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Transformers


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Current

Electric current is the drift of electrons

from an area of negative charge to an area

of positive charge. Current is measured in

Amperes (A).


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Direct Current

In DC (direct current) electrons flow in one

direction, from the negative part of the circuit

towards the positive side. In DC the voltage and

current stays the same at all times.

DC Voltage Over Time

0

1

2

3

4

5

6

7

8

9

1 2 3 4 5 6 7 8 9 10 11 12

Time in Seconds

Voltage

in

Volts


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Alternating Current

An alternating current (AC) circuit is one in

which the direction and amplitude of the

current flow changes at regular intervals.

The polarity of the AC voltage source

changes at regular intervals resulting in a

reversal of the current flow.

Discuss:

Alternations

Cycle

Hertz

Amplitude

Text: pages

122 - 125
http://upload.wikimedia.org/wikipedia/en/e/ea/Sine_wave.svg


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Transformers

What are they?

Why are they used?

How do they work? Real World Examples

Center Tap what?

Calculating: Es/Ep/Is/Ip/Ps/Pp/Ns/Np
http://upload.wikimedia.org/wikipedia/en/1/1c/Polemount-singlephase-closeup.jpg


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What is a Transformer?

A Transformer is a device that is used to

transfer electrical energy from one circuit

to another through electromagnetic

induction.

Typically transformers are used to step up

or down voltage between two circuits

(though there are other uses).


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Electromagnetic Induction

When a conductor passes through, or is

passed by, a magnetic field, a current is

produced in the conductor.

Demo


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How Transformers Work

Alternating current ispassed through the primarycoil (the input) whichcreates a changingmagnetic field in the ironcore.

The changing magneticfield then inducesalternating current of thesame frequency in thesecondary coil (the output).


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How Transformers Work


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Step Up Transformer

A step up transformer has more turns of wire on thesecondary coil, which makes a larger induced voltage inthe secondary coil. It is called a step up transformerbecause the voltage output is larger than the voltage

input. If the secondary coil has twice as many turns of wire

then the output voltage will be twice the input voltage.

Turn Ratio:

1:2


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Step Down Transformer

In a step down transformer the voltage output issmaller than the voltage input. If the secondarycoil has half as many turns of wire then theoutput voltage will be half the input voltage.

(Decreasing the voltage does not decrease thepower. As the voltage goes down, the current

goes up.

Turn Ratio:

2:1


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What is the Turn Ratio?

The turn ratio is all about the windings in theprimary coil vs. the windings in the secondarycoil. If there are more windings on the primarythan the Secondary the transformer will be a

Step down. If there are more windings on thesecondary then the transformer will step downvoltage.

The Ratio of one set of turns to another will helpyou identify how much your voltage and currentwill change. If a Transformer is said to have a2:1 Ratio it means for every 2 turns of wire onthe primary there is 1 on the secondary.


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Real World Uses of Transformers

Utilities distribute electricity over large areas using highvoltages, commonly called transmission voltages.

Transmission voltages are normally in the 35,000 volt to

50,000 volt range. High voltage allows the utility to use

small sizes of wire to transmit high levels of power, or

watts. You can recognize transmission lines because they

are supported by very large steel towers that you see

around utility power plants and substations.


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Real World Uses of Transformers

As this electricity gets closer to its point ofuse it is converted, through the use oftransformers, to a lower voltage normallycalled distribution voltage. Distribution

voltages range from 2,400 to 25,000 voltsdepending upon the utility. Distribution linesare the ones that feed the pole mount andpad mount transformers located closest toyour home or place of business. Thesetransformers convert the distribution voltagesto what we call utilization voltages. They arenormally below 600 volts and are utilized foroperating equipment, including light bulbsand vacuum cleaners in our homes, to motorsand elevators where we work.
http://upload.wikimedia.org/wikipedia/en/1/1c/Polemount-singlephase-closeup.jpg


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Transformer Schematic Symbol


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What is a Center Tap?


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Voltage Primary Vs. Voltage

Secondary

Voltage Primary of Ep is the voltage

across the primary coil.

Voltage Secondary of Es is the voltage

across the secondary coil.

In a Step up transformer Es will be larger

than Ep and in a step down it will be the

exact opposite.


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Turn Ratio and Voltage

The transformer equation relates the

number of turns of wire to the difference in

voltage between the primary and secondary

coils.

Ep/Es = Np/Ns Ep is the voltage in the primary coil.

Es is the voltage in the secondary coil.

Np is the number of turns of wire on the primary coil.

Ns is the number of turns of wire on the secondary coil.

Demo


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Calculating Turn Ratio

The Turn Ratio can be calculated by

dividing the number of Secondary turns by

the number of Primary turns.

Turn Ratio = --------------

N = The number of turns in the coil.

NsNp

i.e. Turn Ratio = 500 turns in Secondary / 100 Turns in Primary

Turn ratio = 5

Ratio = 1:5

Text p. 169-170

i.e. Turn Ratio = 75 turns in Secondary / 750Turns in Primary

Turn ratio = 0.1

Ratio = 10:1Or just divide the large number by thesmaller number


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Current Primary vs. Current

secondary

Current Primary or Ip is the current that

flows through the primary coil.

Current Secondary or Is is the current that

flows through the secondary coil.

The current is inversely proportional to theturns ratio:

Is/Ip = Np/Ns

Demo


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Power Primary vs. Power

Secondary

Power, which is the work that can be done

by a circuit is measured in Watts. It is

calculated by multiplying the voltage by

the current. P = IxE

In a transformer the Pp is equal to the Ps.


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Transformer Formulas


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There are two points to remember

1. Transformers only work with alternating current.Using direct current will create a magnetic field in thecore but it will not be a changing magnetic fieldand so no voltage will be induced in the secondary coil.

2. Using a step up transformer to increase the voltagedoes not give you something for nothing. As the voltagegoes up, the current goes down by the same proportion.The power equation shows that the overall power

remains the same, P = V x I Power = Voltage x Current.


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Oscilloscope

An electronic measuring instrument that

creates a visible two-dimensional graph,

on a screen, of one or more continuously

varying voltages or currents.

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