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Transformer AC Source Alternating current comes from generators, not batteries. • Ideal sinusoidal source The symbol for an AC source uses a sine curve. R DV D V AC V 0 sin t I AC V0 R sin t Internal Flux Faraday’s law links flux to voltage. DV N • Change in voltage to change in flux • Change in flux to change in voltage D M Dt The flux can link one conductor to another. • Iron for better flux link • Minimize losses DVA R N A NB Primary Coil D M Dt An AC voltage source produces a changing voltage. DVA R N NB A D Dt M DV A NA The changing voltage creates an opposing magnetic flux in the iron. Secondary Coil Faraday’s law describes the induced voltage in the second coil. • Assume all field lines from primary go through secondary The induced voltage directly depends on the primary voltage. DVA R N NB A DVB N B D Dt M NB DV A NA Turns Ratio The output voltage depends on the ratio of the turns in the coils. • Symbol represents linked coils If NB > NA then the voltage increases. • Step-up transformer If NB < NA then the voltage decreases. • Step-down transformer DVB NB NA DV A Transformers Commercial transformers wind two coils around a linkng medium. • Single cylinder of air or iron • Connecting bar of iron The schematic symbol represents two coils. AC Adapter An AC adaptor is an example of a step-down transformer. • Convert 120 V AC to 18 V DC Power transmission uses step- up up and step-down transformers. • Power plant: 10 kV to 345 kV • Substation: 345 kV to 7200 V • Power pole: 7200 V to 120 V Power Law Transformers do not create power. • Energy conserved • Losses small compared to resistors For no power loss, current and voltage must maintain same power. • Current decrease for step-up • Current increase for stepdown Pout Pin V out I out V in I in I out V in I in V out V in I in N out N in I out N in N out V in I in next