Chuck Yung
EASA Technical Support Specialist
We've all faced those rush jobs, where a customer is desperate to get his motor back faster than humanly possible. It is our nature to try to do the impossible that's why we are in this business.
We enjoy a challenge and the opportunity to help people. Turning a motor around quickly, when a customer is in need, is rewarding. One of the most time-consuming steps drying the windings after they have been cleaned Ð can be frustratingly slow when an anxious customer is calling. How long does that motor need to bake to be safe?
Most service centers have built in a safety factor, based on some long-forgotten problem we had (or heard about) with a winding that was not properly dried. "Joe" opened the oven door right before lunch and left it open, then someone pulled a stator out a couple of hours later and it megged low. Maybe it failed on the test panel. Of course, "Joe" didn't fess up to what happened. So you told everyone: "From now on, all windings bake 2 hours longer than we used to."
Now you have a safety margin in the procedure that adds unnecessary time to every job through the doors. That may not be a big deal, except when your best customer loses a critical motor.
You probably tried to sell him a spare last time this happened, but he wouldn't go for it. None of that matters now he expects you to get him out of a bind.
Here is a procedure for those emergency repairs, to take the uncertainty out of the bake cycle. This procedure will help in determining how long a bake cycle is really necessary. This saves time and energy both of which cost money.
With the wet winding on the oven cart, attach a lead to the frame and one to a winding lead. RTD wire works fine. These leads should be long enough to reach out of the oven. Use a DC voltmeter to read DC mili-volts. When the voltage is zero, the winding is dry.
How does it work?
The iron frame and copper windings act as two plates of a crude battery. Electrolytic action across the wet insulation causes current to flow. As long as the cell is "wet" there will be voltage.
When the "cell" is dry, so is the insulation. Try it the next time you have a large rush job in your service center. This procedure works for everything except a few newer form coil VPI / B-stage insulation systems.
If you find that hours are cut from your expected drying time, consider using this method to evaluate drying times for normal work. This will save energy and reduce the chance of damage caused by excessive temperatures. After all, every 10 degrees above an insulation systems rating halves the insulation life.
Print