17 July 2012
17 July 2012
Dr Werner Hollstein from Huntsman Advanced Materials has looked at how the latest epoxy and polyurethane encapsulation systems are capable of supporting, or even enabling, new and robust motor designs with the assurance of high cost-efficiency and quality.
According to Hollstein, 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. He says simple varnishing for the electrical insulation and mechanical fixation of rotor and stator windings is typically used, but this tends to result in the following issues:
• Various losses in operation leading to high temperatures and overheating
• Vibrations causing wear and the short-circuiting of windings
• Aggressive oils, chemicals, vapours and humidity attacks damaging windings
Hollsein suggests that 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 where the copper windings, gaps and undercuts are completely impregnated and filled with polymer.
He explains that 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. He says that, 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.
When selecting a suitable encapsulant Huntsman says it is important to choose one which offers high thermal conductivity, precisely defined flow properties and short curing times.
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.
According to Huntsman, 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. It says 20 test cycles were successfully conducted down to temperatures as low as -80°C. 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.
They say that 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 mould 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.
Huntsman claims that amines are the most commonly used curing agents for epoxy cure saying 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 C fulfilling class F.
Hollstein says polyurethanes (PUR) have been used for electrical insulation since the beginning of the 1950s when cost-efficient raw materials became available on an industrial scale. He continues, 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.
He says 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.
Most epoxy based systems are supplied as two separate components. Hollstein claims that, for mass production, an extensive range of equipment is required to process these systems. One-component 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. Huntsman says 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.
Hollstein says that, in summary, 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. He continues; today’s epoxy and 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.
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