In an earlier blog post, we introduced the 6 different current sensing methods used in the current meters and sensors developed by Hioki. Here, we highlight the Rogowski coil method that enables the design of flexible sensors through the use of an air core (or coreless) structure.
In a Rogowski sensor, there is no heat generation, saturation and hysteresis due to magnetic loss, and the insertion impedance is relatively small. When current is measured, the magnetic field that is generated is interlinked with the coreless coil to induce a voltage. This induced voltage is then output as the time derivative (di / dt) of the measured current, and an output signal proportional to the constant current is obtained by passing it through an integrated circuit. A power supply is required to conduct the integration, and in principle, the coil does not respond to DC signals.
In summary, the advantages of using the Rogowski method to measure AC current are:
· The coreless structure is resistant to magnetic saturation so that linearity is maintained, allowing you to measure large currents
· No overheating, saturation or hysteresis due to lack of magnetic loss
· Ability to provide a flexible and thin structure to meet many applications
· Low insertion impedance
The disadvantages are:
· Inability to measure DC
· High susceptibility to noise, which does not make it ideal for high precision testing
One important characteristic to note regarding Hioki flexible AC sensors is the integration method.
Typically, in a Rogowski sensor, especially one designed to pair specifically with a power meter or PQA, the value obtained from the Rogowski coil is output as is to the measuring instrument and then integrated by the device. In other words, integration is not conducted on the sensor side so there is no need for sensor power supply.
In Hioki flexible sensors, however, the integration circuit is located at the sensor side, making it necessary to drive it with a power supply. Some might find this inconvenient, but this design also lets the customer use the flexible sensor with a wide variety of measurement devices independently, such as Memory HiCorders and other data acquisition devices. FYI - the PQ3100 and PQ3198 can supply direct power to the CT704X Flexible AC Current Sensor series without the need for an additional AC power supply or batteries.
The HIOKI philosophy towards current sensors is to contain as much of the processing of signals at the sensor side as possible, making them self-sufficient with minimal vulnerability from external effects such as noise. After powering the sensor, measured signals are integrated and adjusted at the sensor before being output to the main measurement device.
The Rogowski coil method was used to engineer HIOKI CT7044, CT7045, CT7046 , and CT9667 Series AC Flexible Current Sensors, and you can learn about more about the operating principle of the Rogowski method in this Technical Note .
Hioki's core competency in current sensing technologies empowers us to leverage traditional testing principles while overcoming their inherent disadvantages to deliver state-of-the-art devices. You can familiarize yourself with all 6 current sensing methods here.
Basics of Electrical Testing Poster
A new “Basics of Electrical Testing” poster has been authored by HIOKI’s own test and measurement expert and seminar lecturer, Mr. Hideo Ishihara, for the global market.
“It was my pleasure to contribute to the creation of this poster based on the materials I used at seminars, etc., because it is important for everyone to know the basic principles and structure of measuring instruments when using HIOKI products."
Download the PDF here (printable size: A1).