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

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.

The side connected to the input AC voltage source is called the

primary

and has N P turns.

N P N S

The other side, called the

secondary

, is connected to a load and has N S turns.

N S

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

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

N P N S Turns Ratio = N S N P

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 The power input into the primary equals, at best, the power output at the secondary.

Power In = Power Out I

P

V

P

= I

S

V

S

Power

I

P

V

P

= I

S

V

S (This assumes an ideal transformer.) If V S increases, as in a transformer, I S

step up

must decrease. If V S decreases, as in a

down

transformer, I increase. S

step

must

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 s =?

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

V

S

=

V

P

N

S

N

P

V

S

V

S

100v X 500

=

250

= 200V

Power Grid

750,000 volts 7,200 volts 240 volts

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

Transformer Applications Phase Shifting

Transformer Applications Isolation Passes Signal unchanged Prevents Electric Shock

DC Transformer Applications Blocking DC

Transformer Applications Produce Multiple Voltages Multi-Tap Transformer

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

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

Ω

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