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Posted on 25 April 2019

Open Loop Hall Effect Current Transducers

 

 

 

 

 

 

 

Open loop transducers use the simplest implementation of the Hall effect. They provide the smallest, lightest, and most cost effective current measurement solution while also having very low power consumption.

Construction and principle of operation

A current carrying conductor creates a magnetic field. This field is concentrated by a magnetic core. The core has a gap cut through it and a hall generator is used to sense the magnetic flux density in the gap. The control current, IC, and differential amplification are supplied by electronics (Figure 1 above) built into the transducer.

Within the linear region of the hysteresis loop of the material used for the magnetic circuit (Figure 2), the magnetic flux density, B, is proportional to the primary current, IP; and the Hall voltage, VH, is proportional to the flux density. Therefore, the output of the Hall generator is proportional to the primary current, plus the Hall offset voltage, VOH.

Magnetization curve
Figure 2. Magnetization curve

 

The measurement signal is  compensated to remove the offset component and address temperature effects, and amplified to supply the user with the desired output. In the case of low current measurement (< 50 A), multiple turns are recommended or implemented internally to achieve 50 Ampere-turns nominal, providing reasonable flux density levels for measurement.

Advantages and limitations

Open loop transducers measure DC, AC, and complex current waveforms while providing galvanic isolation. As mentioned earlier, the advantages include low cost, small size, lightweight, and low power consumption. Open loop transducers are especially advantageous when measuring high (> 300 A) currents. As with most magnetic based measurement techniques, insertion losses are very low. Primary current overloads can be easily handled although it may result in some magnetization of the core creating an offset shift, called remanence or magnetic offset. Compared to other technologies, the limitations of open loop transducers are moderate bandwidth and response time, a larger gain drift with temperature, and a limitation on the current frequency product (power bandwidth). In many applications, the advantages outweigh the limitations, and an open loop solution is advised.

Nominal and extreme currents

Many open loop transducers are made for nominal currents, IPN, from several amperes to 10 kA, with a peak current rating up to 30 kA. Such transistors are capable of addressing virtually all  industrial requirements. The maximum current an open loop transducer can measure is dependent on the design and material used for the magnetic circuit and on the design of the processing electronics.

Open loop transducers are generally designed such that the maximum measurable current is 200 % to 300 % of the nominal RMS current rating. Even so, open loop transducers can withstand current overloads significantly beyond the maximum measurable value,;for example, 10 times the nominal current. However, as described earlier, this can create a magnetic offset resulting in an additional measurement error, to be removed by applying a dedicated demagnetization procedure.

Output signals

The output of an open loop transducer is generally a voltage directly proportional to the measured current. This voltage is typically equal to 0V without primary current and 4V at the nominal current, IPN. Variations are possible, including different offset, and/or nominal values or a current output.

Measurement accuracy

The typical open loop transducer has an overall accuracy of a few percent. There are a number of error terms that combine to create this error, at nominal temperature (25°C) and across the temperature range.

The accuracy is limited by the combination of:

  • DC offset at zero current (hall generator, electronics)
  • DC magnetic offset (remanent magnetization of core material)
  • Gain error (current source, hall generator, core gap)
  • Linearity (core material, hall generator, electronics)
  • Output noise floor (hall generator, electronics)
  • Bandwidth limitation (attenuation, phase shift, current frequency)
  • Temperature changes also create drift in:
  • DC offset
  • Gain

The location of the primary conductor through the aperture as well as the positioning of the return conductor can affect dynamic performance of the transducer. An optimal routing/position for the primary and return conductors is generally recommended. In addition, high frequency disturbances can affect the transducer output due to capacitive coupling, so the routing and layout of the transducer output must be considered (e.g. twisted and shielded cables, appropriate routing of the output PWB tracks).

Magnetic offset consideration

Depending on the type of transducer and the magnetic material used, the residual flux (BR or remanence) of the magnetic core induces an additional measurement offset referred to as ‘magnetic offset’. The value depends on the previous core magnetization and is at a maximum after the magnetic circuit has been saturated. This might occur after a high overload current.

In the case of a higher current overload (e.g. 1000 % of IP), a larger magnetic offset error may occur. Recovering from this condition requires demagnetization, either by appropriate reversal of the primary current or a dedicated degauss cycle. This process will return the transducer to the initial, pre-overload, performance.

Demagnetization to eliminate magnetic offset

The elimination of magnetic offset requires demagnetization. A degauss cycle requires driving the core through the entire B-H loop with a low frequency AC source, then gradually decreasing the excitation, returning the B-H operating point to the origin (Figure 3). As a minimum measure, one should provide 5 cycles at full amplitude and then decrease the excitation smoothly, no faster than 4 % per cycle, requiring 30 cycles or 500 ms at 60 Hz. For closed-loop devices, additional care must be taken to be sure the compensation coil does not negate the demagnetization effort.

Degauss cycle current

Figure 3. Degauss cycle current

Alternatively, a partial demagnetization of the core is possible by providing an appropriate signal in the opposite polarity of the magnetization. The difficulty is determining the exact amplitude and duration to obtain a satisfactory result. With a well defined application, it may be feasible to determine the required value empirically and apply this correction as necessary.

 

For more information please read:

Handling Core Losses of Open Loop Transducers

Characteristics of Closed Loop Current Transducers

Selecting Voltage Transducers

Basic of Hall Effect Technologies

 

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