Transformers Test Friday 3/30/12

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

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

input output

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

Transformer Designing a Transformer Frequency Voltage Power

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

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

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

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

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

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

Coefficient of Coupling

Mutual Inductance

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.

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

Turns Ratio = N P N S NSNPNSNP

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

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

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

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

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.)

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)

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

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

Power Grid

750,000 volts 240 volts 7,200 volts

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

Phase Shifting Transformer Applications

Isolation Transformer Applications Passes Signal unchanged Prevents Electric Shock

Blocking DC Transformer Applications DC

Produce Multiple Voltages Multi-Tap Transformer Transformer Applications

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

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

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

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