According to the DIN 1319 standard for measuring technology , a measuring transducer is a measuring device ( measuring device , measuring device , ...) that converts an input variable into an output variable according to a fixed relationship. Measuring transducers are specifically referred to as measuring transducers that have the same physical size at the input and output and work without auxiliary power .
They are used when an electrical current or voltage is too great to be measured directly with conventional devices. Inductive transducers enable the galvanic separation of the easily processable electrical signal from dangerously high voltage before it is fed to an ammeter , voltmeter , energy meter , measuring device for active power , etc.
As part of a measuring device, transducers must be designed for small error limits .
Measuring transducers for alternating quantities are primarily used in the power grid as stationary specially designed transformers , both in versions for measuring purposes and for protection purposes. They are dealt with under the keywords current transformers and voltage transformers . They are also available as laboratory devices as well as combined with a display measuring device called clip-on ammeters .
In medium-voltage switchgear in particular , so-called small - signal converters with Rogowski coils are increasingly being used, which instead of a current proportional to the input current output a voltage proportional to the input current derivative according to the time. Due to their small size, these are becoming more and more popular. However, they are not measuring transducers in the sense given above, but measuring transducers; they need electronic support - the output signal is only available after integration.
Electronic current transformers that require auxiliary voltage (s) have been developed for currents including the direct component. There are two known principles of these compensation current transformers:
With Hall probe
The picture opposite shows a possible structure. It contains a magnetic circuit with two windings: One thick-wire with one or a few turns for the load current to be measured and a second thin-wire with a high number of turns for compensation . A magnetic field sensor sits in a small air gap. To this end, Hall probes or field plates used. The voltage from the sensor is fed to a control amplifier. Its output signal is switched to the measuring winding in such a way that it compensates the field of the load current to zero. In this circuit there is also a relatively low-resistance measuring resistor (load, e.g. 100 Ω) from which the load current image can be tapped. This measuring method can be used over a wide frequency range, but with direct current a hysteresis error interferes. The upper limit frequency is largely determined by the control amplifier and is typically in the 100 kHz range.
With fluxgate probe
The setup uses a fluxgate magnetometer as the zero field sensor . The principle of controlling the field to zero by means of a control amplifier and auxiliary winding is also used as described above. The zero field sensor based on the fluxgate principle does not require a slotted core, has no hysteresis behavior and, according to the manufacturer, has a low zero point drift.
For very high-frequency alternating currents and microwaves, transducers with coils are unsuitable because of their inductivity , and thermal converters are often used . Because of their thermal inertia, they do not measure the instantaneous value of voltage or current, but rather generate a voltage that is proportional to the mean power consumed by the sensor, i.e. the square of the current.
- Kurt Bergmann: Electrical measurement technology. 6th edition. Springer, 2008.
- Melchior Stöckl: Electrical measurement technology. 8th edition. Springer, 1987.
- Karl Küpfmüller, Gerhard Kohn: Theoretical electrical engineering and electronics: An introduction. 15th edition. Springer, 2000.
- J. (Isaak) Goldstein: The transducers. Your theory and practice . 2nd Edition. Birkhäuser, Basel 1952 (1st edition. Springer, Berlin 1928; Russian: Ismeritelnye transformatory. Gostech, Moscow 1930).
- DIN 1319-2: 2005-10: Basics of measuring technology - Part 2: Terms for measuring equipment