This method is intended for evaluating the bond strength of a thin film of impregnating varnish on a helical coil. This method does not apply to the testing of winding wires. Since a primary function of varnish is to bond the components of an insulating system, the strength of the varnish film is one measure of performance quality. By measuring the changes in the strength of varnish films as affected by cure, temperature and ageing at high temperature, the quality of varnishes can be compared.
In this test, 1 mm diameter wire in the form of a helical coil is coated with varnish and cured. The force necessary to break this coil as a simple beam is a measure of the varnish film strength.
The behaviour of the varnish on enamelled wire may differ from that exhibited on aluminium wire. The evaluation of compatibility between varnish and enamelled winding wires is not within the scope of this Recommendation.
The method is only one of those which will need to be used to determine the suitability of a varnish for use in practice. The order of merit in which different varnishes are placed may depend on the particular test chosen.
SIS SS IEC 290:1982 history
1982SIS SS IEC 290:1982 Evaluation of the thermal endurance of electrical insulating varnishes by the helical coil bond test