Rogowski Coils

Aim Dynamics offers 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), whereas 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:  Rogowski Coils can be configured for this type of output signal by using a Rogowski coil with an integrator. Aim also offers customizations to these coils, please call our office to inquire on options.
  • AC Amp Input -> 4 to 20mA Output:  These flexible coils output a 4-20mA signal.  Some models are powered by the 4-20mA loop whereas others come with their own external power supply.
  • 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 a Rogowski Coil:
    Input/Output Types
    Basic Understanding
    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 in order 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.
    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?
    Rogowski coil output is fixed regardless of the length of the coil.  Therefore, whether you have selected one of our smallest coils with just a 1 inch opening, or one of the larger coils, there will be no difference in the output signal coming from the coil.

    In other words, a 12-inch length coil (the length of the perimeter) will have the same output as a 36-inch length coil. The benefit is that the same coil can be used for a 2000A circuit or a 200A circuit as long as the integrator or meter is scaled accordingly.
    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 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 were 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 allows 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.
    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 outweighs the few challenges you might experience.  Because they have no core they are: 1) conveniently flexible and 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.
    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.

    You may also want to watch this video by General Manager, Victor Fehlberg about the new "tear drop" Rogowski Coil and how to install it.