DC Electrics - Aircraft Electrical Power Systems — Page 126, Lesson 129

I want to walk you through the monitoring instruments used in a DC electrical system on an aircraft. These are the instruments and warning lights that let the pilot keep an eye on the electrical system's health. We're focusing on DC here — AC instruments are covered in a separate chapter. Let's start with ammeters and voltmeters. In both AC and DC systems, these are most often the moving coil type of instrument. Here's how that moving coil instrument is built. Inside, there's a permanent magnet with a soft iron core placed between its poles. Between those poles, a former — that's a cylindrical frame — is mounted on a spindle, and that spindle is free to rotate inside the magnetic field. A coil of wire is wound around that former. Current flows through that coil. Two hairsprings are fitted to restrain the coil's movement. As the coil rotates, one hairspring gets wound up and the other unwound. Those hairsprings also serve as the electrical connection — they feed current into and out of the coil. The coil and former carry a pointer that moves over a scale as the coil rotates. So when current flows through the coil, it creates its own magnetic field. That field interacts with the main magnetic field from the permanent magnet, and that interaction causes the coil to rotate. The pointer moves across the scale until the torque from the magnetic interaction is balanced by the restoring force of the hairsprings. The greater the current through the coil, the greater the pointer movement. When current reduces, the hairsprings return the pointer back to its zero mark. The key point: the deflection of the pointer is proportional to the current flowing through the coil. That gives you an evenly divided scale — linear, not logarithmic. The whole meter is likely housed inside a case made of soft iron. Why? To prevent stray magnetism from affecting the indication. Stray magnetic fields from other equipment or wiring could distort the reading, so the soft iron case shields the mechanism. Now, to extend the range of the instrument — that is, to let it measure larger values without damaging the delicate moving coil — we use different components depending on whether it's an ammeter or a voltmeter. When this moving coil meter is used as an ammeter, we fit a shunt. A shunt is a resistor of low resistance value. It's connected in parallel with the meter coil, so most of the current bypasses the delicate coil, and only a small, proportional fraction flows through the meter. That allows the instrument to measure large currents while protecting the mechanism. When the same moving coil meter is used as a voltmeter, we fit a multiplier. A multiplier is a resistor of high resistance value, connected in series with the meter coil. That limits the current flowing through the coil for a given voltage, again protecting the mechanism while still allowing it to indicate large voltage values. So in both cases — shunt for ammeter, multiplier for voltmeter — only a proportion of the total current passes through the instrument, protecting the delicate mechanism but still letting it measure large values. And there's a diagram of the moving coil instrument itself — Figure 8.4 — that shows all these parts laid out. That's the core of how we monitor DC electrical power in the aircraft.