David Sattler
L&S Electric
The goal of vacuum pressure impregnation is complete saturation of a winding with insulating resin. As resin penetrates the insulating materials, it causes them to darken. When VPI resin is drained from the winding, all end-turn insulation should be darkened to a uniform shade. Connection insulation should also show uniform darkening. If some insulation is a lighter shade, the coil or jumper is not fully saturated. The winding is not properly protected, and if the problem is not addressed, it is likely to cause premature failure of the unit. This could result in costly warranty work, or at a minimum, will be a failure to provide your customers with the quality they expect and deserve.
When uniform darkening of the winding is not achieved, the most likely cause for the lighter colored coils, or “white” coils, is an influx of air during the pressurizing phase of the VPI process. Because air has a much lower viscosity than resin, given the opportunity, it will rush in to fill voids in the coil before the resin can reach them.
Note the spaces between the conductors in Figure 1. The coils are constructed with radiused rectangular conductors stacked like bricks, often two strands wide and several strands high as shown. Air pockets exist wherever the radiused edges meet. In addition, a tiny layer of air remains between each layer of tape which surrounds the coils. These pocket s of ai r are connected to pockets of air in adjacent coils through the inter-coil or inter-group jumpers. Therefore, if there is a path for air to make its way back into one coil, it may also reach one or more of the coils connected to it.
During the dry vacuum stage of the VPI process, air that occupies the spaces within the coils exits through the layers of tape and is evacuated from the process chamber. Once complete evacuation of air has been achieved (usually after holding the vacuum below an established value for at least an hour), resin floods the winding and begins to soak into the outer layers of tape that cover the coils of the rapidly cooling winding. While the chamber is still under vacuum, the resin only soaks in due to contact with the tape that covers the coils and the limited pressure provided by the column of liquid resin surrounding the winding.
At this point, whether you implement a “wet vacuum” stage or apply pressure as soon as the winding is flooded, once pressure is applied, if care has not been taken to ensure there is no possible avenue for air to enter the winding, the potential for incomplete saturation increases.
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Wet Vacuum or Immediate Pressure?
A "wet vacuum" stage is a wait period of an hour or so that allows resin time to soak into the layers of tape while they are more relaxed than they will be when pressure is applied to the chamber.
Applying pressure as soon as the winding is flooded takes advantage of the lower resin viscosity caused by the heat of the unit being processed. Lower viscosity resin will penetrate more easily than cooler resin with its higher viscosity. The alternative of waiting for a wet vacuum stage will result in higher viscosity resin during the pressure stage because the warmth of the unit will be dissipated into all the resin.
Each method has advantages and disadvantages, but both can be effective.
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For example, if a stator is processed with the core horizontal, and just a small part of the core is left unsubmerged, air will likely make its way between the laminations and into the coils, leaving voids in the coils. Another often overlooked return path for air is through the flexible lead cables during pressurization. Air has an easy path around the wire strands and bundles within the cable jacket of a flexible lead. The submerged end that is connected to the winding must be sealed.
Processing the unit before the flexible lead cables are attached is the best way to prevent the problem, but that is not always practical. Whenever leads are installed before the VPI process, the procedure followed to seal the leads must include a proven method to prevent air from using the leads as a channel to reenter the coils. Unfortunately, very few guides for the VPI process include instructions for how to seal leads to prevent the ingress of air. Most don’t even address the topic.
The Electric Power Research Institute's (EPRI's) Guide for Electric Motor Stator Winding Insulation Design, Testing and VPI Resin Treatment1 states: “Attach motor lead cables and seal each end of each cable with room temperature cure epoxy resin to prevent VPI resin ingress during impregnation.” The Epoxylite Process Specification PS-43F2 Step 10 states: “SEALING OF LEADS: Special care must be taken to seal the stranded lead wire in order to prevent impregnation; otherwise, they will become stiff and brittle.”
These instructions address the importance of sealing leads to prevent stiffening of the lead cables if resin penetrates the cables, but neither addresses the importance of preventing air from flowing through the cables into the winding, which is of even greater concern. While stiff and brittle lead cables are certainly undesirable, as stated above, the ingress of air can prevent thorough penetration of the winding and result in premature failure of the unit.
In the EPRI document above it states, “seal each end of each cable with room temperature cure epoxy.” Sealing each end of each cable is acceptable, even if some air escapes from the cable during the dry vacuum stage of the process. The important thing is that no resin be allowed to make its way into the cable and no air be allowed to make its way into the winding during the pressure stage of the process.
No matter what method you employ to seal the leads, the seal must be complete and of adequate strength to handle the pressure difference between the chamber pressure and the pressure in coils at near full vacuum. Lead seal methods can be tested by submerging a test sample in water and gradually applying compressed air to the unconnected end of the lead cable. If no bubbles form, the lead is sealed properly. (While I have never experienced cable jacket failures due to pressure applied to the inside of the cable while using this testing method, you should always use caution when working with compressed air.)
Fortunately, during the VPI process, the pressure acting on a cable with an unsealed end above the resin level is the same on the inside as on the outside (see Figure 2). In this regard, sealing a lead is not like a futile attempt to wrap tape over the outside of a leaking garden hose. Developing a procedure that works consistently for your process can be done and is well worth the effort.
But, even after a proven method for sealing leads has been developed, do not neglect looking for “white” coils as part of your post process inspections. If “white” coils are observed, try to determine the cause. Was the unit fully submerged, or could air have entered through exposed laminations? Could air have entered through the lead cables? If so, does the lead sealing process need to be executed with greater care, or does the existing process still need improvement?
Regardless of the cause, do not ignore the problem of “white” coils and just hope to “do better next time.” If the “white” coils are caused by air re-entering the windings through the flex lead cables, the affected coils are likely line-lead coils which are subject to the greatest turn to turn and ground voltage stresses of any coils in the winding. Premature failure and costly warranty repairs become real possibilities if the problem is not addressed.
Fortunately, if you observe “white” coils in the winding during inspection, you do not need to start the winding process from scratch. First move the uncured stator to a safe work place. Then, using disposable gloves, carefully remove ties from the connections to gain access to all affected areas (usually where the flex lead cables are attached to the winding). Remove all tape from the leads attached to the coils that are lighter in color. Make sure the connections are exposed to the point where resin will contact them and leave no in-road for air from the lead cable. See Figure 3. Next re-process the winding using double or triple the pressure stage times along with bump cycles (releasing pressure and re-pressurizing). The extra time is needed because the unit is cold and the resin impregnation rate is reduced.
After re-processing, you should observe a winding that is uniformly darkened by the resin. Re-tape the affected connections, using process resin to thoroughly wet the tapes and tie cords as they are applied. (If the people who did the initial winding and lead sealing are required to do this messy re-work, they will quickly understand the importance of developing and following proven techniques for sealing leads. It’s not a job they will want to repeat!)
While every step in the process is important, sealing leads to prevent ingress of air is an often-overlooked step that can significantly reduce the problem of “white” coils during the VPI process. Adding this step to your company’s process could improve the quality of your product, reduce costly warranty work, and ensure your customers are getting the reliable motors they expect and deserve.
- Guide for Electric Motor Stator Winding Insulation Design, Testing and VPI Treatment
EPRI (Electric Power Research Institute) 1009700, Technical Update, October 2004, Section 7
- Process Specification PS-43F
Epoxylite document 8B6052
AVAILABLE IN SPANISH
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