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Refer to figure.
The annex shows the layout of an electrical system for a twin-engined helicopter. What is the purpose of the TRUs?
• A
To convert AC power into DC power at a different (usually lower) voltage.
• B
To convert DC power into AC power at a different (usually higher) voltage.
• C
To separate the DC systems (powered by the generators) from the AC systems (powered by the batteries).
• D
To separate the AC systems (powered by the generators) from the DC systems (powered by the batteries).

Refer to figures.
Electricity can be transmitted through wires in two different ways, via Direct Current (DC) or Alternating Current (AC).

Direct current is the simplest to consider, charge flows from high potential to low potential (conventional) through electrical conductors continuously, all in the same direction.

Alternating current is a little more complex, where the charge flows backwards and forwards through an electrical conductor, with the current increasing in one direction, then in the opposite direction, and so on.

The majority of small helicopters simply uses a DC electrical system (often 14 V or 28 V). Larger helicopters use AC systems (often 3 phase, 115 V, 400 Hz) as they require increased power generation require high loads. They also have some loads onboard that require DC power, so have this capability also.

Electricity can be transferred between the two different types (AC and DC) by the use of rectifiers and inverters.

AC is "rectified" to turn it into DC (think of it that AC is too complex, and it has to be fixed ["rectified"] to turn it into simple direct current). In aircraft, we do not want to turn 115V AC straight into DC, it would be far too much for the 28 V circuits, so first a transformer is used to bring the voltage down to 28 V, and then it is rectified. Aircraft use an integrated Transformer Rectifier Unit (TRU) for these job, to go from high voltage AC to lower voltage DC.

DC can be "inverted" into alternating current, and this is quite intuitive, as AC is just DC, flipped backwards and forwards many times per second, and then smoothed into a sine wave. Aircraft will often have an inverter on board, for operations where the aircraft needs to use a DC power source like batteries to power it's AC consumers. As with the rectification process, the low voltage DC will create low voltage AC, which will need to go through a step-up transformer to reach the usual 115 V AC required.

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