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Transformers Test Friday 3/30/12

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Electromagnetic Induction Induction is the process of producing (inducing) a voltage by passing a wire through a magnetic field. Generator

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In induction one of two things must be happening!! The magnetic field is moving. The wire is moving.

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input output

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AC voltages A transformer works with AC voltages, since the magnetic field must be changing to induce a voltage in the coils.

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Transformer Designing a Transformer Frequency Voltage Power

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Transformer Rating Transformer are rated in Volt-Amperes (VA) Volt Amperes are used to determine the Maximum Current the transformer can handle.

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A transformer consists of two coils of wire wound around a core of soft iron.

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N P N S The side connected to the input AC voltage source is called the primary and has N P turns.

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N S The other side, called the secondary, is connected to a load and has N S turns.

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Core The core is used to increase the magnetic flux and to provide a medium for the flux to pass from one coil to the other

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Coefficient of Coupling The measure of how good the transformer is. Scale of 0 to 1 1 – All the magnetic Flux lines cut the Secondary Winding 0 – None of the magnetic Flux lines cut the Secondary Winding

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Coefficient of Coupling

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Mutual Inductance

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The fact that a change in the current of one coil affects the current and voltage in the second coil is quantified in the property called mutual inductance.

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Turns Ratio The turns ratio of a transformer is the ratio of number of windings of primary side to the secondary side of the transformer. N P N S

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Turns Ratio = N P N S NSNPNSNP

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Voltage Relationships The voltages are related by: N P V P N S V S =

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Voltage Relationships When N S > N P, the transformer is referred to as a step-up transformer. Voltage Increases

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Voltage Relationships When N S < N P, the transformer is referred to as a step-down transformer. Voltage Decreases

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Power I P V P = I S V S The power input into the primary equals, at best, the power output at the secondary. Power In = Power Out

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Power I P V P = I S V S If V S increases, as in a step up transformer, I S must decrease. If V S decreases, as in a step down transformer, I S must increase. (This assumes an ideal transformer.)

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Power Efficiency You don’t get something for nothing!!!! In real transformers, power efficiencies typically range from 90% to 99% (0.9 to 0.99)

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V P =100v N P = 250 N S = 500 V s =? R s =1k Ω

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100v X 500 250 = VSVS V P N S N P = VSVS = 200V VSVS

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Power Grid

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750,000 volts 240 volts 7,200 volts

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Transformer Applications Impedance Matching (Ω) More Power is Transfered!!!

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Phase Shifting Transformer Applications

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Isolation Transformer Applications Passes Signal unchanged Prevents Electric Shock

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Blocking DC Transformer Applications DC

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Produce Multiple Voltages Multi-Tap Transformer Transformer Applications

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Autotransformer Transformer Applications Step-Up or Step-Down No Isolation!!!

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V P =100v R s =1k Ω N P = 250N S = 500

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Ch 12: AC Pages 122 – 128 Answer Question through Chapter Write down Questions and Answers AC Worksheets: 2-1 Lab 1: Book 2 “DC and AC” Problem Worksheets 4 Simulations – Computer Lab Due 3/26 th Test 3/26 th

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Ch 18: Transformers Pages 168 – 174 Answer Question through Chapter Write down Questions and Answers Transformer Worksheets: Lab 2-14 Lab: 29 Old Book Problem Worksheets Video Worksheet – “Generating Electricity” Due 3/26 th Test 3/26 th

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