Core loss

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. 

Core losses or magnetizing losses, which comprise... 

Since the resistive loss would vary in a square proportion of the current, the motor will overheat on lower voltages (drawing higher currents). At higher voltages, while the stator current may decrease, the core losses will be higher. 

For all practical purposes, the core losses may be regarded as proportional to the square of the flux density. With a reduelion in voltage, the flux

flux density B will decrease in the same proportion as the voltage and so will the core losses. 

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. 

For yet larger motors of frame sizes 710 and above, core losses play a more significant role, and require very effective cooling to dissipate the heat generated. Cooling of larger machines, complicated as it is in view of their size and bulk, necessitating core losses to be restricted as low as possible. 

Ignoring the friction and core losses, the torque developed in synchronous watts. 

Stray load loss (hysteresis and eddy current core loss). 

The watts input is the sum of the friction and windage losses, core loss and no-load primary loss (/njR )- The sum of friction, windage and core losses is obtained by... 

Core loss is the magnetizing or hysteresis loss and represents the iron loss of the machine. 

I will be using an E-E core since it has the largest wire area of all the core styles. The large wire area will be needed for all the additional insulating layers required for the VDE certification. No air-gap is needed for bipolar forwardmode converters. The core material is going to be 3C8 (Ferroxcube) or F (Magnetics, Inc.) material. This material will yield reasonable core losses at this frequency. 

I will be using an E-E eore with the neeessary Mylar tape insulating layers to meet the isolation requirements of the safety approval ageneies. At these operating frequeneies, a high-frequency core material will have to be used to minimize the core losses at 1 MHz. The material should be the K material from Magnetics, Inc, 3C85 material from Phillips, or N67 material from Siemans. 

Figure D-4 Curves showing volumetric core loss vs. frequency and B, shown). (Courtesy of Philips Components.)...

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