Eta is the name of the Greek symbol ‘η‘, representing efficiency. The Eta transducer product family has been given this name because of its very low secondary power requirements while also providing exceptional performance. In terms of performance, Eta falls between open loop and closed loop technologies. Because the construction is similar to closed loop transducers, this technology offers no price advantage.
Construction and principle of operation of Eta transducers
Hall effect Eta transducers are similar in construction to closed loop transducers, with the same magnetic circuit geometry, a Hall generator, and secondary winding.
Differences lie in the details of the magnetic core and processing electronics designs, leading to the Eta's specific features. A Hall effect Eta transducer is a mix of open loop and current transformer technologies providing the following characteristics:
• It works as an open loop transducer at low frequencies (up to 2…10 kHz depending on the specific transducer design), with the Hall generator providing a signal proportional to the primary current to be measured
• It works as a current transformer at higher frequencies, where the output current is proportional to the AC primary current
Both the Hall effect and transformer signals are electronically added to form a common output signal.
Eta technology is better suited than others when the following performance is expected:
• Wide bandwidth – DC to 100 kHz or more
• Low power consumption
• Use of a low voltage secondary supply (e.g. +5 V)
Power consumption is minimal because the power supply is not required to drive the secondary coil with compensation current, which also makes them suitable for use with a low voltage secondary supply, as they do not require the voltage ‘headroom’ of a closed loop device.
Indeed, it is very difficult to design closed loop transducers that will operate from low voltage secondary supplies (less than or equal to 5 V) with primary currents exceeding 25 A. This design issue is due to the fact that there is very limited voltage ‘headroom’ to drive a suitable secondary coil and measuring resistance. It is this combination of requirements that are best addressed with Eta technology.
Advantages and limitations of Eta transducers
Eta transducers are capable of measuring DC, AC, and complex current waveforms while ensuring galvanic isolation and low insertion loss. Their significant advantages are the low power consumption and suitability for small secondary supply voltages, such as a unipolar 5 V supply, as with open loop transducers, along with the high bandwidth and fast response time of a current transformer.
These characteristics also lead to the limitations of an open loop transducers at low frequencies - offset and gain drift with temperature and moderate accuracy. With higher frequencies (> 2…10 kHz) the current transformer effect provides very good accuracy and negligible temperature drift. In addition, the flux canceling nature of a current transformer reduces the concern of core losses with high frequency currents.
With Eta technology the major inconveniences are the size of the magnetic circuit, with a large core sized for low frequencies as with open loop transducers, and the need for a large secondary coil for detecting high frequencies, as compared to closed loop transducers. This typically results in a more expensive construction than is required for a similarly rated closed loop transducer.
The LEM Eta transducer range is made for nominal currents IPN from 25 to 150 A. This rather narrow range is not limited by technical issues, but rather by the market itself: for currents less than or equal to 25 A it is possible to work with the higher performance closed loop technology, while for currents greater than 150 A secondary voltage supplies with a greater amplitude (e.g. ±15 V) are generally available, again allowing the use of closed loop transducers. The reduced secondary power consumption of the Eta technology is often not a sufficient asset to promote Eta technology beyond this current range, although there are always exceptions.
For a given Eta transducer, the maximum current which can be measured depends on the limitations set by the open loop or current transformer behavior, at low and high frequencies respectively. This leads to a maximum measurable current of typically 150 to 200 % of the nominal current at low frequencies, and a capability going significantly beyond that at higher frequencies.
The risk of magnetic offset after an unexpected current overload is similar to that of open loop and closed loop transducers, again at the expected frequency of disturbance.
Output Signal of Eta transducers
By design an Eta transducer has a voltage output, although the internal design could be modified or voltage signal post-processed to provide other output types.
Measurement accuracy of Eta transducers
The accuracy of an Eta transducer is dependent on the working frequency:
• For low frequencies (< 2…10 kHz), the overall accuracy is a few percent, as with open loop designs
• For higher frequencies, the overall accuracy is typically below one percent
Dynamic behaviour of Eta transducers
The bandwidth, response time, and di/dt behavior of Eta transducers are very close to those of the closed loop technology, although slight performance reductions may come from the use of a less efficient magnetic circuits (material, design) for high frequency operations.
Measurements carried out on Eta transducers show a typical bandwidth range of DC to 100 kHz or more.
The response to a current step demonstrates the ability to correctly reproduce a transient. It is defined by several parameters such as the delay time, rise time, and reaction time with a particular waveform. Eta transducers show fast reaction time, similar to closed loop designs.
The correct following of di/dt depends on the intrinsic construction of each product and, as mentioned previously, the mounting conditions of the transducer in the circuit to be measured. Depending on the selected Eta transducer model, it is possible to measure di/dt from approximately 50 A/μs to 400 A/μs or more. This feature makes them well suited for the protection of semiconductor devices.
Typical applications of Eta transducers
Eta current transducers are used in numerous industrial applications, generally as an important element of a regulation loop (for current, torque, force, speed, position feedback), but also for current monitoring and display. Examples of applications are similar those of open loop and closed loop transducers.
For more information, please read: