3-lesson notes lec 7(trafo)

7/31/2019 3-Lesson Notes Lec 7(TRAFO)

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Lecture 7


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Autotransformer

Another type of transformer is known as the autotransformer.It consists of a single tapped primary where the center tap is common

to both primary and secondary (not isolated).

some of the turns on the coil are used for the primary and some areused for the secondary.

an example of a variable autotransformer is known as a variac.

A variac is a single coil with a sweeping arm for the center common,

this allows the ratio of primary turns:secondary turns to be alteredeasily.


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Auto-transformer

transformer having a part of its windings common to theprimary and secondary

when a load is connected across b and c, then a currentI2 will flow through the load. The current I2 will producean m.m.f. in the core which will be balanced by a currentI1

flowing in the complete winding


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Auto-transformer

aN

N

V

V

2

1

2

1 ==

The voltages and currents

are related by the sameturns ratio as in a two-winding transformer:

a

1

N

N

I

I

1

2

2

1 ==


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Example

A 1, 100 kVA, 2800/280 V two-winding transformer isconnected as an autotransformer as shown in Fig. E2.6 suchthat more than 2800 V is obtained at the secondary. Theportion ab is the 280 V winding, and the portion bc is the2800 V winding. Compute the kVA rating as anautotransformer.


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Example (cont)

The current rating of the winding are:

AIab

500200

000,100==

AIbc

50

000,2

000,100==

Therefore, for full-load operation of theautotransformer, the terminal currents are:

AIH

500=

AIL

55050500 =+=


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Example (cont)

Now,VL = 2800V and

VH = 3080V

Therefore,

)ans(11001000

5002200|kVA H =

=

)ans(1100

1000

5502000|kVA L =

=

Note: A 1, 100 kVA, two-winding transformer whenconnected as an autotransformer can deliver 1100 kVA.


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Advantages of Auto-transformer

It effects a saving in winding material (copperor aluminum), since the secondary winding ispart of the primary current.

Lower copper loss, therefore efficiency ishigher than in the two winding transformer.

Lower leakage reactances, lower excitingcurrent.

Variable output voltage can be obtained.


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Disadvantages of Auto-Transformer

There is a direct connection between theprimary and secondary sides.

Should an open-circuit develop between pointsb a n d c

, the full mains voltage would beapplied to the secondary.

The short-circuit current is much larger thanfor normal two-winding transformer


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Application of Auto-transformer

Boosting or bucking of a supply voltage by a small

amount. (The smaller difference voltage betweenthe output and input voltages the greater is thesaving of winding material.)

Starting of a.c. machines, e.g. induction motor,where the voltage is raised in two or more stepsfrom a small value to the full supply voltage.

Continuously variable a.c. supply voltages,

normally connected between a low voltage supplyin and a high voltage supply out.

Production of very high voltages, e.g. 275kV and400kV grid system


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Autotransformer (continued)

Same operation as two windings

transformer

Physical connection from primaryto secondary

Sliding connection allows forvariable voltage

Higher kVA delivery than twowindings connection

Figure 9


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Current Transformer

Yet another type of transformer is the current transformer.

Central to all of the AC power transducers is the measurement ofcurrent.

This is accomplished using a current transformer (CT), a "donut"

(toroidal) shaped core through which is threaded the wire whosecurrent is to be measured.

Current transformers are designed to produce either an alternating

current or alternating voltage proportional to the current beingmeasured.


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Potential Transformer

Potential transformers areused by the electrical

industry for smaller powerapplications and metering.

They are a step-down

transformer and range inwattage.

This is a common

Westinghouse 1500VA12,000V transformer.


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Ferroresonant transformer

These transformers use a tank circuit composed of a high-voltageresonant winding and a capacitor to produce a nearly constantaverage output with a varying input.

The ferroresonant approach is attractive due to its lack of activecomponents, relying on the square loop saturation characteristics ofthe tank circuit to absorb variations in average input voltage.

The ferroresonant action is a flux limiter rather than a voltageregulator, but with a fixed supply frequency it can maintain analmost constant average output voltage even as the input voltagevaries widely.

Ferroresonant transformers output either non-sinusoidal (CVN typetransformers) or sinusoidal waveshapes (CVS type transformers).

3-lesson notes lec 7(trafo)

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