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I. Overview:
A Rogowski coil, also called a current measurement coil or differential current sensor, is a toroidal coil uniformly wound on a non-ferromagnetic material. The output signal is the derivative of current with respect to time. By using a circuit that integrates the output voltage signal, the input current can be faithfully reproduced. II. Structure Overview: It contains no ferromagnetic material, has no hysteresis effect, and has virtually zero phase error; it has no magnetic saturation, so the measurement range can extend from a few amperes to hundreds of kiloamperes; it has a simple structure and no direct electrical circuit connection with the measured current; its response bandwidth is 0.1 Hz–1 MHz. Compared with traditional transformers with iron cores, Rogowski coils offer a wide measurement range, high accuracy, stability and reliability, wide response bandwidth, and simultaneous measurement and relay protection functions, and are small, lightweight, safe, and compliant with environmental requirements. Rogowski-coil-based devices feature real-time current measurement, fast response, no saturation, and virtually no phase error, so they can be used in applications with severely distorted signals such as relay protection, silicon-controlled rectification, variable-frequency speed control, and resistance welding.
Rogowski Coil, whose full name is Rogowski coil, is also called an air-core coil because it contains no iron core.
A Rogowski coil is a toroidal coil uniformly wound on a non-ferromagnetic material. The output signal is the derivative of current with respect to time. By using a circuit that integrates the output voltage signal, the input current can be faithfully reproduced. The coil features real-time current measurement, fast response, no saturation, and virtually no phase error, so it can be used in applications with severely distorted signals such as relay protection, silicon-controlled rectification, variable-frequency speed control, and resistance welding, as well as in high-current applications such as electric furnaces, short-circuit testing, and lightning signal acquisition.
With the banana plugs and BNC connectors provided by the integrator, it can be conveniently connected to acquisition boards, oscilloscopes, multimeters, and other measuring instruments.
Rogowski Coil isa flexible current probe utilizing the Rogowski principle，which issuitable to conveniently measure single and three phase AC as well as pulsed DCcurrents. After appropriate signal conditioning,it can be used with digital multimeters, recorders and other suitable equipmentto measure current from very low frequencies up to 1MHz.
The probes comprise a flexible air–cored sensor which can be opened,allowing them to be installed around a primary conductor without interruptingthe circuit. The flexible and lightweight measuring head allows quick and easyinstallation in hard to reach areas and with large conductors.
II. Working Principle:
A Rogowski coil is an air-core toroidal coil, available in flexible and rigid types, that can be directly placed around the conductor being measured to measure AC current. Its basic design principle is shown in the figure:
The theoretical basis for current measurement with a Rogowski coil is Faraday's law of electromagnetic induction and Ampère's circuital law. When the measured current passes along the axis through the center of the Rogowski coil, a corresponding changing magnetic field is generated within the volume enclosed by the toroidal winding, with strength H. From Ampère's circuital law:
∮H·dl=I(t)
From B=μH, e(t)=dΦ/dt, Ф=N∫B·dS, e(t)=M·di/dt, we obtain:
When its cross-section is rectangular, the mutual inductance M and self-inductance L are respectively:
M=μ0Nhln(b/a)/2π
L=μ0N^2hln(b/a)/2π
In the above equations, H is the magnetic field strength inside the coil, B is the magnetic flux density inside the coil, μ0 is the vacuum permeability, N is the number of coil turns, e(t) is the induced voltage across the coil, a and b are the inner and outer diameters of the coil cross-section respectively, and h is the cross-section height.
It can be seen that for a given coil, M is a constant, and the coil output voltage is proportional to di/dt. In other words, the Rogowski coil output voltage is proportional to the derivative of the measured current; passing its output through an integrator yields an output voltage proportional to the primary current.
III. Performance Features:
- Suitable for current testing from milliamperes to megaamperes
- Good linearity
- Wide bandwidth range
- No danger of secondary open circuit
- Strong overcurrent capability
- Low power consumption
- Can measure irregular conductors;
- Easy to install, no need to break the conductor
- Lightweight
Rogowski Coil is an ideal replacement for medium/large current, currenttransformers(CT’s) and has the advantages that it is：
Suitable to measurecurrents from mA to hundreds of kA
Wide dynamic range
Very useful with awkwardshaped conductors or in places with limited available space
No danger fromopen-circuited secondary
Not damaged by largeoverloads
Non-intrusive, no powerdrawn from the main
IV. Integrator Circuit:
According to the law of electromagnetic induction and Ampère's circuital law, the induced electromotive force output by a Rogowski coil is proportional to the rate of change of the primary current. That is, its output voltage is proportional to the derivative of the current. Therefore, integration, the inverse operation of differentiation, can be used to restore the output to a voltage signal proportional to the input primary current; by measuring this signal, the primary current can be reflected more directly.
Except for standard sine waves with a fixed and known frequency, the Rogowski coil must be used with an integrator during measurement. Because Rogowski coils are usually used to measure high-frequency currents or large currents, the measurement environment has significant electromagnetic interference, and special attention is required during measurement. To reduce the influence of interference, integrators with digital output are preferred.
A digital-output integrator completes the integration of the Rogowski coil output voltage signal and AD sampling inside the integrator, and uploads the AD sampling results via optical fiber to a secondary instrument or host computer. This can effectively avoid losses and interference during transmission. During use, keep the analog connection between the integrator and the Rogowski coil as short as possible, and use shielded cables where possible.
V. Brands and Products
| Model | Appearance | |
| SS-280A Series | Maximum peak current: 12kA (SS-0289A) Maximum test frequency: 30MHz Maximum peak voltage: 1.2kV Cable length: 1.5m Probe operating temperature range: -40oC～ 125oC | |
SS-620S Series | Maximum peak current: 12kA Maximum test frequency: Coil length 100mm Type: 25MHz Maximum peak voltage: 5kV Cable length: 3m Probe operating temperature range: -40oC～ 125oC | |
| SS-620M Series | Maximum peak current: 12kA Maximum test frequency: Coil length 200mm Type: 20MHz Maximum peak voltage: 5kV Cable length: 3m Probe operating temperature range: -40oC～ 125oC | |
| SS-290S Series | Maximum peak current: 12kA Maximum test frequency: Coil length 300mm Type: 20MHz Maximum peak voltage: 10kV Cable length: 3m Probe operating temperature range: -40oC～ 125oC | |
| SS-290L Series | Maximum peak current: 12kA Maximum test frequency: Coil length 700mm Type: 10MHz Maximum peak voltage: 10kV Cable length: 3m Probe operating temperature range: -40oC～ 125oC | |
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