Transformer

1

Chapter 9 Transformers

• Function: Transformers are extremely

versatile devices that can be used to either

step up and step down AC voltages or to

step up and step down AC current. They can

also allow AC to pass and block DC.

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9.1 Transformers• 9.1.1 Construction and Operation

Figure 9-1 Simple Transformer

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• The most common type of transformer is

the voltage transformer, which consists of

two windings, the primary winding and the

secondary winding. The windings are not

electrically connected together, which is a

safety feature in AC electrical circuits, but

are wound on the same laminated soft iron

core.

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• If an AC voltage is applied to the primary

winding, the resultant changing flux links

with the secondary winding. The changing

flux is concentrated by the iron core and

causes an EMF to be induced in the

secondary winding. The magnitude of the

EMF is proportional to the ratio of the

number of turns between the primary and

secondary windings.

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Where:

VP = Primary voltage

VS = Secondary voltage

NP = Primary turns

NS = Secondary turns

S

P

S

P

V

V

N

NRatio Turns

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Categorise of Transformer

Figure 9-2 Set up and Step down Transformers

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• Transformers are also extremely efficient

(i.e. the amount of power in is approximately

equal to the amount of power out), and they

are rated in volt-amperes (VA). The following

relationship exists between the turns ratio,

voltage, and current.

• where IS = Secondary Current

• IP = Primary Current

P

S

S

P

S

P

I

I

N

N

V

V

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• If the voltage is stepped up, the current is

stepped down. For example, if a transformer

has a turns ratio of 1:2, and inputs of 240 V

and 5 amps, the outputs will be, respectively:

P

S

P

S

N

N

V

V

volts4802401

2VS

S

P

P

S

N

N

I

I

amps 5.252

1IS

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• Transformers also consist of inductive

components, so it is important that they are

operated at their correct frequency and

voltage. Any under-frequency condition

results in the primary current increasing and

the transformer overheating.

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9.1.2 Types of Transformers

• Three-phase transformers (isolation

transformers).

Figure 9-3 Primary Windings of Three-phase Transformers

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9.1.2 Types of Transformers

• Three-phase transformers (isolation

transformers).

Figure 9-4 Secondary Windings of Three-phase Transformers

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Auto transformers

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Auto transformers (continue)

• Auto transformers are a special type,

since they have no electrical isolation

between the primary and secondary

windings. A single continuous winding is

wound on a laminated iron core, where

part of the winding is used as the primary,

whilst the other part is used as the

secondary, as shown below.

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Auto transformers (continue)

• These transformers can be used to

either step-up or step-down the applied

voltage, depending on the winding

configuration.

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Auto transformers (continue)

• In a step-down device, the whole of the winding

serves as the primary winding, whilst the lower half

of the winding serves as the secondary winding. In

this case, there are fewer turns in the secondary than

in the primary: so the voltage is stepped-down, but

the current is stepped-up. This configuration is

typically used to power aircraft instruments where

the voltage is stepped down from 115 V 400 Hz to 26

VAC.

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Auto transformers (continue)

• The disadvantage of this format is that the

full voltage is placed across the load if the

coil goes open circuit, since there is no

voltage isolation between the two

windings.

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Auto transformers (continue)

• Conversely, in a step-up auto transformer, the

lower half of the coil is used as the primary, and the

entire coil is used as the secondary. In this case, the

secondary has more turns than the primary, so the

transformer steps-up the voltage and steps-down the

current. On aircraft, this arrangement is typically

used in windshield anti-icing systems.

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Auto transformers (continue)

• Conversely, in a step-up auto transformer, the lower

half of the coil is used as the primary, and the entire

coil is used as the secondary. In this case, the

secondary has more turns than the primary, so the

transformer steps-up the voltage and steps-down the

current. On aircraft, this arrangement is typically

used in windshield anti-icing systems.

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Auto transformers (continue)

• If the output from the auto transformer can be

varied via a moveable tapping, as shown

below, it is also known as a variac and is

typically used on the flight deck to control the

intensity of ultra-violet lighting.

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Current transformers • Current transformers differ from the voltage

transformer, because the primary circuit consists

of a supply feeder cable rather than a winding

connected across a supply, as shown below.

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• In this arrangement, the alternating magnetic field associated

with the load current is linked to the current transformer

secondary winding via a laminated soft iron core, through which

the feeder (primary) passes. The secondary current is used to

feed a meter and typically registers the current flowing from an

AC generator to the busbar or load. The secondary current can

additionally be used to supply power meters and to monitor the

load-sharing in an electrical circuit.

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• In AC power generation systems, this type of

transformer can also be used as a sensor in a

differential protection circuit, as shown below.

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9.2 Transformer Rectifier Units

• A transformer rectifier unit (TRU) is used to

convert AC into relatively smooth DC. An

example of a simple TRU circuit is that which

is used in a car battery charger, as shown

below.

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• This device takes the mains 240 VAC and converts it

to approximately 14 VDC to charge the battery. This is

achieved by a transformer, which first steps down the

AC voltage to a reasonable level and then converts it

via a bridge rectifier assembly into DC.

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Figure 9-10 Typical TRUs on Aircraft

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Typical TRUs on Aircraft

• The TRU that is fitted to an aircraft is

typically supplied with 200V 400Hz three-

phase AC, Which is stepped-down

through a three-phase star-star wound

transformer and changed to 28 V DC by a

six-rectifier bridge assembly. The output

from the TRU is then fed to the aircraft's

DC busbars.

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Overheat protection

• When operating, most TRUs are cooled by

air from a thermostatically controlled

cooling fan. If the TRU overheats (150°-

200°) due to fan or other failure, a warning

light illuminates on the flight deck. The

TRU should then be switched off, either

manually or automatically.

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Reverse Current protection

• When the TRUs are operating in parallel with

some other power source, the failure of a

rectifier in a TRU can cause a reverse current

to flow into it and may even cause a fire.

Reverse current protection in the failed TRU is

designed to sense the fault current when it

reaches approximately 1 amp, and disconnect

the TRU automatically from the DC bus bars.

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END OF CHAPTER 9