Current sensor
Current sensors are electrical components with which the current intensity in cables and busbars can usually be measured in a galvanically separated (contactless) manner based on the magnetic flux density triggered by electrical currents .
A distinction is made between sensors that can only detect alternating current and those that can detect direct and alternating currents.
AC current sensors
AC current sensors are called current converters in the narrower sense. They usually consist of a toroidal core and are designed like a transformer . The alternating magnetic field of a conductor through which the alternating current to be detected flows induces an alternating current in the measuring coil which is proportional to the measuring current via the reciprocal number of turns ratio. Current transformers represent current sources and are specified with a maximum permissible so-called load (terminating resistor) to avoid impermissibly high output voltages or core heating. Unused converters are short-circuited.
For small currents, current transformers have their own winding for the alternating current to be measured, for large currents - the ranges go up to a few kiloamps depending on the type - the current transformer is pushed over the solid conductor (e.g. a power rail) so that results in only a single "turn" through the converter.
Further explanation:
- Current clamps designed for alternating currents with a large opening that can be "folded" over the conductor to be measured.
- Ferrite toroidal current transformers in switch mode power supplies, converters or other high-frequency applications
- Rogowski current transformer : coil without a core around the conductor to be measured
By choosing the number of turns ratio of the coils involved, a large measuring range can be covered without significantly affecting the primary circuit. Current transformers based on this principle do not require any external energy to operate.
DC sensors
Due to their principle, direct current sensors can also detect alternating currents and transient processes. You need an auxiliary energy source (supply voltage).
Converter with Hall probe
In principle, current sensors that are equipped with Hall sensors are temperature-sensitive and may have to be temperature-compensated. Very high currents, including high short-term inrush currents, can magnetize the core material used, which leads to measurement errors or zero point deviations (offset errors) due to remanence .
Direct imaging current sensors
These sensors work with a Hall sensor . A slotted toroidal core made of a preferably linear, soft magnetic ferromagnetic material is used, which either encloses the current-carrying conductor or on which a primary winding with a few turns is applied. The Hall sensor itself is housed in the slot (air gap). At the same time, the air gap limits the magnetic flux density, linearizes the relationship between magnetic field and current and thus enables measurements over a large area. The measurement signal of the Hall sensor is proportional to the magnetic field and thus to the current. In the case of very small currents, the mapping is not exactly linear, and the earth's magnetic field may even have a disturbing effect.
Current clamps , which can detect direct and alternating currents , usually work according to this principle . These current clamps consist of a hinged toroidal core.
Compensation current transformer with Hall probe
These sensors have a similar structure to direct imaging sensors. However, a winding is additionally applied to the core surrounding the conductor, through which a compensation current flows, which is generated by an electronic circuit in such a way that the magnetic fields of the measurement current and the compensation current (opposing field) cancel each other out at the sensor. An external measuring resistor looped into the compensation winding via the connection terminals generates a voltage proportional to the current, which forms the output signal.
Compensation converters are more precise than directly imaging Hall current sensors; among other things, they can minimize the influence of the earth's magnetic field with precise measurements and small currents. They have fewer offset and linearity errors. But they are also more expensive and require more auxiliary energy.
Compensation current transformer with soft magnetic probe
These sensors also have a core made of ferromagnetic materials, but this has no air gap, but an integrated soft magnetic sensor. As with the compensation current transformers with Hall sensors, this controls the current through a compensation winding via electronics so that the magnetic flux is zero. These current sensors have much smaller offset, hysteresis and temperature errors than Hall current sensors.
More current sensors
Sensor with reed switch
Here a reed switch is brought axially into the interior of a coil that tightly encloses the glass tube ("reed"). By choosing the number of turns of the coil, it is possible to determine the current at which the reed contact closes. This switching current can, for. B. are indicated by an LED that is switched on by the reed contact.
Thermal sensors
For high-frequency currents, a wire is sometimes used, which is heated by the current to be measured. This heating is measured with the aid of a suitable device, e.g. B. with a hot wire measuring mechanism or - with a corresponding temperature coefficient of the electrical resistance of the wire - a resistance measurement.
In motor circuit breakers , miniature circuit breakers and thermal relays are bimetallic strips used, either by itself flowed from the power or wear a thick filament winding. Such current sensors have a deflection following the effective current and a certain thermal inertia and are therefore particularly suitable for overcurrent protection devices. They are also used in the secondary circuit of current transformers to switch off the main circuit breaker in the event of an overcurrent.
Faraday effect
Here the conductor in which the current is to be measured is surrounded by a transparent optical medium, for example a glass fiber . The direction of polarization of a linearly polarized light beam passing through the medium is rotated by the magnetic field surrounding the conductor due to the Faraday effect . By measuring this rotation of the optical plane, the electrical current in the conductor can be determined. Such fiber optic current sensors, abbreviated FOCS from English fiber optical current sensor , or MOCT from English magneto-optical current transformer , work without any magnetic influence on the conductor and also offer potential separation even at very high electrical voltages. Accordingly, they will u. a. Used in electrical power engineering in the area of high-voltage systems as an alternative to current transformers. Another application is measurements of very high currents in electroplating .
The measuring range of FOCS goes up to 500 kA (pulse currents) and can achieve an accuracy of 0.1%.
Magnetoresistive effect without a toroidal core
In addition to the Hall probes, there are other compact current sensors that use a magnetoresistive effect , but manage without an external toroidal core for magnetic field formation.
The sensor is located in an integrated circuit . The current is conducted through the component or it is placed over the conductor track to be measured on the circuit board , as shown in the adjacent figure. The measuring range is from a few 100 mA to a few amperes.
Types of current sensors
Automation technology
In automation technology , signals with 0–10 V and 4–20 mA are common in order to transmit various physical quantities in analog form. The above current sensors and current transformers are usually not suitable for these signal levels, as they do not offer a sufficiently high burden or do not have the offset value of 4 mA. Current sensors with a 4-20 mA output are therefore offered for automation technology. For this purpose, they have integrated signal processing including scaling, which often outputs its rms value for an alternating current and the mean value for direct current .
Vehicle industry
Current sensors that are used in motor vehicles , for. B. to monitor battery voltage and vehicle electrical system, often have CAN and LIN bus connections that are implemented with microprocessors. Other types provide a PWM output.
Energy Technology
Current transformers and sensors for high voltages often have the design of a post insulator. At low voltage (<1000 V) they are on the power rail. Since they have to withstand the short-circuit current and, in many cases, lightning strikes, they are mechanically stable in order to be able to withstand the magnetic forces. They are often potted or coated with synthetic resin. Moving iron measuring mechanisms are often used to display the current , the scale of which is adapted to the transmission ratio. The secondary current is typically 5 amps. Moving iron measuring mechanisms show the effective value.
Power electronics
Current sensors for soldering on printed circuit boards are typical here, with the measurement current winding as a component or not. There are both those as finished components and user-specific constructions made from ferrite toroidal cores for use in switched-mode power supplies or inverters.
Individual evidence
- ↑ http: //www.hta - bu.bfh.ch/e/enl/enl - faradayd.htm
- ↑ Measurement accuracy of the fiber optic current sensor from ABB (press release ABB)
- ↑ Product presentation at elektroniknet.de