Clip-on ammeter
A clamp-on current meter , also current clamp , clamp or general language clamp meter called, is a measuring device for indirect measurement of electric current based on the magnetic field surrounding the conductor.
While the circuit has to be disconnected for direct measurement in order to switch the ammeter to the electrical line , this is not necessary when measuring with the clamp-on ammeter . Thanks to an iron core that can be divided like pliers , conductors or busbars can be encompassed without having to separate the circuit. Measurements can also be made on systems that cannot be switched off for the purpose of measurement. The measurement is potential-free - it can be measured without contact.
A clip-on ammeter can only measure the current strength of a conductor if it can be grasped individually. This can also be the consumer himself, e.g. B. a straight or ring-shaped fluorescent lamp, provided the inner diameter of the pliers is sufficiently large. If the clamp encloses the entire cable with conductor and return conductor, it only measures leakage currents.
Mode of action
AC current measurement
The transformer principle is used for AC current clamp meters . The current probe functions here as a magnetic transducer / DC converter . In the closed state, the fixed and movable legs of the clamp form the transformer core, the conductor to be measured forms the primary winding and the coil in the measuring device forms the secondary winding. The current in the conductor magnetizes the core and thereby induces a current in the secondary winding that is proportional to the conductor current. The output power of the secondary coil is so high that it can directly operate a (appropriately scaled) measuring device (e.g. with moving iron measuring mechanism ). The energy to drive the measuring mechanism is taken from the circuit to be measured.
Universal current measurement
Clip-on ammeters suitable for direct current can not be built according to the above principle due to the lack of alternating fields . Here Hall sensors or magnetic field-dependent (magnetoresistive) resistors , which can also detect static magnetic fields, are installed in an air gap in the core. The weak signals generated must be amplified electronically. Therefore, these measuring devices must be supplied with energy via batteries or power supply units . These measuring devices are also suitable for alternating currents.
Furthermore, a measuring principle with a compensation method can also be used. A current flowing through a compensation winding is regulated in such a way that the magnetic flux in the core ideally becomes zero. The compensation current is displayed; the flowing current is reduced according to the turns ratio (as with the transformer principle described above ). The advantage here is that any non-linear properties, for example of the core or the Hall sensor, hardly have any influence on the measurement. Here, too, an energy supply is necessary for the regulation and the compensation current.
There is also a historical, rarely used form of clip-on ammeter, which is basically a moving-iron measuring mechanism suitable for alternating and direct currents , the magnetic circuit of which is formed by the clamp. In this respect, it is externally similar to today's design, but no Hall or magnetoresistive sensor is used to determine the field strength , but the force of the magnetic field is displayed mechanically. Different scales were often attached to the measuring mechanisms for direct or alternating current. These devices are only suitable for comparatively large currents and have a high level of measurement inaccuracy . To change the measuring range , separate moving iron measuring mechanisms are inserted into the magnetic circuit.
Display of the measured value
Built-in display
The measured values from current clamps based on the transformer principle can be visualized directly using moving iron measuring mechanisms. Moving coil measuring mechanisms can also be used, but a rectifier must be connected upstream.
Newer versions often have digital displays for which the measured value first has to be converted using electronic circuits. Such devices therefore require (in addition to the measured variable) an additional energy supply.
As mentioned above, current clamps suitable for DC current generally require amplifiers for the weak signals from the magnetic field sensors. Since electronic assemblies and an additional energy supply are already available anyway, they are usually equipped with digital displays because of the little additional effort required.
External display ( Current Probe )
Current clamps without a built-in display are e.g. B. So-called current clamp transformer . Apart from the lack of a display, their structure is basically the same as that of AC current clamps. Instead of a display instrument, there are sockets or cables via which the measured value z. B. is transferred to oscilloscopes , recorders or multimeters . Every device has a fixed transmission ratio , which must be used to calculate the true magnitude of the current. So there are B. a current clamp with a transmission ratio of 100: 1 at a current of 20 A from an output current of 200 mA.
There are also versions with a built-in current-voltage converter (in the simplest case as a passive burden (resistor)) that can be connected directly to a voltage input (common input of an oscilloscope ) (so-called current probes ). Such devices are partially capable of measuring direct and alternating current components at the same time and have cutoff frequencies of up to approximately 100 MHz. They work with a current transformer and a Hall sensor as well as a compensation winding to protect the core from saturation. An additional measuring amplifier is required for operation, which also performs the offset adjustment and demagnetization. Such current clamps are usually connected to the oscilloscope with a BNC cable that is impedance-adjusted.
Digital current clamps contain an A / D converter and are used e.g. B. connected to a PC or smartphone using a USB cable.
Execution of the core
The divisible core of the clip-on ammeters consists of layered electrical sheets ( ferrites also at higher frequencies ) and is usually additionally insulated, since such devices are often used to measure on uninsulated busbars. The contact surfaces of the two core parts are ground and fitted with teeth to keep magnetic losses as low as possible. The moving part of the core is connected to the fixed part by a hinge and can be opened by means of a lever. During the measurement, a closing spring presses the two core parts together.
Measuring range
The measuring range of clip-on ammeters is limited, as the then weak magnetic field cannot be measured with sufficient accuracy and also disturb the residual magnetization and surrounding fields. If you want to measure smaller currents more precisely, you can run several turns of the live cable through the clamp. If you put z. B. the line to be measured in 10 turns around the core, you get ten times the display. Upwards, the possible measuring range is only limited by the size of the clamp opening, since currents in the kA range require correspondingly large conductor cross-sections or busbars and thus sufficiently large clamp openings.
Special design for multi-core cables
Clip-on ammeters are designed to measure a single conductor. Apart from leakage currents, the magnetic fields of the forward and return conductors cancel each other out. In the case of multi-core cables, each core to be measured must therefore be individually accessible, which would require the cable sheath to be removed. So-called multi-sensor clamp-on ammeters have therefore been developed, the mode of operation of which is based on the fact that the magnetic field penetrates to the outside at close range and the signals from differently arranged receiving coils are calculated in such a way that conclusions can be drawn about the current. They are more expensive and less precise, but can measure on power cables with a forward and return conductor.
literature
- Rolf Fischer, Hermann Linse: Electrical engineering for mechanical engineers: with electronics, electrical measurement technology, electrical drives and control technology . 14th edition. Vieweg + Teubner, 2012, ISBN 978-3-8348-1374-9 .
Individual evidence
- ↑ http://www.dennlec.com/images/manuals/tek-am5003-op-manual.pdf Page 33: AM503B + A6312 Current Probe
- ↑ http://docmesure.free.fr/manuels/Tektronix/TM500/AM/AM503/AM503.pdf Service booklet of the AM503 system from 1979
- ↑ Johann Schmidt, article about analog current clamps archive link ( memento of the original from October 6, 2014 in the Internet Archive ) Info: The archive link was automatically inserted and not yet checked. Please check the original and archive link according to the instructions and then remove this notice. Retrieved October 1, 2014
