Resistance limiting/preventing

The reason for the use of zinc as a power-impressed rather than a sacrificial anode is that the high concrete resistivity limits the current output, and a higher driving voltage than that provided by the e.m.f. between zinc and steel in concrete is used to provide the necessary current output. No cementitious overlay is required, although it may be advisable to paint the top surface of the sprayed zinc to prevent atmospheric corrosion of the zinc anode. 

They indicated [211] that, while this equation reflects the state-of-the-art, its major limitations prevent one from obtaining quantitative structure-property relationships based on it. It does not involve the structural parameters of the material it takes no account of the relaxation (prefracture) state and it provides no way to describe any stepwise transitions, discontinuities, and abrupt qualitative changes upon the interaction of a material with its environment. They then devised a new method based on their autooscillation model of the solid state, and showed the promise of this method by using a heat-resistant polyimide as their example. 

Water. Latices should be made with deionized water or condensate water. The resistivity of the water should be at least lO Q. Long-term storage of water should be avoided to prevent bacteria growth. If the ionic nature of the water is poor, problems of poor latex stabiUty and failed redox systems can occur. Antifreeze additives are added to the water when polymerization below 0°C is required (37). Low temperature polymerization is used to limit polymer branching, thereby increasing crystallinity. 

Restraints. A restraint limits thermal reactions at equipment and line stresses or expansion movement at specifically desired locations. It may be defined as a device preventing, resisting, or limiting the free thermal movement of a piping system. Because the appHcation of a restraint reduces the inherent flexibiHty of the piping, its effect on the system is estabHshed through calculation. 

The beryUides continue to be of interest for high temperature aerospace appHcations because of their oxidation resistance, low density, and high strength at elevated temperature (7). The limited strain capacity of the materials, particularly at low temperatures, has thus far prevented actual use. 

The major differences between ac and dc starters are necessitated by the commutation limitation of dc motors, which is the ability of the individual commutator segments to interrupt their share of armature current as each segment moves away from the brushes. Normally 250 to 275 percent of rated current can be commutated safely. Since motor-starting current is limited only by armature resistance, line starting can be used only for veiy small [approximately 1492-W (2-hp)] dc motors. Otheiwise, the commutator would flash over and destroy the motor. External resistance to limit the current must be used in starting to prevent this. 

Manual rheostats can be used in series with the motor armature for the current-limiting func tion. If the rheostat has ample thermal capacity, it can also Be usedto vaiy speed. If this system is used, interlocks should be included to prevent closing of the contactor unless maximum resistance is in the circuit. 

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