Moving-iron meter

A rotary iron or soft iron movement serves to display currents and is used in analog ammeters. It converts the current into a corresponding pointer deflection.

Moving iron works are still in production today, where they are mainly used as control tools in plant and cabinet used. In other applications, they have been largely replaced by digital meters such as the digital multimeter.

Construction

Within a single coil is a fixed iron core and fastened to the pointer axis and with their movable iron core ( the rotary iron). When current flows through the coil, both iron are magnetized in the same direction and therefore repel each other. This causes the movable iron core away from the fixed rotating and causes the pointer to the rash. Here, a spring is stretched until the spring force is equal to the magnetic reluctance. After switching off the current, the spring the pointer back to the neutral position.

The deflection is proportional to the current, therefore, the scale is not linear. Since the two iron cores are magnetized always in the same direction regardless of the polarity and repel, this measuring system is suitable for DC as well as AC.

Since the reluctance force F is proportional to, is a moving-iron measuring the rms or root mean square of the current alternating or pulsating current, even if the waveform of the alternating current deviates from the sine. For the measurement of alternating current, a rectifier is not, as in the moving coil instrument needed.

For AC measurement, the moving iron has a slightly inductive impedance which is dependent on the frequency ( omega L ). Therefore, the moving iron is only suitable for a certain frequency; depending on the region mostly the usual line frequencies of 50 or 60 Hz

Moving-iron instruments have inherently a non-linear scale division. You need to air damping, as they have no internal damping due to lack of a magnet. They are robust against moving coil instruments.

Measurement of high currents

Moving iron works can, in principle, for measuring high currents are interpreted (up to about 200 A) by the winding of appropriately thick wire is made - but the usual is to use a shunt resistor ( shunt ) or current transformer. To measure low currents below 20 mA, they are unsuitable.

Voltage measurement

If a moving iron are used to measure voltage, it produces its coil of thin wire with many turns. Often, a series resistor additionally is still installed, because of the relatively low sensitivity of the instrument falls in this may, however, significant power ( eg 20 mA instrument with a series resistor for 250 V = 5 watts). Alternatively can be used a measuring transformer.

History

The moving iron was invented by the Austrian engineer Friedrich Drexler in 1884.