Rogowski coil
The Rogowski coil is a toroidal coil without a ferromagnetic core and serves as a component of electrical measuring instruments for the measurement of alternating current. Since this coil has no core, it is one of the group of air-core coils.
The basic idea for their design which takes advantage of the alternating currents induced in air coil voltages, had Arthur Prince Chattock 1887. The name is since the release of Walter Rogowski ( 1881-1947 ) known as " Rogowski coil " or " Rogowski current transformer ".
General
The Rogowski coil is made of a conductor wire which is wound as uniformly as possible by a solid body of a non-ferromagnetic material. This structure is referred to as an air coil. The conductor wire of the coil is wound distributed around the whole ring of the coil body, so that both connectors are to each other. The picture to the coil has been realized as an open arc which facilitates insertion into the line to be measured. The second terminal of the coil is magnetically neutral in this case guided to the other end.
To measure a current i1 in a conductor, the Rogowski coil is placed around the current carrying conductor - for example, a cable wire or a busbar. The alternating current flowing through the conductor generates a magnetic field which induces a voltage U2 in the Rogowski coil.
The voltage is measured with high impedance so that the current in the Rogowski coil is almost zero. Under this condition, the following relationship applies:
The mutual inductance M of the Rogowski coil can be calculated as follows:
Where μ0 is the magnetic field constant, N is the number of turns, A is the cross-sectional area of the Rogowski coil and Lm is the average length of the field line in the ring.
As the conductor current to be measured I 1, the time integral of the voltage U2 of the coil must be formed. To obtain a quantity proportional to the current i1. The time integral is formed in an analogous manner with an integrator. For sinusoidal current integration can be omitted - the measured voltage can be calibrated in power units, as it is advanced by only 90 ° phase angle.
Pros and Cons
Advantages of the Rogowski coil over other current methods of measurement are firstly the robustness of the structure. Furthermore, the current to be measured can be within a wide range, even up to the maximum current ( short circuit current), without damaging the transmitter. Nonlinear effects of an iron core are not provided. The magnetic influence of the conductor does not apply to Rogowski coils; However, it is also of conventional current transformers, which have an iron core is small, since the former are operated in a quasi- secondary short circuit.
Can Rogowski coils without interrupting the circuit, that is, without installation works just created and removed again. They are manufactured in many different sizes, so that measurements directly on components on PCBs to measurements on busbars or machine parts ( streams ) a wide range of applications is possible.
Rogowski coils are manufactured in different sensitivities for currents from a few amperes to several 100 kA. They are, depending on the series and sensitivity, suitable for measurements below 1 Hz up to double -digit MHz range. Power surges as they occur eg in converters and other power electronic modules, can thus capture well. For the measurement of harmonics and other high frequency disturbances are, Rogowski coils.
In addition to series with direct voltage output, there are also versions with converter to measure the RMS value. For these to eg direct on 0-5 V or 4-20 mA input - sockets a programmable logic controller.
High short circuit currents, as they occur for example in the electrical industry, causing at Rogowski coils unlike current transformers no high forces and losses. Hysteresis and permanent magnetization, such as occur with Hall probes omitted. Rogowski coils form unlike current transformers no danger to the operator and are not destroyed when they are unconnected.
However, you need to measure current auxiliary equipment such as amplifiers and power supply of the amplifier, whereas conventional current transformers can deliver significant benefits within their rated burden because of the magnetic field converging core, so that measurement works and overcurrent tripping devices can be operated directly.
Another drawback is the location dependency of the measurement accuracy. For an ideal coil without an air gap, the Location of the enclosed conductor is irrelevant. In real Rogowski coils, however, there is an air gap through which the magnetic field unmeasured " escapes ". The calibration of this error is usually carried out for the center position. Said circuit then when measured in the vicinity of the air gap, the leakage magnetic field is greater and there is a negative deviation. The measured current is less than the actual value. If the printed circuit on the opposite side, the opposite situation occurs and there is a positive deviation. The measured current is then greater than the actual value.