Chapter

14

AC Electrics - Transformers

Transformers . . . . . . . . . . . .	. .	. .	. . . . . . . .	. .	. .	.	231
Transformation Ratio . . . . . . . . . .	. .	. .	. . . . . . . .	. .	. .		.231
Power in a Transformer							232
Three Phase Transformers . . . . . . . .	. .	. .	. . . . . . . .	. .	. .		.232
Autotransformers							232
Rectification of Alternating Current							233
Half Wave Rectification							233
Full Wave Rectification . . . . . . . . .	. .	. .	. . . . . . . .	. .	. .	.	234
Three Phase Rectifiers . . . . . . . . .	. .	. .	. . . . . . . .	. .	. .	.	234
Transformer Rectifier Units (TRUs) . . . .	. .	. .	. . . . . . . .	. .	. .	.	236
Inverters . . . . . . . . . . . . . .	. .	. .	. . . . . . . .	. .	. .	.	236
Questions . . . . . . . . . . . . . .	. .	. .	. . . . . . . .	. .	. .		.237
Answers . . . . . . . . . . . . . .	. .	. .	. . . . . . . .	. .	. .	.	238

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14 AC Electrics -Transformers

Transformers - Electrics AC 14

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AC Electrics -Transformers 14

Transformers

One of the biggest advantages that an AC supply has over a DC supply is the ease with which the value of alternating voltage can be raised or lowered with extreme efficiency by the use of Transformers.

A simple transformer would consist of two electrically separate coils wound over iron laminations to form a common core. This forms a completely closed magnetic circuit. See Figure 14.1.

The Primary winding is connected to the AC supply and the output is taken from the Secondary winding.

The alternating voltage and current in the primary winding creates an alternating flux which links across to the secondary winding.

The alternating flux in the secondary winding sets up an EMF of mutual inductance which is available as the output voltage. The output voltage will be 180° out of phase with the input voltage. If a load is placed across the terminals of the secondary winding then a current will flow in the circuit.

AC Electrics - Transformers 14

Figure 14.1 A simple transformer

Transformation Ratio

The Transformation Ratio of a transformer is the ratio of the number of turns of wire on the secondary winding (N2) to the number of turns of wire on the primary winding (N1). The transformation ratio will also allow the determination of input and output voltages by using the formula:

	TRANSFORMATION RATIO (r) =	N2	=	E2
		N1		E1

If the transformation ratio is greater than one, then the transformer is a Step Up transformer. If the ratio is less than one, then the transformer is a Step Down transformer. See Figure 14.2.

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14 AC Electrics -Transformers

Transformers - Electrics AC 14

Figure 14.2 Step up and step down transformers

Power in a Transformer

If we ignore the very small losses that do occur in a transformer, then we can say that the power that goes into a transformer equals the power that comes out of it. The power in either the primary winding or the secondary winding is equal to the product of the voltage times the current in either winding.

Three Phase Transformers

The output of a three phase alternator can be transformed by either:

• 3 SINGLE PHASE TRANSFORMERS

or

• 1 THREE PHASE TRANSFORMER

A three phase transformer consists of the primary and secondary windings of each phase wound on one of three laminated iron limbs.

Autotransformers

Where AC is required for the operation of instruments on the aircraft, an Autotransformer can be used to either step down, or sometimes even step up, the source supply; the supply usually required for instruments is 26 volts AC.

An autotransformer is a single winding on a laminated core to form a closed magnetic circuit. Figure 14.3 illustrates the relationship between the primary and secondary windings in an autotransformer.

It should be noted that part of the winding carries both the primary and secondary current because it is common to both windings.

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