Blake Parker
Technical Education Committee Member
Integrated Power Services
Field drying of windings introduces unique challenges when compared to drying out windings in the service center. Whereas the machines are fully disassembled in the shop, varying levels of disassembly, depending on the design, are possible in the field. NEMA and small frame machines are typically best suited for removal and shipment to a service center for a true steam and bake. Cases exist where access or other concerns make this an exception. While there are mechanical concerns to consider as well, this article covers only the drying of the winding. The techniques listed below broadly cover stator windings, armatures, field poles, wound rotors and rotor poles.
After an event in which a motor winding becomes saturated with moisture, ensure the work area is safe with proper lockout tagout of the machine affected and others affected in the area. A machine that is testing poorly due to condensation is far less affected than one flooded with contamination or salt water. Successfully drying out windings is affected by age, type of winding, degree of contamination and time of exposure. The customer should be made aware of any concerns prior to beginning the process.
Unless condensation or humidity is the sole source of moisture, the motor should be disassembled to the extent possible prior to attempting to dry out the winding. The winding should be cleaned to the extent possible. Depending on the design of the machine, that cleaning may be by hand with solvents, steam, pressure washing or dry ice blasting. In the event of saltwater immersion, it is critical to remove and neutralize the salt residue to prevent corrosion and future issues in the machine. Look for signs of a water line inside the stator, which will help determine the severity of the moisture intrusion. Any contamination that remains after drying of the windings will affect the insulation resistance of the machine and can lead to tracking. Once the machine is clean, prepare it for drying.
Motor Heaters
Factory equipped motor heaters are designed to keep the windings above ambient temperature, preventing them from transitioning through the dew point and accumulating condensation. These heaters were not designed nor intended to dry out a winding that has become saturated with moisture. In some cases, the heaters will improve the insulation resistance of a saturated winding. It is not a reliable method nor a quick means of drying out a winding.
Nitrogen
Nitrogen is often used as a purge gas in industrial/manufacturing environments due to its purity and ability to remove moisture from lines/containment being purged. However, in items that were not designed to be sealed, such as Weather-Protected Type II (WPII) or Open Drip Proof (ODP) motors, the volume of nitrogen needed to effectively purge a motor is often greater than readily available. The rate at which the process occurs and success of drying is largely dependent on the amount of nitrogen available. A large industrial customer had a plumbed in ½" (1.27 cm) nitrogen purge line to a Totally Enclosed Water-to-Air Cooled (TEWAC) machine. After three days of continuous purge, no appreciable change was evident. So, while possible, the volume of nitrogen needed is critical and seldom available.
Dehumidification/Air Conditioning
Depending on the circumstances and environment surrounding the machine, drying of the machine using dehumidified or air-conditioned air may be a good option. This works particularly well when the moisture has not penetrated deep into the insulation system. Rental equipment providers can provide machines capable of moving high volumes of dehumidified/air-conditioned air. The above-mentioned motor, that did not dry out after three days of nitrogen, had positive results after only 24 hours of dehumidification with an air conditioner that circulated a high volume of air. One major advantage of air conditioning is the concern of exposure to heat is reduced. Insulation classes and auxiliary components do not have to be evaluated prior to the application of air conditioning.
Heating
Heating is the most dependable means of moisture removal when windings are thoroughly saturated, and the moisture has penetrated deep into the insulation system. A temporary oven can be constructed from heat resistant materials. (See Figure 1.) It should not be expected that this method will be as fast, efficient or that the same temperatures will be reached as those in a service center’s oven.
The image depicts a large, transparent plastic dome structure, likely a greenhouse, being prepared for installation, with exposed plumbing and construction materials nearby. Note that if you generate content using artificial intelligence (AI), you must check for accuracy.
The materials used to construct the oven are critical. Refer to the following article: easa.com/resources/resource-library/dealing-with-wetflooded-motors in the selection of materials. Use care in selecting the insulating materials ensuring they are rated for the temperatures to which they will be exposed. Most household insulation is not rated for the heat it may be exposed to in this application. Choosing the source of heat is also very important.
Ensure the heat source does not generate smoke or soot that may contaminate the winding. When using a flame fired heater, ensure clearance between the heater and the windings is adequate to prevent burning of the windings. Indirect electric heat can be used but is slower than fan forced heat. Electric fan forced heat is safe and effective but requires very high current often necessitating the use of a generator. The other thing to consider is auxiliary components.
Many components such as terminal strips, electronics and certain sensors must be removed prior to the heating cycle to prevent damage just as you would in the service center prior to utilizing a traditional bake oven.
Electric Current
Note: Many electromechanical professionals, including the author of this article, consider the use of current to dry out windings as too risky. It is very easy to inadvertently overheat the windings regardless of the level of caution used. Runaways in heat can happen very quickly, faster than thermal monitoring can catch. As it is considered an option by some, the author included it in this article.
The use of current to dry out windings has been done for many years. It is the most risky method of drying as the temperature of the copper can reach damaging heat levels before you pick [up the windings with thermal detection devices. This is especially true in field frames or rotor fields where the heat may be excessive on internal turns but not yet high externally.
When using current to dry out a winding, there are a few things to consider. There is difference in inductance between an operating machine on AC and the current draw for DC. When a machine is not running, there is no cooling effect from the air moving through the designed flow paths. Depending on the connection of the machine, it can be difficult to heat all three phases at once. If the wye is connected internally, you can only connect to two phases of the winding at once leaving one phase cold. If it is a delta connected machine machine, then all three phases can be connected in parallel. While not recommended, if this approach is used, having someone monitor the temperature of the windings and the power source is critical to shut the machine down in the event temperatures become excessive. Always start with a reduced current rating from nameplate and slowly increase over several hours until the temperature correlation between the winding and current can be determined.
When is it dry?
An easy way to determine when the stator is dry is to plot the resistance temperature detector (RTD) temperature vs. insulation resistance over time. (See
Figure 2.) Initially, as the temperature increases, the insulation resistance will drop. It is not uncommon to see the insulation resistance drop, sometimes even to zero. Once the insulation resistance starts to climb, the machine is dry. For larger machines, it is recommended to hold the heat for eight hours after the insulation resistance starts to rise. This will ensure the winding is dry and prevents stopping the process prematurely. We know insulation resistance drops with an increase in temperature; therefore, the insulation resistance will rise significantly as the stator cools, provided it is properly dried and no other issues remain.
The diagram illustrates the relationship between temperature (both air and stator) and insulation resistance over time, showing how resistance changes as temperatures rise. Note that if you generate content using artificial intelligence (AI), you must check for accuracy.
Conclusion
Like many electromechanical processes, a careful balance of knowledge and experience is necessary to determine the best path forward when drying out windings in the field. This EASA article easa.com/resources/resource-library/ dealing-with-wetflooded-motors provides valuable insight on drying machines after flooding as well. High volume air, whether heat, air conditioned or clean dry air, is typically the quickest way to improve insulation resistance when impacted by moisture. For windings with greater moisture intrusion, high volume heated air often yields the fastest results but requires a high degree of caution to prevent overheating. Lastly, ensure the asset heaters are energized immediately after dryout to keep the windings dry and ready for service.
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