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TecTalkElectrical Testing of Uncured

Resin Rich or Untreated VPI CoilsHal Miller and Kevin Alewine

Spring 2007

One of the most common questions posed tomica tape suppliers is how to electrically testcoils during the manufacturing, winding and

final test stages during motor manufacturing or re-pair. For completed machines, NEMA MG-1, IEEE

43 and IEEE 95 address the issue clearly, but forgreen, untreated or uncured coils intended for vac-uum pressure impregnation (VPI) or resin rich hotpressing, there are really no official standards. Eachmotor OEM, coil manufacturer or repair center hasits own procedures, but there are always issueswhen more failures than expected arise during thetesting process. In this edition of TecTalk, we willaddress some general guidelines that have beenused in the USA for many years and make some

suggestions from a tape manufacturers perspective.Most technicians and engineers recognize that

a VPI insulation system is very dependent on theresin impregnation process to realize the full proper-ties, thermal, electrical and mechanical, that are ex-pected of the system. To expect an untreated coil to

be held to the same standards as a finished systemis unrealistic and can lead to false conclusions aswell as premature motor failure even if the coil suc-cessfully passes the hipot testing. Incipient damageor pregnant failure can result in an unexpectedly

short service life of the motor. It is difficult to tracethe failure back tothe testing proce-dure as the dam-age is normallyworsened duringthe failure due toadditional carbontracking or, in thecase of catastrophic failure, destruction of insulationmaterials as well as the copper conductor.

From a chemical standpoint, reduced voltage

testing is recommended because these styles of po-rous mica tapes or uncured resin rich mica tapesnormally contain uncured binder, which may beslightly conductive before processing. Secondly, themica paper and the other tape components will ab-sorb moisture from the environment. This furtherweakens the apparent dielectric strength. If the rela-tive humidity is 60% or higher, the unit should beheat dried and cooled to ambient temperature (aslong as it is above the dew point) before testing. Thecondition of the coils can be confirmed by testingwith a megohmmeter. Electrical testing at elevated

temperatures may also increase the conductivity ofthe uncured mica binders.

The first rule of thumb, is to not exceed1000VAC per lap layer of tape. As an example, atypical 4160volt coil is made with 4 layers of micatape. Using this rule, the hipot testing should notexceed 4000VAC or, using the 1.7 conversion factorsuggested in IEEE 43, 6800VDC. A more commonalternative is to use the line voltage of the machineas the maximum. For example, a 4160VAC ma-chine would be tested at 4160VAC or 7070VDC.Some procedures allow for a +/- of 10% with no evi-

To expect an untreatedcoil to be held to thesame standards as a

finished system isunrealistic.

Photo courtesy of Grayson Armature Works

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dent problems. Either of these guidelines is probablyacceptable to most coil manufacturers and fall withinour suggested test ranges.

The frequency of testing during the windingprocess is also debated. Most coil manufacturers

will hipot and/orsurge test thecoils before

shipment as it isin their interestto catch errorsor defective

products before they are shipped for many reasons.They are, therefore, skeptical regarding qualityclaims related to electrical failure. The winding de-partment wants to assure that the coils are not dam-aged during the winding process. The resin treat-ment department wants to know if the stator is goodbefore processing, as that is the point of no return.

All of these concerns can lead to excessive testingof the coils with the resulting potential for damage ifthe voltage ranges are not carefully controlled. Our

suggestion is to test the coils at the coil shop asusual and to test each group once during the wind-ing process. A final test after all connections aremade, including a de-rated surge test, also makessense. This would limit the electrical testing to threetimes in the untreated state. At the suggested test-ing levels, this should not endanger the windings butwill still give confidence in the quality of the coils andthe winding process. Remember, testing does notimprove product quality. Good workers and goodwork practices make good products.

TecTalkis a publication ofTechnical Marketing Von Roll USA, Inc.

Kevin Alewine, EditorOne West Campbell Road Schenectady, NY 12306

800-654-7652 www.vonroll.com

Good workers andgood work practicesmake good products.

Suggested Hipot Test Voltages

Untreated VPI or Uncured Resin Rich Coils

MachineVoltage

Green Hipot Finished Machine

AC AC DC AC

2300 2300 3910 5600 9520

4160 4160 7072 9320 15844

6600 6600 11220 14200 241407200 7200 12240 15400 26180

11000 11000 18700 23000 39100

13800 13800 23460 28600 48620

www.vonroll.com

World LeaderVon Roll is the world leader in fully evaluated insu-

lation systems for electric motors and generators.

With broad testing capabilities for both life testing

and agency approval, Von Roll offers invaluable

assistance in customizing and analyzing systems

for both OEM and aftermarket applications to the

highest voltage levels.

All information in this document is subject to the product liability clause as published in the website www.vonroll.com