Thermal life of enameled round magnet wire with various twist holder configurations

The IEEE (formerly AIEE 57) test procedure for the evaluation of the thermal stability of enameled magnet wire in air¹ did not specify a method of supporting the twisted wire samples during the thermal aging cycles. The ASTM D 2307 method² which supersedes the IEEE document, now mentions one type of holder, the "floating" type, that may be used. There have been numerous methods used in the past to support magnet wire samples by the various commercial and Navy laboratories. In a round-robin test performed by nine laboratories³ in 1958, the method of supporting the samples was optional. The use of various holders has been discussed at various meetings of the AIEE Working Group on Magnet Wire ^{4 6} but no positive action was ever taken to standardize on the type of holder to be used. A series of tests by a United Kingdom laboratory ⁷ in 1960 used three methods of supporting twisted magnet wire samples. These methods consisted of (a) hanging the samples on a rod, (b) clamping both wires at one end in a holder and (c) clamping one wire at one end and providing limited restraint on the other wire. The longest life was reported with the clamp-restraint holder (c), the clamp method (b) resulted in a longer life than with the rod method (a). No mention was made in the report of whether the samples were varnished or unvarnished. In 1960 the Naval Research Laboratory reported on the development of a multiple-clamp type specimen holder.⁸ By the use of this holder (´which is the same as the "Clamp" holder reported later in this report) some of the variables associated with the thermal study programs were eliminated. Another holder was described by McClenahan⁹ which allowed the both ends of the wire to protrude beyond the holder, thus providing essentially a floating holder.