Moving iron measuring mechanism
A moving-iron or soft iron measuring mechanism is used for display of currents and is used in the analog current measuring devices used. It converts the current intensity into a corresponding pointer deflection.
Moving-iron measuring mechanisms are still produced today, and they are mainly used as control instruments in plant and switch cabinet construction. In other applications they have been largely replaced by digital measuring devices such as the digital multimeter .
construction
Within a single coil is a stationary iron core and fastened to the pointer axis and moving with her iron core (the moving iron ). If current flows through the coil, both irons are magnetized in the same direction and therefore repel each other. As a result, the movable iron core rotates away from the fixed one and causes the pointer to deflect. A spring is tensioned until the spring force is equal to the magnetic reluctance force . After switching off the current, the spring returns the pointer to the zero position.
The deflection is not proportional to the strength of the current, so the scale is not linear . Since the two iron cores are always magnetized in the same direction, regardless of polarity, and repel each other, this measuring mechanism is suitable for direct current as well as alternating current .
Since the reluctance force F is proportional to , a moving iron measuring mechanism shows the effective value or root mean square value of the current for alternating or mixed currents, even if the waveform of the alternating current deviates from the sine . For the measurement of alternating current is not, as in the moving-coil , a rectifier needs.
When measuring alternating current, the moving iron measuring mechanism has a slightly inductive impedance , which is dependent on the frequency: (ωL) . Magnetization reversal and eddy current losses in the iron and metal parts lead to less deflection at higher frequencies. At low frequencies (15… 100 Hz) this is irrelevant - at higher frequencies up to 2 kHz the devices are calibrated to the working frequency.
Moving iron gauges have a non-linear scale. You need air damping, as they do not have any self-damping due to the lack of a magnet. They are more robust compared to moving coil measuring mechanisms .
Measurement of high currents
Moving iron measuring mechanisms can in principle be designed to measure high currents (up to approx. 200 A) by making the winding from wire of appropriate thickness. It is also common to use current transformers , which usually transform the measuring current down to the range of 5 A. Shunt resistors (shunts) are unusual due to the difficult temperature and frequency compensation. Moving-iron measuring mechanisms are unsuitable for measuring currents below about 20 mA.
Moving-iron measuring mechanisms tend to be superior to other measuring methods with regard to voltage drop and power loss when it comes to high currents - with their measuring principle, theoretically no voltage drop is necessary for measurement. For example, a 60 A instrument at 60 A has a voltage drop of only 20 mV, which is less than the usual value of 60 mV for a shunt and much less than the 200 mV that typically drops with a digital multimeter at the end of the range.
Voltage measurement
If a moving iron measuring mechanism is to be used to measure voltage, its coil is made from thin wire with many turns. This creates a high inductance and the current and thus the display is highly frequency-dependent. A series resistor is often used to reduce this influence and also to save copper. An example is an instrument which is designed to measure the mains voltage and which requires 220 volts 10 mA to display. It has a series resistor of 20 kOhm, at which 200 volts drops at this measuring voltage and a power loss of 2 watts arises.
To measure high AC voltages ( medium voltage ), measuring transformers ( voltage transformers ) are connected upstream, which usually transform the measurement voltage down to the level of 100 V.
Moving coil instruments and digital voltmeters require currents that are around 3 orders of magnitude lower for voltage measurement than moving iron instruments.
history
The moving iron measuring mechanism was invented by the Austrian engineer Friedrich Drexler in 1884.
Individual evidence
- ↑ http://www.alte-messtechnik.de/technik/dreheisen.php Description including air damping.
- ↑ Panel built-in instrument, 60 A full scale, year of construction approx. 1995.
- ↑ Built-in panel instrument 65 × 65 mm, full deflection 250V, year of construction approx. 1970.
- ↑ Questionnaire from the personal portfolio of Friedrich Drexler (1858 - 1945) . Technical Museum Vienna. Retrieved July 10, 2013.