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Figure 14.3 Autotransformers

Autotransformers are less expensive than two coil transformers because they use less wire; however, they do not electrically isolate the primary and secondary windings and so cannot be used in many circuits for this reason.

Rectification of Alternating Current

A Rectifier is a device which will convert AC into DC. The operation of the Diode Rectifier is described in the Semiconductor chapter, and is a very common device in modern aircraft solid state circuits. It can be used to convert AC to DC or as a “Blocking Diode” (electrical non-return valve) to prevent reverse current flow in a DC system. Some rectifiers are designed to conduct at a predetermined voltage; these rectifiers are called Zener Diodes.

A diode has a high resistance in one direction and a low resistance in the other. The accepted symbol for a diode rectifier and the direction of conventional current flow is shown in

Figure 14.4.

Half Wave Rectification

A diode inserted in the secondary circuit of a transformer will allow current to flow through the load in one direction only. This is termed Half Wave Rectification. The bottom half of the AC waveform is blocked and the frequency of the output is the same as the input, as shown in

Figure 14.4.

Figure 14.4 Single phase half wave rectification

AC Electrics - Transformers 14

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14 AC Electrics -Transformers

Full Wave Rectification

To fill in the gaps between pulses that have been left from half wave rectification a Bridge Rectifier can be used. As can be seen from Figure 14.5 when one half of the bridge circuit is presenting a high resistance to current flow, the other half is allowing it to flow relatively easily. This arrangement is specifically designed to allow the output of the bridge to be of a single polarity. The output can be smoothed to some extent by the addition of a capacitor placed across it.

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Figure 14.5 Single phase full wave rectifier

Three Phase Rectifiers

The rectification of a three phase supply can be effected by using a formation of six rectifiers in a bridge circuit.

This is shown in Figure 14.6. The output of a Three Phase Rectifier is essentially a steady output.

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AC Electrics -Transformers 14

			DC OUTPUT
	+	PHASE A		PHASE B	PHASE C
	-
A				A	14
B				B	- Transformers
					Electrics
C				C
-			+	-	AC
		DC OUTPUT			+
LINE VOLTAGE BETWEEN PHASES A AND B				LINE VOLTAGE BETWEEN PHASES A AND C
		A
		B
		C
		-			+
		LINE VOLTAGE BETWEEN PHASES B AND C

Figure 14.6 Three phase full wave rectifier

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14 AC Electrics -Transformers

Transformer Rectifier Units (TRUs)

Transformers - Electrics AC 14

TRUs convert AC at one voltage to DC at another voltage by combining the transformer and rectifier in one unit (usually 115 V/ 200 V/ 400 Hz/ 3 phase to 28 V DC) to supply the DC needs of an AC distribution system.

TRUs are invariably multi-phase units to achieve a smooth DC output. Indications of TRU output (amps) can be shown on the main electrical panel on the flight deck.

Cooling is achieved by drawing air through the unit which may be monitored for temperature with an overheat warning supplied.

Inverters

An inverter converts DC to AC.

The inverter in a constant frequency AC equipped aircraft is used as a source of emergency supply if the AC generators fail; then, the inverter is powered by the battery.

Inverters are usually “solid state” static inverters, transistorized in modern aircraft, providing constant frequency AC for operation of flight instruments and other essential AC consumers. Rotary and Static inverters are described in the DC section and are not generally used in modern aircraft.

Aircraft which have a frequency wild distribution system (British Aerospace ATP, ATR 42) use inverters to supply their normal constant frequency requirements. This is done by transforming and rectifying the frequency wild into DC, and then supplying the DC to the inverter (static) to give a controlled AC output.

Inverter output can be monitored for voltage and frequency in the same manner as the main generators.

Cooling is accomplished in the same manner as the TRU.

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