Tom Bishop
EASA Technical Support Specialist 

Did you ever wonder if the preheating instruc­tions from solvent varnish and solventless resin (hereafter we’ll use the term “resin” when it ap­plies to both) manufacturers were really all that important? The short answer is, yes, they are. Here we’ll expand on some of the reasons that preheating is a key step in the winding process. 

One of the first benefits of preheating is that it drives out moisture that may have settled on sur­faces, or been absorbed by insulation material. A little known aspect of pre­heating is that it relieves the mechanical stress cracks, termed “crazing,” on the magnet wire insulation coat­ing that occur during coil winding and insertion. Epoxy B-stage materials can be set by preheating, provided the preheating time and tempera­ture meet the epoxy’s curing requirements. Random wind­ings typically don’t use many B-stage materials; however, many of the lacing products for endturns are thermoset­ting. Form coil windings often have B-stage surge ropes, and some felt packings used for endturn coil bracing are B-stage epoxy loaded. 

Further benefits 
During the impregnation process further benefits of pre­heating come into play. The preheated winding and core, upon coming in contact with the cooler resin material, lower the resin’s viscosity. This enhances penetration of the resin into the winding, which is particularly important in the slots, and it aids in drain-and enhances the curing process. 

How long, and at what temperature, should a winding be preheated? The typical range of pre­heating temperatures is 250ºF to 275ºF (120ºC to 135ºC). The entire stator, rotor or armature should be heated until the wind­ing reaches the preheat temperature, and then held at that temperature for at least one hour. The entire unit should then be allowed to cool down to a surface temperature appropriate for the type of resin being used.

The temperatures associated with cool down when using solvent varnishes differ from those for solventless resins. With a solvent varnish the unit being preheated should be allowed to cool down to a surface temperature between 150ºF and 175ºF (65ºC and 80ºC) prior to immersing in the varnish. The cooler temperature prevents degradation of the varnish and aids in avoiding solvent entrapment. The cool down temperature range for solventless resins should be between 120ºF and 140ºF (50ºC and 60ºC).

Process critical in resin curing 
The preheating proce­dure may seem like a waste of time and energy, but it is a critical step in as­ life of the resin. If the winding is too hot, in the case of a solvent varnish the solvent will be evapo­rated, thus increasing the viscosity. This will reduce the capability of the varnish to penetrate into the windings, and the cure time will increase. If you are unaware that an overtemperature of the impreg­nated unit has affected the varnish, you may not realize that the winding has failed to cure during the normal bake cycle. The worst-case scenario is an un­cured winding that fails prematurely because the weak bond strength allows the wires to vibrate until the insulation quickly wears off, and a short circuit occurs. Even if the lack of a full cure is detected prior to assembling the com­plete machine, there is the lost time associated with having to rebake, and for an extended period. A similar situation can occur with sol­ventless resins. The resin viscosity increases, making penetration more difficult. And gelling takes place in the resin, forming gel par­ticles that are like microscopic “meatballs.” 

Test resin periodically 
Periodic testing of the resin will reveal if it has been affected by impregna­tion of units that were too hot. Solvent varnishes will require more solvent, a rather simple cure. How­ever, the solventless resins will require adding compounds that try to break up the microscopic meatballs. The gelled particles can be made smaller and put in suspension, but they do not dis­solve, so the negative impact on the resin, though reduced, is there forever. Further, the additives for the resins are relatively expensive, costing more per unit of volume than the resin itself. 

Another issue relating in part to preheating came to light in the late 1990s. Some electrical apparatus service centers experienced problems with the inverter type magnet wire and solventless resins containing diallyl phthalate (DAP). The difficulties ranged from poor bonding of the wires to no bonding at all. The NEMA Magnet Wire Technical Committee conducted an investigation and a testing program to determine the factors that were contributing to the poor bonding results. The committee found that most of the problems were related to compatibility of the DAP resins and the magnet wire coating. The bonding problem oc­curred with non-inverter and inverter type magnet wire coatings. 

Helps alleviate bonding problems 
A secondary factor was that certain lubricants used on the magnet wire to facilitate manufacture were contributing to a beading effect that resulted in poor bonding. Preheating often helped to alleviate the bonding problem in these cases, as the process evaporated the topical (applied to the surface) lubri­cant from the magnet wire insulation coating. Some magnet wires are “internally lubricated,” that is, the lubri­cant is embedded in the coating. Preheating does not drive off internal lubricants, but it does help to alleviate the bonding issue associated with the DAP resin even if the magnet wire coating is internally lubricated. 

Since the bonding problem was associated with a number of vari­ables, mainly the DAP resins and the magnet wire lubricants, it would be wise to check the com­patibility of your resin with the different brand magnet wires your service center uses. Remember that the bonding issue can apply to non-inverter and inverter type magnet wire. A good way to check for bond is to view the end turns of wind­ings with a portable microscope, with a 30x magnification or better. 

If you detect a complete lack of bond, or beading (like water drops on a waxy surface), then there is a bonding problem. At that point you will want to discuss the issue with your resin manufacturer. And, if satisfactory bonding is found, plan to re-check the wire bond from time to time; manufacturers sometimes change their processes, e.g., the types of lubricant and the wire coatings themselves. In all cases you will be well served to follow the preheating instructions from your resin manufacturer and the guidelines given here. As described earlier in this article, the benefits of preheating extend well beyond bonding and compatibility issues. 

Categories: Winding insulation, Repair procedures & tips, Service center equipment

Tags: Winding insulation Varnish and resins

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