If aluminum wound transformers are properly installed, the total cost may equal or exceed the cost of a copper wound unit. The long-term maintenance costs, however, will always be there, i.e. checking for and correcting loose connections. With the shortage of qualified marine electricians and maintenance/operating personnel, the risks are probably not worth the “perceived” cost savings.
Aluminum windings require extraordinary care during installation says Joseph Payne, vice-president of DC Maritime Technologies Inc., a firm specializing in the engineering and systems integration of marine electrical equipment, with offices in Vancouver (Canada), Seattle and Mobile (USA). While copper windings are the standard for marine grade transformers, some shipyards and owners may consider buying aluminum wound transformers due to the apparent cost savings.
A careful review of the regulations, however, will probably convince most that the added installation costs considerably offset the low purchase price.
Material properties of aluminum (Ref: NVIC 2-89)
Galvanic corrosion can result from aluminum in contact with other metals, such as steel. Aluminum alloys containing copper are particularly subject to corrosion in a damp salt atmosphere, even when not in contact with a dissimilar metal. Most aluminum alloys form a hard, inert oxide coat when a fresh surface is exposed to air. This layer of aluminum oxide has a high electrical resistance and can create a hot spot at connection points. Aluminum exhibits a phenomenon known as creep, which is a plastic deformation that occurs at stresses below yield strength. Periodic tightening of many types of aluminum connections prevents connections from becoming loose. If connections do become loose, the surface contact area is reduced, permitting the oxide coat to form, which in turn causes high-resistance hot spots.
U.S. Coast Guard Regulations
Navigation and Vessel Inspection Circular (NVIC) 2-89 permits the use of aluminum wound transformers as long as they are fully encapsulated; connecting hardware is plated with silver, nickel, or tin after all drilling has been completed; a joint compound is applied to all joint surfaces before assembly to seal out air and improve corrosion resistance, etc.
More information: Joseph Payne