Rogowski Coil

Rogowski Coil

The Rogowski coil is defined as an electrical device that is used to measure alternating current (AC). It is also used to measure the high-speed transient, pulsed current or sinusoidal current. The name Rogowski coil was named after the German physicist Walter Rogowski.
A Rogowski coil is an evenly wounded coil with N number of turn and constant cross-section area A. There is no metal core in a Rogowski coil.
The end terminal of the coil is returned through the central axis of the coil to another end. Therefore, both terminals are at the same end of the coil.
This entire assembly wrapped around the current-carrying conductor whose current we need to measure.

Rogowski coils work on the principle of faraday’s law. It is similar to AC current transformers (CTs). In current transformers, the voltage induced in a secondary coil is proportional to the current flow through the conductor.
The difference between Rogowski coils and AC current transformers is in the core. In Rogowski coils, an air core is used and in the current transformer, a steel core is used.
When current passes through the conductor, it will create a magnetic field. Due to an intersection with a magnetic field, a voltage is induced between the terminals of the Rogowski coil.
The magnitude of voltage is proportional to the current passes through the conductor. Rogowski coils are close pathed. Generally, the output of Rogowski coils is connected with the integrator circuit. So, the coil voltage is then integrated to provide an output voltage that is proportional to the input current signal.

Advantages of Rogowski Coil

1.Rogowski Coil Current Sensor
Rogowski coil current sensors are preferred due to no magnetic saturation, no overheating, or no hysteresis loss. As such there are very low magnetic losses in Rogowski coils. And it has very low insertion impedance.
It is able to sense the current flowing through the conductor. Hence Rogowski coils can also function as current sensors.
Rogowski coils are an air-cored toroidal winding wrapped on a conductor. For large currents, the output does not saturate due to the non-magnetic core.
It can be designed for a wide range of current measurements as well as protection applications. Rogowski coil sensor converts the input current to an output voltage. And it can sense the current flowing the conductor by integrating the output voltage.
There are two types of Rogowski coils; Rigid and Flexible.

2.Simple Mounting
Busbars and irregular shaped cable bundles are common in applications with high power requirements. In these challenging installations, CT’s typically are not able to fit around the monitored conductor leading to an exhausting and time intensive installation. The flexibility of Rogowski coils can save the installer a significant amount of time and physical exertion because of how easily they surround a conductor. Selecting a Rogowski coil instead of a conventional split-core CT can save the installer over two hours per meter point in a challenging installation, which could be the difference between making and losing money on a job.

3.Wide Current Range = Reduced Shipping Costs
A technical limitation of conventional CT’s is the narrow range of current it can measure vs. the size of the CT; as the current range increases the size and weight of the CT required increases dramatically. A typical 100A CT weighs 2lbs, however as the current range expands to 400A and 3,000A the average weight can increase from 5lbs to 20lbs. Considering three CT’s are required to monitor a 3-phase motor, certain applications could require up to 65lbs of shipping weight per meter point; is a serious waste of shipping dollars. Rogowski coils have a current range of 5-5,000A (depending on the max current rating of the meter), which means that any ROGOWSKI COILS VS. Rogowski coil can be installed in any application regardless of the monitored amperage load. Rogowski coils are typically 1lb or less; the lighter shipping weight will help reduce costs

4.Improved Linearity
Conventional CT’s are wound over a magnetic iron core, which makes them more susceptible to saturation leading to linearity error. In order to account for this error, engineers and contractors must adjust the phase shift of the meter to compensate in order to achieve an accurate reading. Rogowski coils are wound over a Benefits of Rogowski Coils over Conventional Split-Core CT’s 2 non-magnetic core, giving them perfect linearity and improved accuracy over wide current ranges, saving time on installation. The air core of Rogowski coil’s eliminates the negative effect of magnetic fields which pose a serious issue to conventional CT’s in similar environments.

Summary of Advantages of Rogowski Coil
The advantages of Rogowski coils include:

  • It can respond to fast-changing currents.
  • The second terminal of the coil is returning to the first terminal. And it makes an open circuit coil. So, there is no danger of opening of the secondary coil.
  • The air is used as a medium. There is no magnetic core is used. So, there is no question of saturation of the core.
  • In this coil, temperature compensation is simple.
  • In order to keep output current constant, the AC current transformer (CT) needs to increase the number of secondary turns for large currents. Hence, for equal rating, the size of the Rogowski coil is small compared to a conventional current transformer.
  • It is available in two types; flexible as well as rigid.

Disadvantages of Rogowski Coil

The disadvantages of Rogowski coils include:

  • To obtain the current waveform, the output of the coil must be pass through the integrator circuit. It needs a power supply of 3V to 24Vdc.
  • It cannot measure the DC current.