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Odours have been generated during commissioning and first time operation of large rotating electrical machines. These emissions were observed in machines from several manufacturers. Subsequent investigations indicated that formaldehyde was present. The main source of the emissions was located in the insulation varnish of the sheets of the stator core. In core sheet varnishes, formaldehyde containing precursors are often used as crosslinking agents. Energy input during the curing process, determines the extent of reversibly bonded formaldehyde after the cure. Weakly bonded formaldehyde is released when the coated sheets are heated during the early operation of the generator.The influence of the curing conditions on the formaldehyde emissions of stator core sheets was investigated in laboratory experiments. Experiments showed a decrease in formaldehyde emissions by increasing the energy input during the curing process. Even if the energy input was high enough to achieve the required mechanical (e.g. cross cut test), chemical (e.g. rubbing alcohol test) and electrical properties (e.g. surface resistivity), the formaldehyde release could still be reduced by further increase of the energy input. Appropriate material properties do not correlate with the amount of releasable formaldehyde in the varnish. This gives an opportunity to modify the curing to reduce or eliminate the emissions without negatively impacting the insulating properties.The influence of the chemistry of the varnishes on the formaldehyde release was also studied. Varnishes based on phenolic and melamine resins give the highest emissions, while the polyester-based varnishes showed no emissions. The temperature dependence and the kinetics of the formaldehyde release of stator core sheets are investigated in model tests. The laboratory experiments on the kinetics of the emissions showed, that at a certain temperature the majority of formaldehyde is emitted over a short time. Later emissions approach zero with continued thermal exposure.Temperature tests on stator core sheets in the range of 30-100 C showed, that at 30 C small amounts of formaldehyde are released. Interestingly, the formaldehyde emission does not increase linearly with increasing temperature, it shows a first maximum at about 60 C and a second maximum at about 100 C. This implies chemical reactions associated with definite activation energies that are required for splitting off the formaldehyde.Measures can be taken, during lamination processing, to avoid improper formaldehyde emission in large rotating electrical machines (safety measures and venting in existing machines, optimization of the curing process, development of formaldehyde free coatings for stator core laminations).

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