|Flexible Rogowski Coils||
|Flexible Rogowski coils are useful for measuring electric current in large or awkwardly-shaped conductors, where space round the conductor is restricted or where a lightweight transducer is needed which can be suspended on the conductor.|
Flexible coils are generally more convenient to use than rigid coils but are less accurate. (1% compared with 0.1% for a rigid coil). They are better than rigid coils for high-frequency measurements. The coils are wound on a flexible former. Two sizes are commonly used: 'Standard' coils use a 7mm diameter former, 'thin' coils are wound on a 3.2mm former. The overall cross-section of the coil depends on whether it is screened and the thickness of the insulation.
|Aligning the ends:
The coils are fitted by wrapping them round the conductor to be measured and bringing the ends together. In practice it is very difficult to align the ends without having a gap in the winding. The effect of a gap is to make the coil sensitive to interference from adjacent conductors or other sources of magnetic fields. More information on gaps in windings.
Various methods are used to ensure that the ends are aligned accurately. Coils with screw-together or push- together ends incorporate an additional winding to compensate for the gap and minimise interference from external magnetic fields. Coils with overlapping ends are easier to build but the rejection of stray magnetic fields in some situations is not so good. See Testing Coils.
| Push-together ends are the standard locating
method. They are useful for coils that have to be fitted and removed
repeatedly. The diameter of the free end is reasonably small and can be
threaded through confined spaces. The ends should be pushed together firmly
enough until they 'click'.
Push-together ends are used with 1000 Series Coils.
are better for permanent or semi-permanent installations
Screw-together ends are used with 1000 Series Coils.
|Overlapping Ends are
easier to make but for some situations they are more prone to pick-up from
stray magnetic fields. With thin cross-section coils the pick-up effect is not
as bad as for standard section coils.
Overlapping ends are used with 4000 series coils.
| Thin Coils:
For conductors with limited space around them coils can be wound with a thinner cross section than those shown above. With a very thin winding most of the cross section of the coil is insulation and there is not much point in making the winding any thinner. Also thin coils have a low output and are less suitable for measuring low currents.
Flexible coils come in lengths ranging from about 250mm to several metres. They can be manuactured either with or without an electrostatic screen. If the coil is long enough it can be wrapped several times round the conductor to increase the output. The output is proportional to the number of wraps.
Electrical connection to the coil is at one end only. The other end is 'free' and can be threaded round awkwardly-shaped conductors or conductors in confined spaces. Flexible coils do not have to fit tightly round the conductor and their output is not excessively sensitive to position in relation to the conductor. They must, however, encircle the conductor completely.
A typical mutual inductance for a flexible coil is 200 - 300nH. Depending on the integrator these coils can be used to measure currents from less than 1A to more than 1MA and at frequencies of up to several hundred kHz. Special low output windings can measure higher frequencies.
Coils are individually calibrated to give a mutual inductance value. For coils of the same design the individual mutual inductance values can vary by a few percent between coils.
Alternatively coils can be made interchangeable. In this case the coil is 'loaded' with a series resistor to compensate for the variation in mutual inductance. An integrator used with interchangeable coils has to be designed appropriately.
(i) Coils are affected by temperature due to the change in resistance of the winding. For an increase in temperature the output of a coil/integrator combination decreases. The temperature coefficient depends on the length of the coil and the input impedance of the integrator and can vary over a range from -0.003%/°C to -0.17%/°C.
(ii) Coils are also affected by thermal expansion of the silicone rubber former material. The effect of thermal expansion is complex and the output of the coil can either increase or decrease depending on its design. As a general rule, coils wound with thin wire (0.1mm) have an output that increases with increasing temperature and coils wound with thicker wire (>0.2mm) have an output that decreases with increasing temperature. If coils are heated at too high a temperature the output of the coil can be permanently affected.
More Information on Temperature Effects
An electrostatic screen is sometimes useful to reduce noise with very low current measurements or for minimising capacitive pick-up with high-frequency measurements.
Polyester braid provides maximum flexibility and a generally attractive finish, but less good electrical insulation. The normal covering is polyolefin sleeving which gives better insulation but a less flexible coil. Conform Sleeve can be used on longer coils. It is a glass fibre / silicone rubber composite with excellent electrical properties..
For large, fast current pulses we can use a special winding to reduce the coil output voltage to an acceptable level. This also gives a better high-frequency response.
|The effect of temperature on the output of a Rogowski coil measuring system||Download PDF file|
|The effect of some wiinding defects on coil output - Basic theory and concepts||Download PDF File|
|More about flexible coils||Download PDF file|
|Specification 1000 Series Coils||Download PDF file|
|Specification 4000 Series Coils||Download PDF file|