|Integrators for Rogowski Coils|
|Since the output from the coil is proportional to the rate of change of current an integrator is essential to give the correct current waveform.|
Active integrators are generally much more versatile than passive integrators They can be used for low currents (less than an amp) and low frequencies (less than 0.1Hz.) as well as for currents of more than 1 million amps and frequencies approaching 1MHz. The low-frequency performance of a transducer is determined by the integrator design.
A passive integrator is basically a resistor/capacitor network. Passive integrators are only really suitable for large fast current pulses (big dI/dt) because they need a high voltage from the coil to give acceptable accuracy and their low-frequency capability is poor. They have been used with lightning test equipment and rail gun measurements. They have the advantage of not needing a power supply.
An integrator is characterised by its time constant (RC) where R is the integrating resistor and C is the integrating capacitor. By using different values of R and C the characteristics of the complete transducer (coil + integrator) can be varied over an enormous range. For example a typical flexible coil can be used to make current measurements from a few mA to more than a million amps simply by changing these two components in the integrator.
As a general rule, for a measuring system consisting of a coil and an integrator, the low-frequency behaviour is determined by the design of the integrator and the high-frequency performance depends on the properties of the coil. The picture shows a typical low-frequency amplitude and phase response. In the example shown the effective droop is -0.2%/msec.
For any particular design the amplitude and phase response can be predicted reliably and used to make corrections to the measurements. Where necessary the LF response can be 'tailored' to suit specific requirements, for example very low phase error.
|Click on the picture for an enlargement|
Unfortunately it is NOT possible to measure continuous direct currents with a Rogowski coil but the question is frequently asked and different people mean different things by 'DC'. This is what you can do:
(1) Measure the DC offsets on AC transients.
(2) Measure uni-directional current pulses as well as oscillatory pulses.
(3) Measure the AC part of waveforms that contain a DC component such as a full-wave rectified current.
(4) Measure the ripple superimposed on a DC current
(5) Measure very low frequencies (less than 0.1Hz).
It is also possible to measure a direct current by measuring the AC currents feeding the rectification system that supplies the DC. This can be quite an accurate method. We have provided a system that uses this principle to measure alternator rotor currents. The technique has also been used for monitoring currents in an aluminium electrolysis plant.
We have a range of integrators to suit different requirements. They differ in the way they are powered (mains, battery, own power supply) and in the type of output (voltage waveform, current waveform, RMS, 4-20mA DC output). Some of the integrators have several sensitivities which are selected by a switch. Some people build their own including digital integrators. Where appropriate we can add other features such as RMS output, overload indication, and summing circuits. We have a range of integrators to suit different requirements. Our range of integrators is described on the Products Page. Below are just two examples.
One of the range of integrators offered by Rocoil. This unit is powered from the mains or internal re-chargeable battery. It can be designed for use with either flexible or rigid coils. Each channel can have up to five sensitivity ranges selected by a switch. A single channel version in similar style is also available. Specification Type 7000 Integrators
|Click on the picture for an enlargement|
Integrator (type SIPCB):
A single integrator on a PCB with two sensitivity ranges, suitable for OEM use. Connections to the board (coil input, output, power) are via a screw terminal strip. The board requires positive and negative supplies which can be up to ±18V DC. Board dimensions are approximately 50mm x 50mm.
|See Product List for more integrators.||Click on the picture for an enlargement|
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