Steel core losses

Soft magnetic materials are characterized by high permeabiUty and low coercivity. There are sis principal groups of commercially important soft magnetic materials iron and low carbon steels, iron—siUcon alloys, iron—aluminum and iron—aluminum—silicon alloys, nickel—iron alloys, iron-cobalt alloys, and ferrites. In addition, iron-boron-based amorphous soft magnetic alloys are commercially available. Some have properties similar to the best grades of the permalloys whereas others exhibit core losses substantially below those of the oriented siUcon steels. Table 1 summarizes the properties of some of these materials. 

Table 2. Core Loss of Grain-Oriented Silicon Steels, W/kg ...

Core-Loss Limits. In the United States, flat-roUed, electrical steel is available in the following classes (12) nonoriented, fiiUy processed nonoriented, semiprocessed nonoriented, fiiU-hard and grain-oriented, fiiUy processed. Loss limits are quoted at 1.5 T (1.5 x lO" G). The loss limits at 1.7 T (1.7 X ICf G) of the fourth class and of the high induction grades are shown in Table 2. Typical appHcations include use for transformers, generators, stators, motors, ballasts, and relays. 

The steel of laminations plays a very significant role in determining the heating and the power factor of a motor. See Section 1.6.2A(iv). A better design with a judicious choice of flux density, steel of laminations and its thickness are essential design parameters for a motor to limit the core losses to a low level. 

For a lower range of motors, say up to a frame size of 355, the silicon steel normally used for stator and rotor core laminations is universally 0.5-0.65 mm thick and possesses a high content of silicon for achieving better electromagnetic properties. The average content of silicon in such sheets is of the order of 1.3-0.8% and a core loss of roughly 2.3-3.6 W/kg, determined al a flux density of I W[ym and a frequency of 50 Hz. For medium-sized motors, in frames 400-710, silicon steel with a still better content of silicon, of the order of 1.3-1.8% having lower losses of the order of 2.3-1.8 W/kg is prefeired, with a thickness of lamination of 0.5-0.35 mm. 

However, up to 30% of the power consumption of a motor can be attributed to no-load losses because of windage (by cooling fan and air drag), friction in the bearings, and core losses that comprise hysteresis and eddy current losses in the motor magnetic circuit. Load losses include stator and rotor losses (resistance of materials used in the stator, rotor bars, magnetic steel circuit) and stray load losses such as current losses in the windings.f ... 

Silicon steel sheets with low electromagnetic core loss are supplied with a surface layer of low thermal expansion material such as boron nitride. This can be done by ion implantation, ion plating, chemical vapor deposition (CVD), and secondary recrystallization annealing in a nonoxidative atmosphere [72 to 77]. 

In the present study, the variation of magnetic core losses with temperature of a single readily-available soft magnetic material was obtained. For convenience, the core was constructed from the silicon steel punchings of a commercial 400 cycle per second power transformer, A... 

The presence of tubercles is usually obvious. Friable brown and orange nodular encrustations on mild steel and cast iron cooling water components are almost always tubercles (Figs. 3.12 through 3.14). The presence of a crust, shell, core, cavity, and corroded floor are definitive (Fig. 3.3). Careful analysis can provide considerable information concerning growth, chemical composition, and associated metal loss. 

Where B = induction magnetic flux (gauss), and r = surface resistivity (fl-cm per lamination). Therefore, interlaminar loss decreases with increasing surface resistivity. Figure 2 shows the surface resistivity required for t = 0.023 cm transformer core steel to maintain predicted interlaminar eddy current power loss, P, at B = 15 kG, and/= 60 Hz. 

Instead of cutting a piece of rebar to get an isolated probe, an external preconditioned corroding piece of steel can be mounted into the structure, usually contained in a concrete core (Figure 17.3) that is put into a hole drilled in the structure. This measurement arrangement allows the same information to be obtained as from the isolated piece of rebar. Experience from several years (Figure 17.7) has shown that in this way correct corrosion rates (when compared to weight-loss measurements) can be obtained and the prediction of future corrosion rates for a given exposure condition becomes possible [15]. 

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