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Nickel Alloys |
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When we talk about nickel alloys for rotating electrical equipment, we are talking about two alloys, 49% Ni and 80% Ni, with the balance being primarily pure iron in both cases. These alloys are chosen for their very high permeability at low to moderate inductions, and for their low core losses. These characteristics make them ideal not only for motors, but particularly for synchros and resolvers. These materials are significantly higher in cost than silicon steels, and require careful handling to achieve acceptable properties. The 80% nickel alloy is not suitable for high flux densities (Above about 8000 Gauss) due to saturation at that level. The difference between the two alloys is one of degree rather than type, with the 80% Ni having the same advantages as 49%, only more so. Nickel alloys require a very careful annealing cycle in order to be useful. The required temperature is over 2000 F, and special precautions must be taken to avoid welding the laminations together at that temperature. Usually, Aluminum oxide powder is placed between the laminations, or a magnesium oxide solution can be applied to the parts and allowed to dry. In addition, a very pure atmosphere of dry hydrogen is required. Finally, as if that wasn't enough, the properties of the annealed parts are very sensitive to stress. Bending or even dropping the laminations can completely destroy their value. For this reason, the flatness of the finished part must be controlled closely to avoid stresses when put into stacks. Surface insulation for nickel alloy laminations must be added as part of the annealing process. Most commonly, air or steam is introduced into the retort at about 900 F, which produces a blue- black film integral with the surface of the part. Insulation is not available on the raw material as is the case for silicon steel. The previous discussion is not meant to imply that users of the nickel alloys should expect problems, and in fact the material is used extensively in certain applications. Due to the high value of the finished parts, however, the prospective user of nickel laminations should exercise care in selecting a vendor with experience with nickel alloys. (Particularly an annealing vendor. Joe's Heat Treat and Muffler Shop around the corner is not recommended). |
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