What is Rogowski Coil : Design, Working & Its Applications

Rogowski Coil is one of the commonly used devices for the measurement of AC current. Just like other devices, like clamp meter, multimeter, etc. This coil can also be used to measure AC current. The Rogowski coil is a sort of helical winding or wire resembling a large spring. The spring is wounded such that, one end of the spring is sent back to the beginning end through the center part of the spring. With this, both the ends of the coil come to the same end. This coil is mostly used for measuring AC currents and works on the concept of Faraday’s law of electromagnetic induction.

Rogowski Coil Circuit

In this coil circuit, it is wounded such that, beginning from one end, the coil is wounded in a helical shape, and again the other end is brought out inside the hollow gap of the helically shaped coil, and both the ends of the coil form at one point.

Rogowski Coil Theory

It is used to measure AC currents. It works on the concept of Faraday’s Law of electromagnetic induction. For whatever the current to be measured, which is flowing in a wire, the Rogowski coil is placed around the wire, covering the wire. Due to electromagnetic induction, the current flowing in the wire which is to be measured induces an emf in the Rogowski coil as per Faraday’s law of electromagnetic induction.

After the induction of emf in the Rogowski coil, the current can be measured by using a further measuring device like a clamp meter. We can even use a CRO to measure this current and voltage induced at the Rogowski coil. In Rogowski’s coil design, the coil is wounded in a helical shape, such that, both the ends of the coil come to the same point. Then this coil is wrapped around the wire for the current is to be measured.

Rogowski Coil Formula

The emf induced in the Rogowski coil is given by

E= M*(di/dt)

Where E is the emf induced at the ends of the Rogowski coil, M is the mutual inductance of the coil, and di/dt is the rate of change of current through the coil. It should be noted that M is the mutual inductance, but not self-inductance. When we are considering the mutual inductance, other factors such as coupling constants, dot convention, etc. should also be considered.

Once the E is measured, the current can be measured by using a fundamental RC circuit, or a simple clamp meter which again works on the principle of Faraday’s law of electromagnetic induction.

Working Principle of Rogowski Coil

As shown in the figure, the helically shaped coil is a coil. The cylindrical coil is the conductor, for which the current is to be measured. When the coil is wrapped around the conductor, the current flowing in the conductor induces an emf in the coil, due to Faraday’s Law of electromagnetic induction. The induced emf depends on the number of turns and mutual inductance of the coil.

The emf is measured by using an RC circuit as shown in the figure. The RC circuit acts as an integrator circuit to measure the voltage. We can even measure the voltage directly, by using a CRO or by a simple clamp meter.

Rogowski Coil Versus Hall effect

In the coil, the measured current has to be AC in nature. Due to its alternating nature, a relative displacement is obtained between the coil and magnetic field. This is the basic law of Faraday’s Induction principle. But if the current flowing is DC the coil cannot measure the current. In such cases, the emf induced in the core would be static in nature.

So to measure the static emf, Hall effect-based sensors are used. Basically, Hall effects sensors can be used to detect static emf. Hence to measure AC voltage, the coil is used, and to measure DC voltage hall effects sensors are used. Both these principles can be found in the clamp meter which measures both AC and DC currents.

Rogowski Coil Testing

In case of any faults, the coil can be easily tested by the impedance-based method. For any open circuit faults, the impedance measured will be very high. And for any short circuit in the winding, the impedance measured will be very low. So based upon the impedance value, the type of fault and testing of the coil can be done.

Rogowski Coil Accuracy

The Coil is highly accurate since it measures the AC current based on Faraday’s law. There would be minute losses due to the air gap between the primary and secondary winding, which can be ignored.

Advantages and Disadvantages

The advantages are

•  It is highly accurate and easy to use.
• The circuit for which the current is measured, need not be interrupted
• Efficient is very high

The disadvantages are

• It measures only AC currents
• External means for measuring current is required. The coil itself cannot measure the current

Applications

Since the Rogowski coil is used to measure AC currents, it has numerous applications. It is used in clamp meter, multimeter, CRO probes, Signal probes, Digital storage oscilloscopes, etc.

Hence we have seen the operating principle and working of Rogowski coils. Generally, this is used only to measure AC currents. It would be interesting to know whether the coil can be used to measure other forms of alternating currents like a square wave, trapezoidal, etc?