01. May 2016

Current sensors for power electronics applications

Current sensors are electromechanical components that detect electric current (AC or DC) in an electrically conducting cable or busbar and generate a signal proportional to it. The generated signal could be an analogue voltage, a current or even a digital output. These measurement signals are used for precision monitoring and control of power semiconductors and other components in power electronics systems. The sensors described in this article are based on the proven Hall effect principle and have been developed in close co-operation with key users in the industrial and utilities sectors, and represent an optimised solution that fulfils the requirements of modern power systems.

In very simple terms Hall effect current sensors measure the current in a conductor by sensing the magnetic field it produces, in an electrically isolated manner. These measurements are of two types:

  • direct­mapping current sensing (open-loop technology) for applications where the requirements in terms of precision are less stringent.
  • compensation current sensing (closed-loop technology) for demanding measurement tasks

For open-loop sensors (Fig.1a), the primary current’s magnetic field is concentrated in a magnetically soft toroid. A Hall element that generates a voltage proportional to the magnetic field or to the current is positioned in the toroid’s air gap. The Hall voltage is amplified and delivers a mapping of the primary current as an output signal. One advantage of these sensors is the simple design. However, the temperature dependency of the Hall element and the amplification (offset and gain drift) influence the precision.

Closed-loop sensors (Fig.1b) have a design similar to that of direct mapping sensors. The Hall voltage, however, is not used directly as the measurement signal: instead, it is used to regulate a secondary current. The secondary current flows through a coil with N windings and generates a magnetic compensation field in the toroid. If the secondary current multiplied by N is exactly the same as the primary current, the two magnetic fields cancel each other out in the toroid.

The Hall element always regulates the magnetic flux to zero, and the secondary current is simultaneously the sensor’s output signal.

These sensors consume more current, but work very precisely (with an accuracy better than 1%) throughout the entire industrial temperature range (typically -40°C to +85°C), which allows use in thermally critical applications. They can measure direct or alternating currents, including signals with complex waveforms, over a frequency range from 0 to 50 kHz. Because the devices use proven Hall-effect technology to sense the magnetic field created by the current flowing in a conductor, they provide non-contact, galvanically isolated measurements with high immunity to interference from the magnetic fields of external current-carrying conductors.

Application example

In a typical application example, the water industry is using HARTING current sensors to monitor the power feed lines to the large wastewater control pumps in pumping stations in water treatment plant (Fig.2). As a result, it is possible to detect whether an electric pump’s performance is deteriorating (from pump jam and suction loss, for example) before it overheats or blocks.

The sensing unit is integrated into the overall SCADA management and control system of the pumping station, where it acts to maximise efficiency, saving time and reducing costs by prompting preventative maintenance, with consequent benefits in reduced pump breakdowns and the resulting call-out costs.

Product range

HARTING’s HCS current sensors (Fig.3) are based on the open-loop measurement principle, and provide a direct representation of the primary current with an accuracy of up to ±1%. The family has recently been extended by the addition of the HCSE (Hall-effect current sensors Eco) family, consisting of four models with a current range from 100 A to 800 A.

For applications which require a higher accuracy, HARTING offers a full range of panel mountable closed-loop versions for a measurement range from 200 to 2000 A. Here the sensors provide a direct representation of the primary current with an accuracy of up to ±0.5%. In addition, they operate with alternating currents over a wider frequency range of 0 to 100 kHz. The operating temperature range is also extended from    -40°C to +85°C.

The housing material and potting mass have a flammability rating to UL94 V0, and the devices meet the requirements of EN Standard 50 178: Electronic equipment for use in power installations. They also conform to the EN 50155 standard for rail applications.

A further advantage for developers is that the sensors are easy to integrate into existing applications, as they have standard footprints and installation dimensions.

HARTING offers standard pre-assembled output measuring connection termination technology to allow fast, economical and reliable assembly. In addition they are very application flexible in supporting customer specific requirements for low to medium volume demands.

Conclusion

The current sensors described in this article offer some key technical performance benefits against alternative solutions: notably, high immunity to interference; an extended operating temperature range; and a more stable housing construction for harsh environment applications. In addition, HARTING offers the capability to provide customised solutions for low volume requirements, backed up by independently accredited laboratory test services to guarantee that a product matches the performance of an application’s key operating conditions before full deployment.


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