transformers

# Transformers - Transformers Physics 232 Transformers A transformer is a device for increasing decreasing or isolating ac voltages A transformer is

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Transformers Physics 232 1 A transformer is a device for increasing, decreasing, or isolating ac voltages. Power Substation Pole-mounted transformer ac-dc converter Transformers A transformer is basically two coils of wire wrapped around each other, or wrapped around an iron core. When an ac voltage is applied to the primary coil, it induces an ac voltage in the secondary coil. A “step up” transformer increases the output voltage in the secondary coil; a “step down” transformer reduces it. Schematic: of a transformer circuit The output will be under some load resistance, R L,, that defines the current in both the primary and the secondary. A set of coupled coils with some indication of coupling material The ac voltage in the primary coil causes a magnetic flux change ∆Φ B Φ ε B P P = -N . ∆t The changing flux (which is efficiently “carried” in the transformer core) induces an ac voltage in the secondary coil given by Φ ε B S S = -N . ∆t Dividing the two equations gives the transformer equation ε ε S S S P P P N = = . N V V For a step-up transformer, N S > N P and V S > V P (the voltage is stepped up).

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Transformers Physics 232 2 For a step-down transformer, N S < N P and V S < V P (the voltage is stepped down). Transformers only work with ac voltages; a dc voltage does not produce the necessary changing flux. A step-up transformer increases the voltage. Is this an example of “getting something for nothing?” No. Current decreases. Also, even though transformers are extremely efficient, some power (and therefore energy) is lost. If the transformer is 100% efficient (no resistance in the
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## This note was uploaded on 08/25/2009 for the course PHY 213 taught by Professor Cao during the Summer '08 term at Kentucky.

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Transformers - Transformers Physics 232 Transformers A transformer is a device for increasing decreasing or isolating ac voltages A transformer is

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