Common misconceptions about how to dry wet motors

Two mistaken ideas about how to dry wet windings have persisted for years. One is that heating the windings with a welding machine is good way to dry out an electric motor. The other is that windings should not be dried at oven temperatures above 180 F.

09/03/2013


Figure 1: Drying windings with a welder or other dc power source. Courtesy: EASATwo mistaken ideas about how to dry wet windings have persisted for years. One is that heating the windings with a welding machine is good way to dry out an electric motor. The other is that windings should not be dried at oven temperatures above 180 F (82 C).

Welding machine method. Before using a welder or other dc power source for to dry out an electric motor, make sure you know what you're getting into.

For starters most electric motors large enough to warrant consideration have three leads–one per phase. Internally, they are connected either wye (Y) or delta (∆). (Incidentally, the terms wye and delta come from the Greek letters they resemble.)

With the welder applying current from T1 to T3, only two phases of a wye-connected motor are heated. If the internal connection is delta, one phase is heated with four times the wattage of the other two phases. In both cases, the welder leads must be periodically moved to heat the entire winding evenly.

If you apply dc 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's a lot 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 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.

Oven temperature myth. 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 the windings could somehow be heated 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 Standard 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 1,900 EASA service centers steam-clean and then bake stator windings—mostly at oven temperatures of 250 to 300 F (120 to 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.

Chuck Yung is a senior technical support specialist at the Electrical Apparatus Service Association, Inc. (EASA).



Anonymous , 09/23/13 03:52 PM:

Good article.
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