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Posted on 19 April 2019

Characteristics of Closed Loop Current Transducers

 

 

 

 

 

 

 

Advantages and limitations

Closed loop transducers are capable of measuring DC, AC and complex current waveforms while ensuring galvanic isolation. The advantages of this design include high accuracy and linearity, low gain drift, wide bandwidth, and fast response time. Another advantage is an output current signal that is easily scalable and well suited to high noise environments; nevertheless, closed loop transducers are available in voltage output configurations. Also, as with most magnetic based measurement techniques, insertion losses are very low.

The main limitations of the closed loop technology are the high current consumption from the secondary supply (which must provide the compensation as well as bias current), the larger dimensions (more noticeable on high current transducers), a more expensive construction compared with the simpler open loop designs, and a limited output voltage due to the internal voltage drops across the output stage and secondary coil resistance.

However, depending on the application requirements, the advantages often outweigh the limitations, and the accuracy and response of a closed loop solution is desirable over other alternatives.

Nominal and extreme currents

Closed loop current transducers are available in models with nominal currents from 2 A to more than 20 kA. Specific  designs even allow current measurements up to 500 kA. With closed loop technology, the maximum measurable peak current is typically 150 to 300 % of the nominal current rating. For a given closed loop transducer, the maximum peak current which can be measured can be defined in three different ways:

• From DC to mid-frequencies, the maximum measurable current is limited by the ability of the electronics to drive compensation current, IS, in the secondary coil; this limit is based on the available supply voltage, internal voltage drops, and current through the total series resistance. Exceeding this limit at low frequency will result in "electronic saturation"

• Also, each transducer is designed for a specific measurable current range and exceeding the transducer ratings will result in non-nominal magnetic effects (excessive fringing) that does not allow the electronics to properly compensate the loop, resulting in "magnetic saturation"

• For transient currents at higher frequencies, the transducer operates as a current transformer and the current can reach higher values, limited by magnetic (pulse duration in ampere seconds) and thermal (core losses related to ampere-hertz) constraints; the transducer user should consult the manufacturer when considering working in the transformer mode beyond the limits set in the product's datasheets.

In some specific transducer types, primary currents exceeding the peak measuring range can create abnormal, though non-destructive, results due to the unique electronic configurations. In case of current overload, too long of an overload duration (e.g. > 1 ms) may in some cases start to overheat the snubber which protects the transducer against short time overloads.

Output signal – Measurement resistance

The majority of closed loop transducers have a current output that can be easily converted to a voltage for measurement by adding a measurement resistor in series with the output.

Measurement accuracy

Due to the closed loop working principle, operating at nearly zero flux (some flux remains due to system loop gain and magnetic leakage phenomena), closed loop current transducers have excellent linearity and minimal gain drift over a wide measuring and temperature range, with total accuracy typically remaining below 1 %.

At ambient temperature, the accuracy is given by the combination of:

• Output offset at zero primary current (IP = 0)

• Non-linearity of the Hall generator, electronics, and magnetics

• Gain tolerance (tolerance of the number of secondary coil turns)

• Tolerance of the measuring resistor, R(internal or external)

Temperature changes imply:

• Offset drift (or with respect to the reference voltage, if appropriate)

• Drift of the measuring resistor value, RM

While the above factors may be simple to assess for DC current, AC signals and complex waveforms may have their total accuracy affected by transducer bandwidth limitations, possibly introducing frequency attenuations and phase shifts.

To make the best use of the transducer, the mounting conditions must be such that it optimizes the primary to secondary magnetic coupling, specifically for AC signals where the transducer works as a current transformer. The designer must consider both the primary conductor (wire or busbar being measured) and other conductors in close proximity, such as the return conductor or the conductors of other phases.

Additionally, the routing of the transducer output wires, or paths of the PWB tracks, at the transducer output should limit high frequency disturbances created by external conductors. The output wiring should have minimal loop area, to minimize di/dt effects, and long runs parallel to power wires must be avoided, to limit capacitive coupling and minimize dv/dt effects.

Observations regarding magnetic offset

In standard working conditions, a closed loop transducer is always working near zero flux, either when the low frequency Hall based closed loop is effective or when the high frequency current transformer is working. However, this does not imply that closed loop transducers are not at risk of having a permanent magnetic offset.

• If a low or medium frequency primary current exceeds the measuring range (based on supply voltage, transducer parameters, and measuring resistor value), the electronics can no longer drive sufficient secondary coil current to maintain the zero flux condition

• If one or both of the secondary supply voltages are missing, disabling the electronic compensation process, there will no longer be zero flux compensation

• When an external conductor creates localized core saturation, not totally detected by the Hall generator and compensated by the electronics, the total flux in different areas of the core will be non-zero.

 If any of these conditions occur the result could be magnetic offset, resulting in an additional measurement error. This can be corrected with demagnetization.

With compensated devices, care must be taken to ensure the compensation does not negate the demagnetization effort. Ideally, the output can be disconnected to open the compensation loop. If this is not possible, disabling the power supplies accomplishes the same goal if a low frequency is used for excitation, to avoid the current transformer effect.

Bandwidth and core losses

Closed loop transducers demonstrate excellent bandwidth characteristics. Typically the bandwidth is from DC to 200 kHz,  while transducers with advanced design achieve  a bandwidth better than DC to 300 kHz. Nominal current can not be considered over the full frequency range. To keep the transducer losses constant, current value shall be decreased while working frequency increases.

While the current transformer effect of closed loop transducers provides excellent high frequency performance, they are still subject to core losses due to hysteresis and eddy current losses. As with open loop transducers, care must be taken when attempting long term measurement of high currents at high frequencies.

Response time and di/dt behavior

The response time of a transducer characterizes how it will respond to a step current with a controlled rate of change, called di/dt, following. It is defined by several parameters such as the delay time, rise time, and reaction time.

Closed loop transducers show fast reaction times, typically better than 1 μs. The correct following of di/dt depends on the intrinsic construction of each product and the mounting conditions of the transducer within the circuit to be measured.

Dependent on the closed loop transducer model, it is possible to measure a di/dt of 50 to 400 A/μs or more. This makes them well suited for the short-circuit protection of semiconductors in power equipment. 

Typical applications

Closed loop transducers are particularly well suited for industrial applications that require high accuracy, wide bandwidth, and fast response time. They are often used as the key element of a regulation loop for the control of current, torque, force, speed and/or position, as well as for the protection of semiconductor devices.

Applications are identical to those for open loop transducers, except higher performance results can be expected:

• Frequency inverters and 3-phase drives, for the control of the output phase and DC bus currents as well as protection of the power semiconductors from fault conditions such as output short-circuits

• Converters for servo-motors frequently used in robotics, for high performance speed and position control

• Special wide bandwidth power supplies for special equipment, such as radar

Other applications include energy management systems, switching power supplies, electrolysis equipment, lasers, rectifiers for electrolysis and, finally, many applications for laboratories or test and control benches.

 

For more information, please read:

Introduction to Closed Loop Hall Effect Current Transducers

Transducers - Basic Principles of Selection

 Open Loop Hall Effect Current Transducers

Handling Core Losses of Open Loop Transducers

 

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