Whether you send a motor in a for a repair it is going to eventually go into a baking oven. For purposes of this article, we are not referring to the higher temperature "burn-off" ovens used in the rewind process - we are referring to just baking ovens. And how your repair vendor bakes that electric motor is key to high quality, reliable repairs. So why is baking necessary?
Why Baking a Motor is Necessary
There are two different processes that involve baking: cleaning and curing.
HECO repairs always involve careful cleaning of the motor in order to remove every trace of contaminant such as dust, grease, and oil. When you pressure wash or steam clean a motor, the windings are going to be wet and that moisture must be baked out at high heat. If not, the remaining moisture could lead to failure or you might end up varnishing over that trapped moisture if the motor needs to be rewound. And as you probably already know, moisture and electric motors just don't get along well.
And that leads us to the second time that baking and electric motor is required: curing resin. Whether the motor is a full rewind or a recondition it may require either a varnish dip or a VPI (Vacuum Pressure Impregnation) process, Either way, the resin must be heat cured. The curing process usually takes significantly longer to complete than baking the moisture out (partial days instead versus hours). It is vital that the resin insulation is fully cured and that it has not been applied over trapped moisture, or there is a risk of failure that we at HECO just aren't willing to accept.
The Process of Baking a Motor
At HECO, we use gas-fired forced air ovens for the baking process. Based on our extensive experience, they are ideal for parts with a significant amount of mass, such as electric motor stators and windings.
The baking cycle time used depends primarily on the mass and size of the item, with larger items and more mass requiring more time. At HECO, soaking time begins after the part has reached the target temperature. A very precise temperature probe known as a thermocouple is attached to the middle area of the part's mass to track its temperature inside the oven. Once that middle of the part has reached the target temperature, then soaking will begin.
For example, if a motor has been steam cleaned, it might need to soak 12 to 14 hours at a temperature of 220 - 250F (depending on mass and size). At HECO, heating process time does not start until the temperature probe indicates that the temperature at the center of the motor mass is between 220 - 250F. From that point, the heating time for the motor begins.
When a rewind has been performed, or when just performing a varnish dip on a reconditioned motor, we not only bake parts of the motor after cleaning but we also bake it as part of the curing process. The heating time needed is dependent not only on the mass and size of the motor, but also on the manufacturer guidelines for the type of resin used (which is usually polyester or epoxy).
At HECO, however, we don't like to leave anything to chance. That's why we include sacrificial stator coils with the rewind process. These sacrificial coils go through the same steps as the full motor rewind, but are evaluated at various points during process to ensure that the insulation is void-free and fully cured. That means there will be a sacrificial coil included in the oven during the baking process. The coil will be cut open after baking is complete to verify the quality of insulation achieved.
Keep in mind that the baking process -- whether it's for cleaning or curing -- takes time (from hours to days). Nothing can be done to hurry the process, and it's impossible for a technician to work on a motor that is baking. This is one of the reasons why a high quality repair may take a little longer, but will pay off in better reliability and performance.