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What is Rogowski Coils and How Does it Work?
What is a Rogowski Coil?
Aim Dynamics offers a wide range of Rogowski coils (also called rope CTs or flexible current transformers) from several manufacturers. Rogowski coils are more versatile than other types of current transformers. They don’t have a core that saturates like a standard solid or split-core CT. The lack of core allows them to be used in a much wider range of amperages.
Additionally, they are flexible and generally thinner than traditional CTs, allowing them to be installed in some places where a regular solid or split-core CT wouldn’t fit. Often Rogowski coils are sold in sets of three with one integrator that magnifies and adjusts the signal. Without the integrator, the output of Rogowski coils is a very low AC voltage signal.
An output signal of 333mV, 4-20mA, 0-5Vdc, 0-10Vdc, 100mA, 1A, or 5A can be achieved with the actively-powered integrator. Some power meter manufacturers can accept the native output of the Rogowski coil (without the powered integrator). In contrast, less flexible meters require the signal to be amplified and the phase corrected via the use of an integrator.
To help you find what you’re looking for, we’ve broken down our products into five categories:
AC Amp Input -> 333mV Voltage Output:
To help you find what you’re looking for, we’ve broken down our products into five categories:
AC Amp Input -> 4 to 20mA Output:
These flexible coils output a 4-20mA signal. The 4-20mA loop powers some models whereas others come with their external power supply.
AC Amp Input -> 5 Amp Output:
With the actively-powered integrator, these Rogowski coil kits are configured to output a 5A signal.
AC Amp Input -> 0-5Vdc/0-10Vdc Output:
These Rogowski coils transduce the input signal to a 0-5Vdc or 0-10Vdc signal.
AC Amp Input -> Native Rogowski Coil Output:
Rogowski coils that do not have an integrator attached will output a very low voltage signal with a 90-degree phase shift. Some meters can handle this native output and do not require signal adjusting outside of the meter.
Aim Dynamics carries Rogowski Coils from a range of manufacturers including:
Things to consider when selecting Hall effect current sensors:
Input/Output Types
How Does a Rogowski Coil Work?
All Rogowski coils measure alternating current (AC). The native output is a very low voltage, and the phase is retarded (or advanced) 90 degrees. The output is often passed through an integrator to magnify, transduce, or otherwise alter the signal type.
Common output signal types (produced by integrators) include 333 mV AC, 4-20 mA DC, 5V DC, 10V DC, and 5A AC.
Some meter manufacturers have built-in integrators, such that the Rogowski coils can be used natively. This is what is meant above by the category AC Amp Input -> Native Rogowski Coil Output.
Aim Dynamics carries current transformers in every possible output selection including transformers that feature AC and DC outputs.
Application
Coil Uses
Rogowski coils have a large number of current monitoring applications including:
- Power monitoring
- Precision welding systems
- Arc melting furnaces
- Electromagnetic launchers
- Electric power transmission
- Pulsed power
Rogowski coils are especially helpful when the frequency of the AC input is in the kHz range (where normal CTs often have trouble), but they are also commonly used at 50/60 Hz, and in some cases, in the MHz range.
They can also be used in short-circuit testing of electric generators, motor protection systems, measuring harmonic current content, and as sensors in protection systems of electrical plants.
Coil Length
Does Size Matter?
Hall effect current sensors that have a bipolar output, e.g. +/- 5V, almost always require a bipolar power supply, e.g. +/-15 Vdc. There are a few ways around this – LEM, for example, provides a series of bidirectional sensors that operate on a +5Vdc power supply. This is achieved via the use of an offset in the output.
For example, +2.5V might be the output at 0A on the primary. -100A might result in an output of 1.0V, and +100A would therefore have a 3.5V output. This “strange” output can often be handled by more advanced PLC units, or the output can be designed into a circuit.
Aim sells split-core DC sensing devices that require only a +12Vdc power supply. This is more convenient in systems where a +12Vdc power supply is all that’s available. See the AIMH040-xxxA-VT and AIMH020-xxxA-VT series for more information.
Disadvantages of Rogowski Coil
The Rogowski coils disadvantages that include:
- To obtain the current waveform, the output of the coil must pass through an integrator circuit. It requires a 3V to 24Vdc power supply.
- DC current cannot be measured.
Frequently Asked Questions
Here are some of the FAQs we get asked about Rogowski Coils:
What are Rogowski Coils?
Rogowski coils are similar to current transformers, except that they have no core. The operation is based on a very simple principle: a wound coil without a magnetic core is placed around the current-carrying conductor; the variable magnetic field produced by the current induces a voltage in the coil.
The output voltage is proportional to the variation of the current sensed, but the phase is shifted 90 degrees. After an integrator circuit is applied, the phase shift is accounted for, and the output is typically magnified or transduced at this point.
Using precision winding techniques, developed especially for this purpose, coil manufacturers work hard so that the output signal is not influenced by the position of the conductor inside the Rogowski coil. Proper wrapping technique minimizes the impact of external magnetic fields produced (e.g. neighboring current conductors).
Why is it called a Rogowski Coil?
Named after their inventor, Walter Rogowski, the Rogowski style of coils was first developed in 1912. They basically function as a measurement tool that determines the current moving through a conductor that has been inserted within the flexible Rogowski loop.
Rogowski coils allow the observer to accurately examine the changes in current over a given period. To utilize the output, you may need to “integrate” the raw voltage output.
How do I install a Rogowski Coil?
Rogowski coils are flexible, thin rope-like loops that can be uncoupled to fasten around a conductor. Unlike conventional CTs, Rogowski coils do not have a ferrite/iron core but rather a continuous single spiral winding which allows them to be formed into an elliptical shape to match the conductor profile.
What are the true benefits of selecting a Rogowski Coil over a traditional current transformer?
The Rogowski coil is a useful alternative because it:
– Enables the consumer to span a broad range of amperages and frequencies from Hz to MHz. – Is often less expensive to implement than CTs capable of similar measurement, which enables you to lower the cost of construction. – Offers great physical flexibility as well as a smaller size (than CTs capable of similar measurement).
Disadvantages may include:
“Integration” if your measuring device doesn’t support Rogowski coils passively. In these cases, active power is usually required.
The benefits of using Rogowski coils typically outweigh the few challenges you might experience. Because they have no core they are:
1) conveniently flexible.
2) lightweight and manageable (lower cost of shipping, etc.). The coils are often made in large circumferences to allow the measurement of wide bus bars or multiple cables (in the same phase).
They can also be used to detect small AC changes in large DC loads because no saturation occurs (DC usually saturates the cores of traditional CTs too fast). They can handle fast current changes in as short of a frequency as several nanoseconds.