These two terms are often used in the motor repair industry to describe a type of insulation finishing process used on rewound electric motors. When a motor is rewound, it must be processed in some type of varnish in order to hold the windings together. This varnish, also known as resin, helps to bond the new winding, insulation material and core iron together in order to insure proper function.
VPI means Vacuum Pressure Impregnation. With this process, the apparatus is placed in a vacuum tank and the resin is then pumped in. The resin is drawn into the smallest voids in the coils, and after an oven baking process, effectively bonds all components of the winding together as one insulation system. This process is commonly used in larger motors with formed or manufactured coils. These motors typically operate on 2300 volts and above.
Dip and bake is exactly what it sounds like. A finished winding is submerged in a vat of special resin. Following the resin soaking process, the stator or armature is placed in a baking oven to cure. This is designed to hold the windings in place and often up to three cycles may be performed. Stators and armatures that normally operate in the 115/230/460 voltage range are often dipped and baked.