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AC Transformers Source: OSHA
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Flux Change Creates Emf
Rapidly changing field high induced emf unchanging field zero induced emf
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How a Transformer Works
AC current in the primary coils creates a changing magnetic field. The magnetic flux inside the secondary coils changes Changing flux induces emf in the secondary coils
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Potential Proportional to Loops
Same flux F through both sets of windings Each loop adds emf Emf’s V are proportional to the number of loops N V1 N1 V2 N2 =
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Energy is Conserved Ideally: power in = power out V1I1 = V2I2
Realistic: power in > power out Efficiencies usually around 95%
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Transformer Summary Power in power out loops higher V, lower I
loops lower V, higher I
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Question A “step-down” transformer converts input at 120 V to output at 20 V. If the input circuit has 1100 W, how much power is available at the output? 200 W. 660 W. 1100 W. 2400 W.
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General Physics L18_Faraday
Group Work A transformer with 1000 primary windings converts AC at 110 V (primary) to 220 V (secondary). Which is greater: the voltage in the primary circuit or the voltage in the secondary circuit? Where will the number of loops be greater: in the primary or in the secondary?
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Group Work A transformer with 1000 primary windings converts AC at 110 V (primary) to 220 V (secondary). What is the voltage ratio V2/V1? What is the loops ratio N2/N1? How many loops are in the secondary circuit (N2)?
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Group Work A transformer with 1000 primary windings converts AC at 110 V (primary) to 220 V (secondary). What is the current ratio I2/I1?
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Formulas for Transformers
Loops: V1 V2 N1 N2 = Power: V1I1 = V2I2 Potential: V2 = V1 I1 I2 = V1 N2 N1 Current: I2 = I1 V1 V2 = I1 N1 N2
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Example A transformer with 5000 primary coils and 100 secondary coils has an input voltage of 50 kV. What is the output voltage? Vout = Vin Nout/Nin = (50 kV)(100/5000) = (50 kV)/50 = 1 kv = 1,000 V
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