Importância do SURGE-Test

8/13/2019 Importncia do SURGE-Test

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Forget the controversy. These tests clearly are non-destructive in nature.

Understanding the advantages these methods have over others can make them

mighty powerful tools in your PdM program.

Before any company investigates electrical predictive maintenance (PdM)

instrumentation, it should know the strengths of its equipment's insulation, thevoltages its motors are exposed to daily, how a motor typically fails and where

these faults typically exist. Only then can you really make a decision as to which

electrical PdM equipment is the most appropriate for your operations.

How a motor typically fails

The motor stator has two main insulating systems that include the ground wall and

turn-to-turn insulation. When this insulation is in a good condition it can

withstand the normal day-to-day voltage spikes that exist during starting and

stopping. Over time, this insulation will deteriorate as a result of mechanical

movement of the windings, torque transients, heat, contamination, and other

environmental contaminates. Once the dielectric strength of this insulation fallsbelow the incoming voltage spikes, another failure mechanism is introduced:

ozone.

Ozone is a very corrosive gas that will quickly deteriorate insulation. Although the

motor will continue to run when this failure mechanism is introduced, as it sees

continual voltage spikes, the deterioration rate will accelerate. Eventually, the

dielectric strength of the insulation will fall below operating voltage or deteriorate

to the point that copper wire will touch turn-to-turn. At this point a turn-to-turn

or hard welded short has developed.

According to "Transient Model for Induction Machines with Stator Winding Turn

Faults" written for IEEE by Rangarajan M. Tallam, Tom G. Habetler and Ronald

G. Harley, when a hard welded turn-to-turn short develops, the shorted windings

will develop high circulating currents. These currents, which can be in the order of

1620 times full-load amps, create excessive heat that the insulation cannot

withstand. This intense amount of heat will burn quickly through the insulation-

causing motor failure within minutes.

A study performed at Oregon State University, by Dr. Ernesto Wiedenbrug,

looked at a motor specially designed with a turn-to-turn fault by installing two

wires connected to turn one and turn two of the same phase. These wires were thenbrought out to a switch. The motor was placed on a dynamometer and run at

about 80% load. When the turn-to-turn short was engaged through the switch, the

motor began visibly smoking within 45 seconds. While most motors will not run

for long with a turn-to-turn short, some exceptions do exist. A motor with a high

resistance or floating ground will run with a shorted phase, but once a second

phase shorts, the motor will fail catastrophically.

Recommended tests The tests listed on the next page are recommended in off-line

field testing:

Kelvin Method Winding Meg-Ohm


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Polarization Index (PI) Step-Voltage Surge

Each of these test methods evaluates a different section of the motor. Brief

descriptions of the first three tests are given in order to offer a complete array oftesting information. The nature of high-voltage testing and the necessity of the

Step-Voltage and Surge methods, however, remain the main focus of this article.

Kelvin Method Winding...

The Kelvin Method Winding test measures the resistance of the copper wire of the

motor circuit. If tested in a PdM application, the test is typically performed from

the Motor Control Center (MCC). This test finds issues with miss connections,

shorts, opens, unbalanced turn count in one phase to another and different size

diameter copper in one phase to another. This test is very valuable and should be

performed for predictive maintenance, troubleshooting and quality assurance.

Meg-Ohm Test...

The Meg-Ohm Test applies a DC potential (typically operating voltage) to the

windings while holding the case to ground. Table I shows the recommended test

voltages for different voltage class motors. Meg-Ohm testing is typically utilized to

find grounded motors. It also is a very valuable PdM tool for finding wet and dirty

motors. It's not typically used for quality assurance because of the low voltage level

at which the test is performed.

Polarization Index (PI ) Test...

This test is much like the Meg-Ohm Test, but it is performed for 10 minutes. Over

this time period, the molecules in the slot liner paper polarize. When the molecules

polarize, the insulation resistance values should increase over the10- minute

period. If the resistance increases during this time, it's an indication of good

ground wall insulation with no moisture or contamination.

Insulation testing

Until now we have only discussed the low-voltage tests. Upon successfully

completing these tests the following is known: the winding resistance is balanced.

That means the motor has no shorts, opens or miss connections and the Meg-Ohmand PI indicate that the motor is both clean and dry. These tests, however, still


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have not confirmed that the motor is capable of starting or running for any length

of time. The main reason for performing predictive maintenance on a motor is to

learn if it will continue to provide uninterrupted service. Because low-voltage

testing is not performed at the voltage a motor typically sees, it can't provide this

information.

Many articles have discussed the voltage spikes motors see during starting and

stopping. As stated in "Turn Insulation Capability of Large AC Motors, Part I

Surge Monitoring," by B.K. Gupta, B.A. Lloyd, G.C. Stone, and S.R Campbell

(IEEE Transactions on Energy Conversion, Vol. EC-2, No. 4, December 1987),

these voltage spikes can be in the order of 5 PU (Per Unit):

Calculating this formula for a 480V three phase motor, the PU would be 391.9

volts, or approximately 1960 volts on startup. Logically, if the motor is tested to

only operating voltage or below the operating voltage, the user can not be sure if

the spikes have caused damage to the motor's insulation that will interrupt service.

The other issue is that the turn-to-turn insulation has not been evaluated. In

addition, the Meg-Ohm and PI do not evaluate the ground wall insulation for

strength or the ability to withstand the high voltages it sees during daily operation.

The winding resistance test is only evaluating the motor circuit and not the

insulation.

The most effective way to ensure the motor will start and continue to provide

reliable service is to test it at the voltages the motor sees during normal operation-

which includes starting and stopping. This is accomplished with two tests: Step-

Voltage and Surge. These methods evaluate the ground wall and turn-to-turn

insulation respectively.

Step-Voltage Test

This DC Test is performed to a voltage that a motor typically sees during starting

and stopping. The test voltages, governed by IEEE, are reflected in Table II.

The DC voltage is applied to all three phases

of the winding and raised slowly to a preprogrammed voltage step level and held

for a predetermined time period. It is then raised to the next voltage step and held

for the appropriate time period. This process continues until the target test voltage

is reached. Typical steps for a 4160V motor are 1000-volt increments, holding at

minute intervals. For motors less than 4160V, the step voltages should be 500 volts(see Fig. 1).


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During a Surge Test, the equipment will charge up a capacitor inside the unit and

dissipate it into one phase while holding the other two phases to ground. Then,

automatically, the test unit will slowly increase the voltage from 0 volts to the

target test voltage. This generates a waveform, in a shape based upon theinductance of the coil that is displayed on the test equipment screen. If the target

test voltage is attained without any frequency change in the waveform, the turn-to-

turn insulation integrity has been realized. Fig. 2 is a graphical representation of

the waveform at one-third, two-thirds and full voltage of one phase. This is what a

waveform will look like when the insulation is in a good condition.


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If, at any time, the test equipment sees weak insulation between the turns, the

waveform will shift to the left as shown in Fig. 3. The white line on the graph shows

the failed waveform at about 1000 volts.

Surge testing theory

When the capacitor is discharged into the winding, it is performed at a very fast

rise time (.1 micro second). This produces a nonlinear voltage drop across the

turns, producing a potential difference between the turns in succession. As the rise

time slows, the operator will notice that the voltage potential difference between

the turns is dramatically reduced. This is in contrast to any other signal utilized to

diagnose motor issues. No DC test (or AC tests such as an inductance, capacitance,

impedance, phase angle or HiPot) will produce this potential difference between

the turns.

Physics provides us with Paschen's Law, which states that two bare wires placed

next to one another just a thickness of a hair away need a minimum of 325 volts to

jump the air gap between the two conductors. These two concepts are the core

reason why Surge testing is the natural choice for testing turn-to-turn insulation.The main reason is that if the test equipment doesn't produce a potential difference

between the turns above Paschen's Law, the current cannot flow through the fault.

If current can't flow through the fault, it will continue through all the coils and not

show a difference.


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When Surge Testing a coil with weak insulation turn-to-turn, the voltage applied

can jump across the weak insulation. Removing these bypassed turns from the

circuit reduces the inductance of the circuit and causes the waveform frequency to

ring faster. This will produce the frequency shift to the left in the waveform.

Fortunately, advancements in technology have led to refinements in the analysis ofwaveforms, to the point that some test units automatically recognize failures (see

sidebar).

Surge comparison In the past the Surge Test has been called a "Surge Comparison

Test." Although some individuals believe the Surge Test still needs to be

performed in this manner, it really depends on what is being analyzed.

For finding weak insulation, surge comparison is not necessary. As previously

noted, weak insulation is diagnosed by a frequency shift to the left and is compared

to successive waveforms within one phase. If, however, the following list reflects

problems you're seeking to uncover and eliminate, a comparison of each phase is

recommended.

Shorts Opens Different size diameter copper between phases Unbalanced turn count between phases Reversed coils Shorted laminations

Here again, as referenced in the accompanying sidebar, instrumentation thatautomatically detects these problems is now available.


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Older vs. newer equipment

Just like computers, high-voltage test equipment has changed vastly over the past

20 years.

Today's equipment incorporates modern, high-speed electronic evaluation of

changes to resistance, leakage current, leakage current versus time, voltage, step-voltage, dielectricabsorption, frequency response, wave shape, corona inception

voltage (C.I.V.) and more to detect faults at or under the levels of energy exposed

to the motor during operation. Microprocessor- controlled instantaneous trips

allow winding conditions to be evaluated without compromising dielectric

integrity. Moreover, the addition of field-developed PASS/FAIL test criteria now

makes this testing extremely repeatable.

One of the greatest advances in high-voltage testing has come from via solid-state,

highvoltage power supplies replacing the heavy step-up transformer. This has

resulted in big improvements to equipment portability. Every test is now digitized

and compared to the previously applied pulse. If any weakness is detected, the testis instantaneously stopped, preserving dielectric. The level of weakness is stored

for future reference, in the memory bank.

What to look for

When evaluating electrical PdM equipment, keep in mind that every manufacturer

is slightly different. Test units, though, should be able to perform the following

safety checks to ensure that your motors aren't damaged during testing:

1. Acceptable Meg-Ohm readings should be obtained.2. Acceptable PI Test should be performed.3. The test unit should evaluate the Meg-Ohm readings at the end of each step.

If the motor does not meet the criteria the test set should automatically stop

the test.

4. Current leakage should be monitored continuously and the unit shouldautomatically stop the test if an over current leakage condition exists.

Typical over current trip settings are 1, 10, 100 and 1000 micro amps of

current leakage.

5. Micro arc detection is crucial; if the test sees a tiny arc the unit shouldautomatically stop the test.

6. Real-time display on the screen is a must; this allows the operator to see thevoltage and current while the test is in operation. If the operator sees anyabnormal condition, he/she can stop the test.

Case study: Step-Voltage testing

Exelon Nuclear, Limerick Station

The Station Predictive Maintenance program at Limerick routinely performs

electrical testing of large motors at a two-year frequency. This testing consists of

winding resistance, insulation resistance, PI Capacitance/dissipation factor and DC

step-voltage testing to 20kV. The resulting data has been tracked and trended for

almost 20 years.

On a few occasions during 2002, Operations personnel reported that an "acrid"odor was present at the 1C Circulating Water Pump Motor. The PdM group had


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been tracking this motor on a "watch" list that came about as a result of an

increasing trend in leakage current detected by DC step-voltage testing from 1997

to 2002 (see Fig. 4).

As part of its increased troubleshooting activities, the Limerick Station PdM team

monitored the motor through the summer of 2002, utilizing acoustic monitoring

and vibration and winding temperature/RTD monitoring on a monthly basis. In

September 2002, an action request was made to replace the motor in the winter,

based upon the electrical testing results, increasing vibration at stator slot

frequencies and higher acoustic/ultrasonic "noise."

Once the motor was removed, it showed high leakage current on the "A" phasemotor winding compared to the other two windings. After cleaning, a visual

inspection of the winding identified partial discharge at the junction where the

core slot winding tap transitions to the end winding/knuckle tape. Investigation

revealed a lack of "proper" corona suppression tape at this critical junction point

in the winding.

Among the lessons learned from this event was the fact that tracking and trending

leakage current versus applied voltage on a DC Step- Voltage Test, as presented by

the Baker AWA offline tester, can and does indicate potential problems in the

winding. Furthermore, when this data is combined with other predictive

technologies, it will allow for proactive replacement of a motor prior to an in-service failure.


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Case study: Surge testing

Pulp & Paper OperationA 2300V form wound motor at a pulp and paper plant was found to have weak

turn-to-turn insulation. Of all the tests performed on this motor, the only one that

found the turn-to-turn weakness was the Surge Test. The controversy around

surge testing, though, is that after finding a problem with insulation, could the

tester have so degraded the motor that it would not run?

This Pulp & Paper industry case

study easily puts this myth to rest. The motor in question was immediately put

back in service after testing. It was started up and ran for the four months

required until it could be shut down and removed for repair. Again, as noted inFig. 5, the Surge Test was the only method to identify the insulation weakness. The


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problem was well above line voltage, so other lowvoltage tests would not have

approached this threshold. (The surge summary in F ig. 5 highl ights the faul t

weaknesses found with the tester.)

This particular Pulp & Paper site motor takes about 6-7 hours to change. Thus, it

could have cost about $42,000 in downtime had the Surge Test not found theproblem. Interestingly, 80% of all electrical motor failures begin with weak

insulation turn-to-turn. The Surge Test is clearly the best method available to find

this problem. That's why it is so important to perform this type of non-destructive

testing on all motors.

Summary

The Step-Voltage and Surge Tests are necessary for an effective PdM program.

They identify problems that low-voltage tests can't find.

As the case studies in this article have shown, both of these tests are non-

destructive in that the tested units were returned to service until the next availabletime could be scheduled to replace them.

Finally, these tests are performed at voltage levels a motor is exposed to during

normal operation. If a motor cannot pass the Step-Voltage and Surge Tests, you

can bank on the fact that it is approaching the end of its service life. Consequently,

provisions should be made as soon as feasibly possible to have that motor removed

before unscheduled downtime occurs.

Joe Geiman holds a B.S. from Colorado State University in I ndustr ial Technology

Management. He travels extensively within the Western and Southeastern regions of

the Uni ted States and has tested and analyzed hundreds of motors for a var iety of

industr ies. Telephone: (800) 752-8272 or (970) 282-1200; e-mail :

[email protected]
mailto:[email protected]:[email protected]:[email protected]

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Importância do SURGE-Test