Reliable Thanks to Silicone Resins

1. It optimizes the electrical insulation.

2. It mechanically reinforces the entire winding as the resin bonds the individual windings and wire layers together. Fixed in this way, the windings can no longer move with respect to one another, thus eliminating vibrations and abrasion. This prevents damage to the primary insulation of the copper wire windings. The primary insulation generally consists of a very thin lacquer coat.

3. The impregnation promotes heat dissipation from the copper winding to the stator laminations, thereby improving thermal management and increasing the heat resistance of the stator – an important effect, since overheating is one of main causes of motor failure.

4. The resin protects the winding against moisture, salt spray and other external media.

Manufacturers of electric motors can adjust the properties of the impregnation to their needs by choosing a suitable resin. “With many electric motors, it is quite sufficient to impregnate the windings with an organic resin, such as an unsaturated polyester or epoxy resin,” said WACKER chemist Christian Ochs. However, for motors expected to face considerable stress from heat or media, optimum protection can be achieved with a phenyl silicone resin.

Cured phenyl silicone resins are thermally and chemically resistant. They age more slowly than organic resins under the effects of temperature and weathering. Their high durability is especially salient in the impregnation of windings particularly of motors that are subject to strong electrical currents during operation and are compact in size without active cooling. As a result, silicone resin impregnation imparts high operational reliability to the motors even under overload conditions.

As a one-component formulation, SILRES^® H62 C contains an inhibitor which slows down the curing reaction. This is advantageous if the resin is applied to the windings by traditional dip coating or vacuum-pressure impregnation (VPI). These processes are mainly used for large machines, for example for impregnating railroad, streetcar and subway traction motors, and for traction motors of diesel-electric haul trucks for open-cast mining. For this, the liquid resin must remain in a processable state during impregnation of the winding, i.e. it must not gel or cure too fast. In the automotive industry, on the other hand, the low curing rate of the one-component silicone proves to be a disadvantage – the slow curing thwarts the goal of impregnating large quantities of stator windings in a short time.

“Our customers from the automotive industry prompted us to develop a ready-to-use phenyl silicone resin formulation that would permit a rapid and automated impregnation process, similar to the established one-component product as far as the impregnation effect is concerned,” said WACKER manager Dr. Klaus Angermaier. This became possible thanks to SILRES^® H60. This impregnation resin is a two-component formulation based on the same silicone resin prepolymers used in the one-component product.

Since the new SILRES^® H60 grade is a two-component product, developers were able to keep the curing time short. For example, an inhibitor as required for one-component silicone impregnation resins is no longer necessary. This eliminated a factor that limits the curing rate. Moreover, besides the platinum catalyst, the experts at WACKER introduced another accelerator for the hydrosilylation reaction. The additional accelerator chiefly becomes active during the initial stage of thermal curing, i.e. it significantly speeds up gelling.

Thanks to its rapid curing, SILRES^® H60 is ideal for trickle impregnation of copper round-wire windings. The new resin, because of its short gel time, is not suitable for dip coating or VPI.

For trickle impregnation, the stator is clamped on a mandrel at a slight angle and rotated around its own longitudinal axis. The impregnation resin is continually dripped through multiple dispensing nozzles onto the winding head of the continuously rotating stator. Gravity and capillary forces draw the liquid into the winding and, together with the rotational movement, ensure that the resin is evenly distributed in the stator. At the same time, the rotation keeps the liquid resin in the coil, preventing the impregnation resin from flowing unused out of the stator. After trickle application, the still-rotating stator passes through a heating zone, where the resin is gelled and cured.

This offers a fast, economical and neat process for achieving a high impregnation quality with low resin-application rates. Trickle impregnation is performed in fully automated systems. The art of the resin processor consists in correctly setting all the process parameters of the trickle impregnation unit – from the application rate of the resin, through the tilt angle of the winding and the rotational speed, to the temperature and timing programs for curing. Modern trickle impregnation units can be easily integrated into electric-motor manufacturing lines.

“SILRES^® H60, for the first time, offers electric motor manufacturers a phenyl silicone resin for winding impregnation that can be processed and cured as rapidly as an organic resin,” said Dr. Angermaier, summing up the advantages of the new product. “It will open the door to rapid large-scale production of highly reliable and durable drive motors.”