Hysteresis Loss When a magnetic field is passed through a core the core

Hysteresis loss when a magnetic field is passed

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Hysteresis Loss When a magnetic field is passed through a core, the core material becomes magnetized. To become magnetized, the domains or magnetic units within the core must align them- selves with the external field. If the direction of the field is reversed, the domains must turn so that their poles are aligned with the new direction of the external field. This process takes energy. Power transformers normally operate from either 60-cycle-per- second or 400-cycle-per-second alternating current. Each tiny domain must realign itself twice each cycle or a total of 120 times a second when 60-cycle alternating current is used. The energy used to turn each domain is dissipated as heat within the iron core. This loss is called hysteresis loss, and can be thought of as resulting from a kind of friction. When you increase the frequency of the applied voltage, greater motion of the magnetic elements of the core material are caused and thus a greater loss through heat dissipation. Hysteresis can be minimized by proper core selection. Gener- ally air-core transformers are used for frequencies above audio range - 20KH - because, with an air core, hysteresis loss is kept to a minimum. imum. You can assume, then, that iron cores are generally used with frequencies in the audio range. Transformer Ratings Most transformers have ratings on the nameplates according to their capabilities for handling: 1. voltage 2. current 3. power Voltage Handling Capacity The voltage rating depends on the :onstruction of the primary and secondary windings. The type and thickness of the insula- tion between the windings determines how much voltage the wind- ings can stand before the insulation breaks down and wires short. The thicker (and better) the insulation, the more voltage the transformer can handle. 107
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Narrative Frequency Ten-V A frequency higher than the rated value can be applied to a trans- former without serious damage. For example, a 400-cycle voltage can be applied to a 60-cycle transformer. Transformer efficiency will be seriously reduced, but no damage is likely to occur. However, if you apply a 60-cycle source to a transformer rated at 400 cycles,, you may have circuit trouble. Let's see why! When you increase frequency, XL increases and current decreases; therefore, a higher frequency source results in less current and a small reduction in power but no damage to the transformer. Now, if you decrease frequency to 60 cycles for a 400-cycle trans- former, then X L decreases and current increases. If current ex- ceeds the current handling capacity of the transformer, significant damage results. Current-Handling Capacity The current-handling capacity is determined largely by the diameter of the wire used for the windings. If current in the windings is excessive, there is too much power dissipated in heat and the insulation on the wires is damaged. If excessive current is permitted to flow for too long a time, the transformer will be permanently damaged.
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