Current sensors play an important role in modern drive control in high-speed trains and trams. With their help, the torque in traction motors is regulated by traction converters. Here, a sensor system monitors converters’ input, intermediate and output circuits. Hall-effect current sensors enable the potential-free measurement of currents.
The role current sensors play is demonstrated using the example of modern drive control. Traction converters regulate the torque in the traction motors of rail vehicles. The input, intermediate and output circuits on such converters are monitored by means of current sensors. These detect any deviation in power, "report" it and, if necessary, trigger responses that protect power semiconductors. They therefore provide a reliable basis for monitoring drives and controls in frequency converters.
From trams to high-speed trains
HARTING supplies Hall-effect current sensors in different sizes and designs – thus enabling tailor-made solutions for exact current measurement. The devices detect DC, AC or mixed currents and work very precisely: measurement accuracy is +/- 0.4 per cent and the measuring range is 0-4,200 A. The measurement is based on the Hall effect (see box), with whose help the conductor’s magnetic field can be imaged. Therefore, current sensors effect measurements galvanically separated via the magnetic field flowing through the conductor current. Depending on the required accuracy, two measurement principles are employed: closed-loop and open-loop. For demanding measuring tasks, the closed-loop measuring principle (compensation principle) (see box) is usually used. If fewer requirements are imposed on measurement accuracy, open-loop current sensors (direct-imaging current sensors) (see box) can also be used.
Specially for rail technology
The current sensors of the HCMR series are specially designed for the requirements of the railway sector and are based on the compensation principle (closed loop). The black housings of this current sensor series for railway technology are characterised by their special ruggedness. Particularly high demands were placed on the connection technology in light of the required wide temperature range and the demands on vibration safety. The result? Depending on the application, HARTING can offer different rail-suitable sensor connections. In addition, different cable lengths suitable for the assembly of signal cables are available. Ready-made sets of sensors and cabling simplify both on-site installation and the procurement process.
External influences on the sensors can be minimised by internal EMC shielding – in addition to the protection already provided by the core geometry. The permissible temperature range of the HARTING current sensors is tested under laboratory conditions and ranges from -40 to +85° C. Consequently, the devices can be used in all climate zones from India to Siberia, and even tests in a tropical atmosphere were unable to faze them. By way of example, the current sensors survived continuous operation in over 90 percent humidity perfectly.
All requirements in railway sector met
HARTING current sensors are vibration-proof, resistant to fire loads and meet all relevant railway requirements: EN 50 155 (Electronic equipment on rail vehicles), NF F 16-101 I3 F1 (housing and casting compound self-extinguishing) and UL 94-V0 (self-extinguishing = less than 10 seconds afterburning). HARTING current sensors are available in eight sizes for rated currents up to 2,500 A, whereby the measuring range can also be larger. The current sensors can also easily meet customer-specific requirements. As a result, variants with different sensor resolutions can be provided at any time. The transformation ratio with which the output signal is amplified is highly flexible and can e.g. be 1:3000, 1:4000 or 1:5000.
Excess and deficit
The Hall effect describes the occurrence of a voltage on a current-carrying conductor located in a magnetic field. The Hall voltage occurring at the conductor drops perpendicular to the current flow and to the magnetic field direction. The Lorenz force caused by the magnetic field deflects the electrons perpendicular to their direction of motion. Thus an excess of electrons occurs on one side of the conductor while a deficit occurs on the other. The Hall voltage can be measured at this potential gradient. It increases linearly with the strength of the magnetic field and behaves anti-proportional to the charge carrier density. So-called Hall sensors use this behaviour to provide a simple measurement of the magnetic flux density.
A question of the requirement
In direct measurement using the open-loop sensor, the magnetic field of the primary current to be measured is concentrated in a magnetic toroidal core. A Hall element is placed in the air gap of the core and generates a voltage proportional to the magnetic field or to the current. This Hall voltage is amplified and serves as an output signal for the reproduction of the primary current.
The second variant, the closed-loop sensor, measures more accurately. Here, the Hall voltage is used to generate a current that flows through a coil placed on the toroidal core. This current results in a magnetic compensation field to the field of the primary current, so that the magnetic fields continuously cancel each other out. The secondary current is proportional to the primary current and serves as the output signal to the sensor.