AC Electrics -Transformers — Page 240, Lesson 253

I want to walk you through the next part of AC Electrics — we've covered half-wave rectification, and now we're going to fill in those gaps between pulses using full-wave rectification. Let's start with the single-phase case. To fill in the gaps between pulses that half-wave rectification leaves behind, we can use a Bridge Rectifier. Here's how it works: as you can see 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 — so all the output is either positive or negative, not alternating. The output can be smoothed to some extent by the addition of a capacitor placed across it. That capacitor stores charge during the peaks and releases it during the gaps, reducing the ripple. Now let's move to three-phase rectifiers. The rectification of a three-phase supply can be effected by using a formation of six rectifiers in a bridge circuit. That's shown in Figure 14.6. The output of a three-phase rectifier is essentially a steady output — much smoother than single-phase, because with three phases overlapping, there's very little gap between pulses. Now, in aircraft, we combine the transformer and rectifier into one unit called a Transformer Rectifier Unit, or TRU. A TRU converts AC at one voltage to DC at another voltage by combining the transformer and rectifier in one unit. A typical example you'll see is 115 volts AC, 200 volts AC, 400 hertz, three-phase input, converted to 28 volts DC output. That's used to supply the DC needs of an AC distribution system. TRUs are invariably multi-phase units — that's to achieve a smooth DC output. Indications of TRU output, measured in amps, can be shown on the main electrical panel on the flight deck. So the flight crew can see how much current the TRU is delivering. Cooling is achieved by drawing air through the unit, and that airflow may be monitored for temperature, with an overheat warning supplied if things get too hot. Now let's talk about Inverters. An inverter does the opposite of a rectifier — it converts DC to AC. In a constant-frequency AC equipped aircraft, the inverter is used as a source of emergency supply if the AC generators fail. In that case, 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. There are also rotary inverters, but those are described in the DC section and are not generally used in modern aircraft. Some aircraft have a frequency wild distribution system — examples include the British Aerospace ATP and the ATR 42. Those aircraft use inverters to supply their normal constant frequency requirements. Here's how that works: they transform and rectify the frequency wild AC into DC, and then supply that DC to a static inverter to give a controlled AC output. So the process is: frequency wild AC → transformer → rectifier → DC → static inverter → constant frequency AC. Inverter output can be monitored for voltage and frequency in the same manner as the main generators. And cooling is accomplished in the same manner as the TRU — air drawn through the unit, with overheat monitoring.