What burnoff oven temperature is "right"?

Electrical Apparatus, Dec 1998 by Nailen, Richard L

BURNOFF OVEN TEMPERAture has long been a concern to repair centers facing the need to maintain the efficiency of rewound motors. Several studies made during the past 20 years have shown increased core loss caused by excessive lamination heating during oven stripping of a failed winding. (A European researcher has reported results shown in the graph on the following page.)

But how much is "excessive"? A common answer is: "Stay below 650 deg F" (or 700 deg F, or some other number in that vicinity). That, unfortunately, is seldom helpful. Here's why:

Damage to the insulating coating on some motor laminations can occur at 600 deg F-700 deg F. With others, even 1000 deg F causes no harm. It depends upon the coating chemistry, unknown to service personnel and not easily determined in the field.

The consequences of such damage will vary widely with the specific motor design, such as the relative proportion of eddy current loss to total core loss, of core loss to total motor loss, and of back iron depth and tooth configuration.

Setting any fixed temperature limit poses this question: What temperature are we concerned with? The only one of importance is that reached by each individual lamination in the core itself. And how damaging that may be depends upon how long-and how completely-the laminations are exposed to it.

What's usually measured and readily controlled is not the temperature within the core itself, though, but that of some point on the core surface or (much more commonly) that of some point in the air within the oven chamber.

Any such measurements can be gravely misleading, for three reasons:

Even after a long period of heating, the temperature on or within a laminated core can differ markedly from the oven chamber temperature.

Temperatures on various core surfaces can vary considerably from that within the core structure. Core shape-particularly the depth below slots-will have an influence.

Temperatures at various locations within the oven are also subject to wide variation. One close to the core may be quite different, either above or below, from what's being monitored externally.

How the oven is loaded can greatly distort the expected thermal patterns. To save energy, time, and labor, a number of different cores may be placed in the oven at once. They may be of considerably different sizes and configurations. A small core will heat much faster than a large one. Cores may be stacked vertically such that the bores line up to create a chimney effect, with strong internal air circulation making the inner temperature differ from that on the outer surfaces. Insulation in those on the bottom may flare up, shielded by those on top such that a water mist cooling system may not be fully effective.

Some shop managers declare they have no trouble burning out a winding at 450 deg F or 500 deg F, while others claim they can't do the job below 700 deg F or more. Both may be correct. Not only is burnoff time a factor; the way the oven operates, the way it's loaded, and the way temperature is being measured can account for the apparent contradiction. The process is subject to many important variables.

Be wary, then, of specific, universally applied temperature limits. Remember that only the lamination temperature is important, and even then the limit isn't necessarily the same for all cores. Use your best judgment. Be cautious-but rely on core testing before and after stripping to see if damage has been done. Don't just rely on what the oven thermometer says, any more than you would in the kitchen.

One other bit of kitchen lore: Remember how, to keep a cake from falling, Grandma never opened the oven door while it was baking? Avoid the temptation to open that burnoff oven door to see how the stripping operation is going. It can upset your temperature regulation.