New Industry Products

LEM Announces New Benchmark For Accuracy In Rogowski Coil Sensors

October 14, 2009 by Jeff Shepard

LEM announced the introduction of the first ac current sensors family of components based on patent-pending Perfect Loop Technology. According to the company, after calibration, the new RT series achieves absolute accuracy of better than 0.65% – including the position error – which LEM says makes them the first split-core Rogowski coils to be suitable for use in Class 1 power devices.

An imperfect coil structure induces an unbalanced geometry and increases sensitivity to the position of the measured conductor within the sensor, or to the proximity of external electric cables. The RT series of sensors is said to overcomes the problem of asymmetry resulting from the discontinuity at the sensor opening, which is inherent in conventional split-core Rogowski coils. According to the company, whilst other manufacturers use complex and costly clasp designs to minimize the effects of discontinuity, LEM states that its engineers have invented a unique, patent-pending magnetic coupling technology that allows a perfect extension of the magnetic flux at the loop opening which compensates for coil asymmetry.

In addition, LEM has developed an advanced coil-winding process which are said to produce exceptionally regular windings, to further enhance sensor symmetry, accuracy and immunity to electromagnetic interference.

Sensors based on conventional Rogowski coil technology are said to combine almost perfect linearity, with no saturation or upper limit in the current rating, a large frequency range which includes 50/60 Hz, and an ultra-compact and flexible form. LEM’s RT sensors retain these key benefits whilst overcoming susceptibility to asymmetry caused by discontinuity at the sensor opening, and minimizing unevenness in the coil winding.

The Rogowski coil principle provides very precise detection of the rate of change (derivative) of the primary current that induces a proportionate voltage at the terminals of the coil. A simple electronic integrator circuit, typically within the device to which the sensor is connected, is used to convert the voltage signal into an output signal that is proportional to the primary current. This eliminates the need for an additional power supply, as the RT sensor is self-powered.

According to the company, the very thin, light and flexible format of RT sensors allows them to be fitted into applications for which traditional current transformers are typically too heavy and bulky, especially when measuring high currents. Their split-core construction allows them to easily wrap around the conductor without dismantling cables or shutting down operation. What the company describes as the benchmark accuracy of RT series sensors provide enhanced performance in current and power monitors as well as energy meters.

The 5mm gauge RT sensors are currently available with sensing apertures with diameters of 55 or 125mm.