01 April 2013
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Thermosetting resins for stator encapsulation in industrial motors

Electric motors and generators are of increasing importance for industrial and automotive applications but they present a number of challenges that design engineers need to overcome. In this
article, Dr Werner Hollstein addresses these challenges and looks at how the latest encapsulation systems are capable of supporting robust motor designs.

In the on-going challenge to produce motors and generators that offer more power output, higher integration density, higher reliability, resistance to harsh conditions and noise reduction, thermosetting resin systems for full stator or end-turn encapsulation provide the ideal solution.

Today's epoxy and polyurethane (PUR) systems offer the necessary material characteristics which cover the need for high heat dissipation, electrical insulation, mechanical fixation, damping and protection from aggressive chemicals, vapours and humidity.

Huntsman Advanced Material's products are specifically designed for different processing and application technologies such as vacuum casting and automated pressure gelation which allow short cycle times and high throughput. The presented technologies, using the latest resin systems can contribute tremendously to new motor designs for e-mobility and industrial applications where reliability, high cost-efficiency and quality are key.

Motors and Generators

Emerging challenges for the development of motors and generators for industrial and automotive applications include size reduction, higher integration density, more power output, greater reliability and endurance, resistance to harsh environments and noise reduction. Simple varnishing for the electrical insulation and mechanical fixation of rotor and stator windings is typically used, but this tends to result in certain issues: various losses in operation lead to high temperatures and overheating; vibrations cause wear and the short-circuiting of windings; and aggressive oils, chemicals, vapours and humidity can damage windings.

As the load on the motor increases, so do the operational losses mentioned above. Thermosetting resin systems for encapsulation and impregnation provide an answer to these problems.

One option is a fully encapsulated stator here the copper windings, gaps and undercuts are completely impregnated and filled with polymer (see figure 1)

A sealing core is placed in the middle of the stator to assure vacuum tightness and to prevent the resin from contaminating the metal laminations. The liquid resin system is degassed and potted, preferably under vacuum, into the stator. It is essential that the resin system has a low viscosity and sufficient latency to enable fast filling and impregnation. Following this,
the oven curing needs to be optimised and controlled to minimise volume shrinkage and mechanical stresses.

Another option is the encapsulation of the end-turns. In general, more than 60% of lost heat is produced in the end-turns of a stator. Therefore it's most effective to only pot the gap between the end-turns and housing (see figure 2).

When selecting a suitable encapsulant it is important to choose one which offers high thermal conductivity,precisely defined flow properties and short curing times.

Full stator encapsulation

Formulated epoxy resin systems are irreplaceable in many electrical applications. They offer excellent electrical insulation, good mechanical characteristics, chemical resistance and
thermal endurance. Processing at temperatures between 60 to 80°C significantly reduces the viscosities of these systems, allowing higher filler loadings and fast filling properties. Final curing requires temperatures above 100°C. Araldite CW 229-3/Aradur HW 229-1 is a good example of a prefilled resin system which provides high crack and thermal shock resistance. Temperature cycle tests using an embedded metal insert with an edge radius 1mm have been successful-ly conducted down to temperatures as low as -80°C. In these 20 tests, the impregnation capability was proven to be good, with a heat conductivity of 0.7 W/m K. Thermal endurance in long-term ageing tests (IEC 60216) resulted in a thermal index of more than 180°C (class H).
Even 200°C was determined as a relative temperature index (RTI) following UK746B. Therefore, good heat dissipation, reliable electrical insulation and thermal endurance for highly loaded
motors and generators are ensured.

To respond to demands for short cycle times, Araldite CW 229-3/Aradur HW 229-1 is also available with higher reactivity, making the mandatory need for the post-curing of normal systems obsolete. This non-post-cure (NPC) system is suited to the automated pressure gelation (APG) process, offering additional advantages of shorter moulding times and lower mold temperatures.
If heat dissipation is the most important requirement for stator encapsulation, Araldite XB 2710/Aradur XB 2711 provides a good solution, facilitating heat conductivity at 1.5 W/m K and
the assurance of high thermal conductivity. In offering similar properties to Araldite CW 229- 3/Aradur HW 229-1, this system also offers excellent crack resistance and low coefficient of thermal expansion. Amines are the most commonly used curing agents for epoxy cure. However, the reactivity of these systems allows them to also cure at room temperature. No ovens are needed and therefore processing equipment is much simpler and lower in cost. Araldite XB 2252/Aradur XB 2253 is a cold curing epoxy resin system with excellent flowability and impregnation features.
Thermal endurance is exceptionally high with a thermal index of 180 degrees C fulfilling class F. Araldite CW1312/Aradur HY 1300 is another example of a resilient cold curing system which exhibits good resistance to thermal ageing and good thermal shock resistance, with proven performance in class B applications. PURs have been used for electrical insulation since the beginning of the 1950s when costefficient raw materials became available on an industrial scale. The chemical reaction of a polyol and an isocyanate results in a polymer with urethane linkages. If crosslinking occurs in three dimensions, the resulting polymer belongs to the class of elastomers and thermosets. The curing reaction is fast and exothermal at room temperature and no ovens are needed. Because of the large variety of polyols, isocyanates, modifiers and fillers, PURs can be tailored for a broad range of applications, including full stator encapsulation.

Arathane CW 5631/HY 5610 is easy to process and has good impregnation capabilities; these are exceptional features for a PU system. The flame retardancy UL94 V-0 is met for the cured material, heat conductivity is in the range of 0.6 W/m K and excellent heat ageing resistance is provided. Another advantage of using Arathane CW 5631/HY 5610 is its adaptability, which allows the full encapsulation of different sizes and designs of stators simply by changing the mix ratio. With the reduction down to 100:19 pbw the shore hardness is significantly reduced from D80 to D55 and importantly, the system's excellent crack resistance properties remain unaffected.

End-turn encapsulation

Most epoxy based systems are supplied as two separate components. For mass production an extensive range of equipment is required to process these systems. Onecomponent products are much simpler to process and significantly reduce machinery needs. Existing one-component epoxies are in commercial use as adhesives, sealants, moulding compounds and impregnation and casting resins. Aratherm CW 2731 has been developed for the encapsulation of motor and generator end-turns. This 'pasty' single component epoxy is prefilled with a special type of filler to achieve a high heat conductivity of 3.0 W/m K. It requires no pre-heating, homogenisation or degassing and flowability can easily be adjusted to fill the gaps between the wires and housing.

Oven curing is not needed if the heat capacity of the pre-heated stators is high enough to keep the temperature above 150°C for an hour.

Another important material property is the high Tg of 160°C which ensures consistency over the whole range of operational temperatures and excellent thermal durability. The low coefficient of thermal expansion minimises the thermal mismatch of materials and prevents cracking and delamination.

Paul Gay

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