Chuck Yung
EASA Senior Technical Support Specialist
Flooding in the aftermath of tropical storms (hurricanes, monsoons and cyclones) with heavy rainfall will often shut down hundreds of plants along the Gulf Coast from Florida to Texas and other places around the world.
To get them up and running again, maintenance departments and motor repairers face the daunting task of cleaning muck and moisture from many thousands of electric motors and generators. See Figure 1. The process in such situations can take weeks, if not months, and requires special clean-up procedures for motors contaminated by saltwater.
Although the problems are huge, affected plants can get back in production more quickly by working closely with service center professionals and following a few tips that will make the cleanup more manageable. These include prioritizing motors and generators for repair or replacement, storing contaminated machines properly, and using proven methods to flush away saltwater contamination. Constructing temporary ovens on site or at the service center can also add capacity for drying the insulation systems of flooded motors.
Understanding the problem
The harm done to motors and generators by flooding extends beyond rusted shafts and contaminated bearings and lubricants. Even brief intrusion of moisture can compromise the insulation system, making the windings vulnerable to ground failures. Saltwater flooding poses additional problems. Unless thoroughly flushed from the equipment before it dries, the residual salt will rust the steel laminations of the stator and rotor cores. It may also corrode the copper windings and aluminum or copper rotor cages. The result, predictably, will be lots of motor failures – some occurring years after the storm.
How to proceed
Begin by prioritizing motors by size and availability. Older motors are often good candidates for replacement with more energy efficient models. The horsepower (kW) break will vary from plant to plant, depending on the application, annual usage, energy costs, and other factors. But, considering the real possibility that your regular vendors may be backlogged with work, somewhere between 100 and 200 hp (75 and 150 kW) may be a reasonable place to draw the repair-replace line. By replacing those smaller motors with readily available energy-efficient models, you’ll free up capacity for your service center to concentrate on the larger ones that it makes more sense to repair.
Two ways to clean
Once you decide which motors to save, process those with open enclosures first. In cases of freshwater contamination, disassemble the motor and clean the stator windings and rotor with a pressure-washer. If the insulation resistance is acceptable after the windings have been thoroughly cleaned and dried, apply a fresh coat of varnish and process the motor as usual (new bearings, balance the rotor, etc.). Windings that fail the insulation resistance test should be put through another cleaning and drying cycle and tested again. Stators that fail the second insulation resistance test should be rewound or replaced.
Saltwater contamination requires a more thorough cleaning process to reduce the possibility that salt residue will rust the laminations or corrode the windings. To accomplish this, clean the stator and rotor windings and insulation systems using the “saltwater flush procedure” described below. For best results, immerse stators and rotors in the freshwater tank before the saltwater dries.
For the same reason, do not disassemble contaminated TEFC or explosion-proof motors until there is room for them in the immersion tank. This will keep them full of water and prevent salt from drying on internal parts. If it will be a while before these motors can be cleaned, place them on their sides, with the lead openings up, and keep them filled with fresh water.
Saltwater flush procedure
This procedure offers the best chance for removing saltwater from contaminated windings. As mentioned earlier, it works best if you do not allow the windings to dry first. The sooner the windings are immersed in the tank, the better the results.
The process is straightforward:
- Immerse stators and rotors in freshwater for 8 hours.
- Continuously agitate the water.
- Exchange water in the tank with freshwater at rate of at least 20 - 50 gallons per minute (75 - 190 l/min).
Tank construction. Select a container that will hold enough water to completely immerse a good number of stators and rotors and drill a drain hole of at least 2” (50 mm) in diameter near the top. Weld a pipe nipple to the drain hole and plumb it to a storm drain or other suitable place. Field expedient containers for this purpose include modified shipping containers, dumpsters, or even swimming pools.
Next, route a 3/4” (20 mm) or larger supply pipe into the top of the tank (roughly centered), down the inside wall, and across the length of the bottom. Cap the end of the pipe and then drill holes at a slight upward angle along both sides of pipe to serve as water jets. The hole size should be appropriate for the available water pressure, but no more than 1/8” (3 mm) in diameter. The more holes you drill, the smaller they will have to be (see Figure 2).
Flush procedure. Place the stators and rotors in the tank and fill it with freshwater. Process each batch for 8 hours, continuously exchanging the water in the tank at a rate of at least 20 - 50 gallons per minute (75 - 190 l/min). At the end of the cycle, remove and pressure-wash the stators and rotors, and then dry them thoroughly in a bake oven or temporary field oven (see Figure 3).
Finally, test the insulation resistance to ground. If the test results are acceptable, apply a dip-and-bake varnish treatment before reassembling the motor. If the motor fails the insulation resistance test, bake it again and repeat the insulation test. Motors that fail the insulation resistance test a second time should be rewound. Per IEEE Std. 43 and IEC 60034-27-4, the minimum resistance to ground is 5 megohms for random windings, or 100 megohms for form coil windings.
The bottleneck
For most service centers, the bake oven is the single biggest bottleneck. Even the largest oven will only hold so many motors, and the drying time for each batch can take 12 hours or longer. Imagine the backlog after a disaster, with hundreds of motors to process.
It is possible (but not very efficient) to dry windings by draping larger motors with tarps and applying external heat sources. Another way is to dry the windings is to energize them with a welder or other DC power source. The drawback here is that someone has to monitor the current and winding temperature and periodically move the welder leads to heat all three phases evenly if the winding is not connected wye-delta. Welding machines also have a duty cycle that’s a lot shorter than the two or three days it might take to dry out a large motor.
A better way to increase baking capacity is to build one or more temporary ovens that can dry motor and generator windings safely and efficiently. This approach is especially useful for drying large stators, which take a long time to heat to the required temperature, occupy the entire oven, and delay the processing of other motors. If necessary, temporary ovens can even be constructed on site. This can save the time and labor required to remove the motor from service, transport it, and later reinstall it.
Materials. Energy-shield (the hard-sided foam insulation that home builders install between the exterior frame and siding/brick) and aluminum duct tape are ideal for building temporary ovens–no matter what size or shape you might need. A stock item at most construction-supply super stores, energy-shield has a layer of aluminum foil on both sides and exceptionally good insulating value (R-29) for its thickness. The 4’ x 8’ (1.2 m x 2.4 m) sheets are lightweight and easy to cut with a safety knife. They ‘re also reusable–as long as you store them where they won’t be damaged. Thickness of 1” (25mm) or greater keeps the heat in with minimal looses.
Oven construction. For motors with very large frames, box the motor by placing energy-shield directly on the frame, including the top. Seal the joints with aluminum duct-tape.
Placing the energy-shield directly on the frame minimizes the volume of air that must be heated. This reduces drying time because the insulation minimizes heat loss.
Heat sources. To heat the temporary oven, force air through it from an alternate heat source. If you use a torpedo heater (see Figure 3), position it to blow hot air directly into the center of the bore. Energy calculations for oven design are complex. For this purpose, 100,000 BTU (106,000 kJ) per 1200 ft3 (34 m3) of oven volume will be adequate to heat the oven and contents within a reasonable time.
Temperature control. For an accurate record of winding temperature, directly monitor the motor’s RTDs, if it has them. If RTDs are not readily available, use HVAC instruments or candy thermometers to monitor temperature in each quadrant of the oven. The key is to keep the heat uniform within the motor and not to exceed part temperatures of 250°F (120°C).
Because heat rises, it might seem reasonable to open exhaust ports at the top to let it out. But as those familiar with old-fashioned wood stoves can tell you, the best way to control oven temperature is to open or close dampers (exhaust ports) near all four corners on both sides (see Figure 3).
To raise the temperature at one corner, for instance, open that damper farther. The increased flow of hot air through that area will raise the temperature. The ability to regulate temperature in this way greatly improves the drying process as compared with traditional methods such as a DC current source or tarps.
How long to bake?
The bake cycle should be long enough to dry the windings completely. If it’s too short, you’ll need to repeat the process. If it’s too long, you’ll waste both time and energy. If the winding has RTDs, 6-8 hours at 200°F (93°C) should be sufficient. For windings not equipped with RTDs, here is a method to determine how long the bake cycle should be.
All you need are two lengths of RTD wire or similar small lead wire long enough to reach out of the oven and a DC voltmeter capable of reading millivolts. With the wet winding on the oven cart, attach one lead to the stator frame and the other to a winding lead. Finally, connect the free end of each lead to the DC voltmeter. You can be sure the windings are completely dry when the voltage on the millivolt scale reaches zero.
This procedure is one that many service centers use when they have large rush jobs to process. It often cuts hours from expected drying times, even for normal work. It also reduces the chance of damage that might result from excessive temperatures.
How it works. Like the setup, the principle behind this procedure is fairly simple. The steel core and copper windings function as two plates of a crude battery. Electrolytic action across the wet insulation causes current to flow. As long as the cell is “wet”, it produces voltage. When the cell is dry, so is the insulation.
Note: This procedure works for everything except some form coil VPI insulation systems. Some of these windings are sealed so well that they may exclude moisture from the insulation, keeping the “wet cell” battery from developing.
Conclusion
There is very little anyone can do to protect all equipment from the effects of a hurricane. Hopefully, the procedures outlined here will speed the recovery for the plants in affected areas, as well as for the local populations that depend upon them both for employment and products. In better times, these procedures also can facilitate plant-service center partnerships and maximize uptime.
Common misconceptions about how to dry wet motors
Two mistaken ideas about how to dry wet windings have persisted for years. The first is that heating the windings with a welding machine is good way to dry out an electric motor. Before using a welder or other DC power source for this purpose, make sure you know what you’re getting into.
Most electric motors large enough to warrant consideration have three leads–one per phase. Internally, they are connected either wye (Y) or delta (∆) with the exception of 6-lead wye-delta motors, as shown below. (Incidentally, the terms wye and delta come from the Greek letters that they resemble.)
If you apply DC current to any two leads of a delta winding, two phases will be in series, and the third will be in parallel with them. That means one phase will carry twice as much current as the series pair, so it will get much hotter. For the wye connection, only two phases carry current, leaving the third phase cold.
Whether the winding is connected wye or delta, someone must monitor the current and winding temperature, and periodically move the welder leads. Otherwise, parts of the winding may not dry completely, if at all. Welding machines also have a duty cycle that is significantly shorter than the two or three days it might take to dry out one winding.
Welding machines are useful when both ends of each phase are brought out as six separate leads. An ohmmeter will confirm three separate circuits. In that case, the three phases can be connected in parallel or series, depending on the capacity of the welding machine, and dried simultaneously.
Another misconception holds that windings should not be dried at oven temperatures above 180°F (82°C), for fear that trapped moisture will burst the insulation. That might be a valid concern if we could somehow heat a winding instantly to above boiling temperature. The reality is that windings, like anything else placed in an oven, heat up very slowly. Moisture will get out the same way it got in. As the temperature of the winding slowly increases, the moisture (just as slowly) will evaporate. Although IEEE Std. 43-1974 included an annex with information that may have perpetuated this belief, it was dropped in the next revision cycle.
Every day more than 1,700 EASA service centers steam-clean and then bake stator windings–mostly at oven temperatures of 250 - 300°F (120 - 150°C). Even though many of them repair thousands of motors annually, there is no evidence that this process has damaged a single winding. Burst insulation due to oven temperatures above 212°F (100°C) is simply not a concern.
ANSI/EASA AR100
More information on this topic can be found in ANSI/EASA AR100
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