Measuring High Pulsed Current with Rogowski Coils

Current measurement is probably familiar to everyone. As early as in junior high school physics lab classes, teachers taught us to use an ammeter (Figure 1) to measure current in a circuit. The current ranges are generally 0–0.6 A and 0–3 A. Because the ammeter has very low internal resistance, when measuring circuit current, the ammeter is allowed to be connected directly in series in the circuit; that is, the ammeter will not affect the current in the circuit. However, this also creates a problem: if the current to be measured is very large (exceeds the ammeter range) or the conductor structure or shape is irregular, the ammeter is not allowed to be connected in series in the circuit. Then what method can solve this difficult problem? This problem does not stump imaginative scientists. Next, we will learn about another current measurement tool—the Rogowski coil, also called a Rogowski probe. It is now widely used in testing semiconductor switching parameters with high di/dt, plasma current, pulsed current, RF current, and variable-frequency current.

In 1912, W. Rogowski and other scientists, using the principle of electromagnetic induction, designed an air-core coil for current measurement; the coil was named after the inventor. The Rogowski coil is very simple to make, as shown in Figure 2: it is an air-core solenoid wound on a non-magnetic former. When the current to be measured changes, a closed varying magnetic field is generated inside the solenoid; after the varying magnetic field passes through the turns of the coil, a voltage is induced across the coil terminals. The magnitude of this output voltage and its frequency of variation reflect the variation of the current to be measured. After the output signal is integrated by a digital integrator, the current value to be measured can be reconstructed. The advantages of this coil are that the current it can measure ranges from a few milliamperes to tens of thousands of amperes, it can measure objects with irregular conductor structures, and the coil can be designed in different lengths and thicknesses according to the size of the object to be measured. In addition, the coil does not require an external power supply; that is, it is a passive measurement method.

　　In magnetic confinement fusion experiments, Rogowski coils have been widely used. For example, the measurement of plasma currents of millions of amperes uses this type of air-core solenoid. As shown in the previous figure, a layer of high-temperature cable is tightly wound on a stainless-steel tube, and then the coil is installed on the path through which the current passes. The white object in Figure 3 is a section of a Rogowski coil; in reality, the coil encircles the current once, and the remaining part is not captured in the photo. When the current passes through the center of this coil, a voltage is induced across the coil terminals; after the output voltage is processed, it represents the plasma current to be measured. Therefore, any current passing through this coil can be fully measured. The subfigure below shows the evolution of the plasma current during the experiment, with the current varying from 0 to a maximum of millions of amperes. These coils are all wound by hand; if interested, you can also try your hand under a teacher's guidance and experience the fun of electromagnetic induction.