an insulation weakness that will eventually lead to a catastrophic failure.

Normal Voltage Spikes During Operation

Motors see steep voltage spikes during normal operation all day long. They come from breakers and contactors being opened and closed, starters, power circuits being switched, VSDs or inverter drives etc. They can also come from lightning strikes, faulty breakers or malfunction of switching gear. Many of these spikes can be much higher in voltage than what is used during a surge test.

Use of Surge Testers versus Low Voltage Testers

Surge testers have been used in some form or another since the 1950s and became common in the 1990s.  They are used worldwide in increasing numbers by the vast majority of motor shops and motor and coil manufacturers. They are considered critical to their operations.  Industrial users of motors and generators use surge testers for predictive and preventive maintenance purposes and reliability programs.  Not finding turn-to-turn, coil-to-coil or phase-to-phase weaknesses or faults result in critical motors failing while in operation with major ramifications, both financial and other.

Low voltage measurements and tests are used by industrial end users to analyze and monitor the condition of rotating equipment.  Some argue that they can be a substitution for surge tests.  This has been proven wrong by people using both low voltage and high voltage test equipment. Low voltage measurements are very useful in various applications, but they can be difficult to interpret, and they can produce false results when used to find winding insulation weaknesses. There is no substitute for tests performed at high voltages. Weaknesses are found that simply cannot be detected or measured at low voltages.

Over-Voltage Surge Tests Are Not Destructive When Done Correctly

When an inter-turn weakness is found, there will be arcing or discharges from one turn to another.  If the surge test is run for too long (too many pulses are applied) while arcing, carbon tracking could result and the arc-over will occur at a lower voltage next time.

  • This is not the way surge tests should be done. The surge voltage should be taken up to the test voltage fairly fast and the test should be terminated quickly. The IEEE 522 standard requires a minimum of 5 pulses at voltage.
  • All Electrom Instruments iTIG Surge Testers automatically control and limit the surge voltage and the number of pulses using our proprietary technology.
  • If a weakness is detected, the insulation is not good in the first place. The test has served its purpose and found a fault or weakness during testing. The motor should be scheduled for repair or replacement or considered for a rewind.

Results from an Arc Test Experiment

In one typical example Electrom repeatedly tested a 35 year old used 460V 4-pole motor (made in 1980, picture below).  Normal test voltage for surge and DC hipot for this motor is 1920V.

  • The surge test voltage was raised until the windings arced. Arc voltage: 8000V
  • 5 tests were done before the arc voltage dropped to 7500V.
  • This was followed by 4 tests that did not arc at 6500V.
  • DC hipot tests were done successfully to 7000V after the surge tests.

Bottom Line

A surge test does not shorten the life of a winding because the test pulses produce only a small percentage of all the spikes a motor is designed to absorb during its lifetime. Also, the surge test voltages are far below those for which new and used windings are designed.

If a weakness is found, the tester has served its purpose of detecting a serious defect in the motor that will lead to a catastrophic failure later unless the situation is fixed.

If high voltage surge testing were proven to cause a problem, or motors’ lives were shortened as a result of the stresses from surge testing, the practice would have been discontinued long ago.

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