Chuck Yung
EASA Senior Technical Support Specialist
When rewinding field coils, there are a couple of common problems that make life difficult for the service center. One problem occurs when the newly rewound set of shunt fields is returned to the service center, roasted again. What causes this, and whether we can improve our winding procedure, has been the subject of much discussion.
We all realize that something caused that first failure. If the subsequent failure looks a lot like it — the shunt fields were charred in both cases — then it seems logical that the underlying cause could be the same. Sometimes it is.
Let’s look at why the coil is burnt. Well sure, it got too hot. But why? Is it because the shunts are energized when the machine is otherwise shut down? If so, the fields can be automatically de-energized after “x” minutes of machine inactivity. Installing a timed relay in the controls will avoid future problems. It may be that the customer is able to trace the problem to a new operator, who needs more training.
Treat the cause
If the customer is certain that nothing has changed at their end, then our concern is how to improve the winding. A natural first response is to use insulation of a higher temperature class. On the surface that seems logical, but why is it suddenly necessary? If the coil is getting too hot, treat the cause not the symptom.
Most of us wind a shunt coil on a form, remove and tape the coil, then install it on the pole iron. Depending on how well the coil fits, heat transfer from the coil to the laminated pole may vary widely. A good solution is to “pot” the gap between coil and pole iron, using a thermally conductive epoxy (see Figure 1). Not just any epoxy will do. Some epoxies are designed to insulate, others to transfer heat efficiently. For a field coil, our goal is to transfer heat from the coil as quickly as possible. The laminated pole iron dissipates heat efficiently, so transferring heat from the coil to the pole iron will go a long way towards extending field coil life.
With large coils wound with rectangular wire, brush epoxy or varnish onto each layer as it is wound. This ensures efficient heat transfer, and increases mechanical strength — an important consideration when winding rotating coils.
Place poles face pown
Whether the field coil is dipped or VPI-processed, it should be potted to the pole iron while still warm from the oven. Placing the poles face down on the oven cart allows the potting process to be accomplished with very little handling. Seal the side that’s down, to prevent epoxy from leaking out and making a mess. Some winders use tape or duct-seal for this purpose. Others use silicone (Reminder: Use only electrical-grade silicone in a DC machine!) Some winders place a felt collar for the coil to shoulder against, when placing the coil on the pole iron. Experiment to see what works best for you.
Pour epoxy slowly
Don’t pour all the epoxy at once: a small leak will result in a big mess. Instead, pour a small amount of epoxy into each coil and let it cure. That first partial-pour will seal any small leaks. As soon as that small quantity of epoxy sets, finish the pour for each coil. This technique will prevent a small leak from turning into a big mess. (It is more efficient to pot the whole set of coils at the same time; so preheat them, pour a small quantity of potting compound into each to make sure the bottom is sealed, then finish the pour for each coil.)
Some repairers install the fields in the frame, and VPI or dip the assembly. There are a couple of concerns with that method. First, the VPI penetrates between the shims, pole iron and frame. It bonds everything together, and makes it very difficult to remove the poles during the next repair.
When the varnish penetrates between the pole, shims and frame, it acts as a nonmagnetic “shim” and may change the field / interpole strength. Because the thermal expansion characteristics of varnish differ from those of the steel pole, frame and bolts, stresses may cause thread damage to the bolt-holes. Tapped holes in laminations are not as strong as tapped holes in a solid block of steel, so why risk it?
If coil shape is a concern, temporarily install the poles with mylar placed between each coil and frame. Use sacrificial bolts, to avoid having to clean the originals. (In some cases, non-magnetic bolts are used. Be sure to replace these in kind.) Dip the frame with coils installed. After baking, remove the coils (while hot) and clean the frame and iron. Tap the boltholes to remove varnish from the threads.
Pot the fields as described above, and re-install them —this time with the shims and correct bolts.
This procedure will improve field coil life. A coil rewound without careful attention to thermally conductive potting will have seriously reduced life. For applications where heavy shock loads are present, this procedure also improves coil survivability. A major manufacturer of dragline equipment pots all field coils for that reason.
Tip: When rewinding a coil that originally had aluminum wire, select a copper wire to match the resistance per 1000 feet as closely as possible. Because the wire is smaller (usually 2 sizes) the resulting coil size would be smaller. That reduces the mean turn length, which in turn reduce coil resistance. To avoid this, wind half the turns and compare the coil length to the original. Install a spacer on each end of the coil so that when the remaining turns are installed the total coil length matches the original. This method closely approximates the mean turn length of the original coil, keeping the total coil resistance close to the original. Do not change the turns! That would affect the ampere-turns.
Print