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Principles of current acquisition

If purely ohmic loads such as incandescent lamps or heaters are operated on a conventional 230 V network, no distortions are produced on the power supply network. As non-linear loads increase as a result of phase angle-controlled regulation modules, pure sinusoidal waves are distorted and take on a trapezoidal waveform.

Many current and voltage transducers on the market are only calibrated for sinusoidal alternating currents and only determine the true r.m.s. value by mean-value generation. However, MCR current transducers are true r.m.s. value measuring transducers. They accept all waveforms regardless of the form factor.

True r.m.s. value acquisition according to the transformer principle (RMS)

True r.m.s. value acquisition according to the transformer principle (RMS)  

True r.m.s. value acquisition according to the transformer principle (RMS)

A magnetic flux which changes over time produces an induced voltage at the terminals of a coil.

A circuit arrangement consisting of two electrically isolated but magnetically coupled circuits is known as a transformer.

This is a simple and commonly used option for current transfer.

True r.m.s. value acquisition according to the Rogowski principle (TRMS)

True r.m.s. value acquisition according to the Rogowski principle (TRMS)  

True r.m.s. value acquisition according to the Rogowski principle (TRMS)

The Rogowski principle is a special method used to measure sinusoidal and non-sinusoidal alternating currents in a transformer.

A non-ferrous induction coil (air-core coil), known as the Rogowski coil, measures the magnetic voltage along a closed circumference around a live conductor.

The output signal of the Rogowski coil is then conditioned so as to obtain an exact replica of the primary current.

True r.m.s. value acquisition with a Hall sensor (TRMS)

True r.m.s. value acquisition with a Hall sensor (TRMS)  

True r.m.s. value acquisition with a Hall sensor (TRMS)

The magnetic flux generated by the primary current IP is condensed in the magnetic circuit and measured in the air gap using a Hall sensor.

The output signal of the Hall sensor is then conditioned so that an exact replica of the primary current is obtained.

Mean-value generation

True r.m.s value - root mean square value

The true r.m.s. value of an alternating current corresponds to the steady-state value that results from the instantaneous values of this current. This steady-state value generates the same thermal work in an ohmic resistor as a direct current of identical magnitude.

The term "true r.m.s. value" simply means that distorted direct and pulsating currents can be acquired. The measuring transducer is compatible with any waveform.

  • Irms = Is/√2
  • Urms = Us/√2

 

Arithmetic average value  

Arithmetic average value

Arithmetic average value

The arithmetic average value is used to measure direct currents or filter a DC component out from a pulsating current.

Applying the arithmetic average value to a symmetrical alternating current would result in a measured value of "0".

The arithmetic average value enables direct currents to be made available at the output in the form of standard analog signals.

The polarity can be evaluated by means of a bipolar output signal. For a 230 V/50 Hz power supply network, this results in the following with regard to the voltage levels:

  • Urms = 230 V
  • US = 325 V
  • Uavg = 0 V

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