If you are dealing with more than a couple tens of amperes of AC current and want to make flexible current measurements, consider a Rogowski current probe.
A Rogowski coil is an electrical transducer used for measuring AC currents such as high-speed transients, pulsed currents of a power device, or power line sinusoidal currents at 50 or 60 Hz. The Rogowski coil has a flexible clip-around sensor coil that can easily be wrapped around the current-carrying conductor for measurement and can measure up to a couple thousand amperes of very large currents without an increase in transducer size.
How does a Rogowski coil work?
The theory of operation behind the Rogowski coil is based on Faraday’s Law which states that the total electromotive force induced in a closed circuit is proportional to the time rate of change of the total magnetic flux linking the circuit.
The Rogowski coil is similar to an AC current transformer in that a voltage is induced into a secondary coil that is proportional to the current flow through an isolated conductor. The key difference is that the Rogowski coil has an air core as opposed to the current transformer, which relies on a high-permeability steel core to magnetically couple with a secondary winding. The air core design has a lower insertion impedance, which enables a faster signal response and a very linear signal voltage.
An air-cored coil is placed around the current-carrying conductor in a toroidal fashion and the magnetic field produced by the AC current induces a voltage in the coil. The Rogowski coil produces a voltage that is proportional to the rate of change (derivative) of the current enclosed by the coil-loop. The coil voltage is then integrated in order for the probe to provide an output voltage that is proportional to the input current signal.
Rogowski coil current probes offer many advantages over different types of current transducers or sensing techniques.
- Large current measurement without core saturation -- Rogowski coils have the capability to measure large currents (a very wide range from a few mA to more than a few hundred kA) without saturating the core because the probe employs non-magnetic “air” core. The upper range of the measurable current is limited by either the maximum input voltage of a measuring instrument or by the voltage breakdown limits of the coil or the integrator circuit elements. Unlike other current transducers, which get bulkier and heavier as the measurable current range grows, the Rogowski coil remains the same small size coil independent of the amplitude of current being measured. This makes the Rogowski coil the most effective measurement tool for making several hundreds or even thousands of amperes of large AC current measurements.
- Very flexible to use -- The lightweight clip-around sensor coil is flexible and easy to wrap around a current-carrying conductor. It can easily be inserted into hard-to-reach components in the circuit. Most Rogowski coils are thin enough to fit between the legs of a T0-220 or TO-247 power semiconductor package without needing an additional loop of wire to connect the current probe. This also gives an advantage in achieving high signal integrity measurement.
- Wide bandwidth up to >30 MHz -- This enables the Rogowski coil to measure the very rapidly changing current signal – e.g., several thousand A/usec. High bandwidth characteristic allows for analyzing high-order harmonics in systems operating at high switching frequencies, or accurately monitoring switching waveforms with rapid rise- or fall-times.
- Non-intrusive or lossless measurement -- The Rogowski coil draws extremely little current from the DUT because of low insertion impedance. The impedance injected into the DUT due to the probe is only a few pico-Henries, which enables a faster signal response and very linear signal voltage.
- Low cost Compared to a hall effect sensor/transformer current probe, the Rogowski coil typically comes in at lower price point.
- AC only -- Rogowski cannot handle DC current. It is AC only.
- Sensitivity - Rogowski coil has a lower sensitivity compared to a current transformer due to the absence of a high permeability magnetic core.
Rogowski coil current probes have a large number of applications in broad power industries and power measurement applications. The following are some examples of Rogowski coil applications:
- Flexible current measurement of power devices such as MOSFET or IGBT device as small as TO-220 or TO-247 package or around the terminals of large power modules
- To measure power losses in power semiconductors
- To monitor currents in small inductors, capacitors, snubber circuits, etc.
- To measure small AC current on a conductor with high DC current or in the presence of a high DC magnetic field.
- To measure high frequency sinusoidal, pulsed, or transient currents from power line frequency to RF applications
- To measure current in motor drives and, in particular, power quality measurements in VSD, UPS or SMPS circuits
- To evaluate switching performance of power semiconductor switches (double pulse tester).
- Power distribution line monitoring or utilities pole probe monitoring
- Smart grid applications
- Plasma current measurement
There are a number of different ways of measuring electric current where each method has advantages and limitations.
The Rogowski coil is similar to an AC current transformer in that a voltage is induced into a secondary coil that is proportional to the current flow through an isolated conductor. However, Rogowski coils have the capability to measure large currents (very wide range from a few mA to more than a few kA) without saturation because of its non-magnetic “air” core. The air core design also has a lower insertion impedance to enable a faster signal response and a very linear signal voltage and is very cost effective compared to its hall effect sensor/current transformer counterpart. This makes the Rogowski coil the most effective measurement tool to make several hundreds or thousands of amperes of large AC current measurement.