The performance of current and voltage transducers are often limited by the characteristics of the magnetic core material itself (e.g. remanence, hysteresis, non-linearity, losses, saturation), so the design of an air-core, or coreless, transducer is often considered.
In this case the following issues have to be taken into account:
- the measurement of DC current requires the use of a field sensing element; due to the absence of a field focused area (e.g. gap), a highly sensitive field sensing device must be considered (e.g. GMR, Hall cell), ideally in an array around the conductor.
- when available, a magnetic circuit can be used as a shield to external magnetic field disturbances (e.g. earth’s field, external conductors); with air-core technologies the sensitivity to external disturbances must be managed in a different way, for example an array of field sensors instead of a single sensor or, when coils are considered, special design execution such as the Rogowski method of routing the return wire; the ability to accurately measure the desired current while also rejecting external fields is a significant challenge for air-core technologies.
Two efficient air-core transducer technologies are presented here, both related to AC measurements: the LEM~flex and PRiME™(1) technologies. (1) patented and licensed to LEM by Suparule Ltd.
Basic working principle and sensitivity
LEM~flex and PRiME™ technologies both work on the same basic principle; a pick-up coil is magnetically coupled with the flux created by the current to be measured. A voltage is induced on the pick-up coil proportional to the derivative of flux and thus proportional to the derivative of the current to be measured. Because the derivative of DC is zero these technologies are only useful for the measurement of AC or pulsed currents.
Figure 1. Rogowski AC current measurement
The instantaneous voltage induced in the pick-up coil is typically:
where i(t) is the primary current [A] and L12 is the mutual inductance [H] between the primary and pick-up coil.
For a sinusoidal current, we have:
As shown in this example, where a sinusoidal current i(t) creates a phase delayed (cosine) voltage EOUT(t), reproducing the waveform of the measured current requires the integration of the induced voltage. Therefore, the current transducer includes an integration function in the processing electronics.
In the LEM~flex and PRiME™ transducer datasheets the value of sensitivity (S12) is provided, linking the amplitude of a sinusoidal current to the amplitude of the transducer output voltage at a specific frequency.
The same sensitivity parameter can also be used to link the RMS values of a sine wave primary current and the corresponding sine wave output voltage, namely:
To give an order of magnitude, the typical sensitivity is:
• LEM~ flex probe: S12 = 2.0 [μVs/A]
• PRiME™: S12 = 1.0 [μVs/A]
Using our examples, LEM~flex is a lightweight measuring head combined with remote electronics (distance between head and electronics can be as great as 4 meters, or 12 feet). This, along with all of the previous described attributes, lead to a device suitable for use in a wide range of applications:
- Measuring currents in busbar sets, in particular in induction heating equipment.
- Frequency converters, variable speed drives and generators.
- Control of power semiconductors.
- Analysis of the current distribution in mains networks.
- Analysis of harmonics, power measurements, measurement of the peak load in the mains, and in UPS.
- Switched mode power supplies.
- Low or medium voltage distribution installations.
- Power electronics installations.
- Sensing devices for watt meters and network analyzers installed by electric power distribution companies.
- Electrical maintenance, repair and machine installation and start-up applications.
- Connection to most measuring instruments including multimeters, oscilloscopes, recording devices, data loggers, etc.
Care must be exercised in properly identifying errors based on reading and those based on range.
PRIMETM on the other hand has no theoretical maximum limit to the measuring range, but the typical dynamic range is 1000:1, corresponding to the ratio of the maximum to minimum measurable current with a given transducer. The accuracy is typically specified as a percentage of reading, above 10 % of the nominal rated current, leading to a highly accurate solution when the current is only a fraction of the nominal current. The accuracy is generally better than 0.8 % of reading and the gain variation due to temperature is low, typically 0.01 %/K.
Figure 2. PrimeTM Current Transducer
The output voltage is directly proportional to the measured current and provides accurate phase information. The level of sensitivity depends on the required measuring range and supply voltage.
The aperture sizes for existing transducers range from 20 mm to 160 mm in diameter, in both ring and split format. There is no theoretical limit to the size of the aperture. With split versions, installation and measurement can be performed without mechanical or electrical interruption of the current carrying conductor, while also ensuring galvanic isolation.
The bandwidth of PRiMETM has both a high frequency and a low frequency cut off. The high frequency limit is dependent on the resonant frequency of the sensors while the low frequency limit is a function of the integrator design. Products are designed for a given bandwidth, typically 5 to 100 kHz, but an upper limit in the MHz range appears to be feasible.
Figure 3. Processing of Coil Signals
Advantages and Limitations of PRiMETM Technology
- Capable of measuring AC and pulsed DC currents.
- Wide current measuring range, capable of withstanding high overload.
- Accuracy given in percent of reading; high accuracy over a wide measuring range.
- Large bandwidth, not including DC.
- Lightweight in comparison with current transformers or transducers.
- On-board electronics that can potentially be merged with the users electronics.
- Provides an isolated output signal (e.g. 4-20 mA, 0-10 V) usable with PLCs without conditioning.
- Requires a power supply, but has low current consumption requirements.
This makes PRiMETM transducers suitable for portable applications and power quality monitoring where weight and battery life are a concern. This performance also make them suitable replacements for current transformers.
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