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11 Jan 2007

One of the problems with a switch-mode power supply, whether it is a variable speed drive or a DC to AC Inverter, is that the output voltage sine wave contains lots of high-frequency interference caused by the fast switching of power semiconductors. When these are connected to an inductive load they can cause a number of undesirable side effects, including reducing motor life.

Considering a motor load, there can be a high level of audible noise, current loops through a motor shaft leading to over-heating and premature bearing failure, overheating due to excessive currents and insulation breakdown caused by high voltages. The problems are made worse by the use of long cable runs between the motor and the converter, for instance in applications where several motors are connected in parallel to one variable speed drive. REO Sinusoidal Filter with a choice of terminations.

One possible solution to these problems is to fit a sinusoidal filter, which is in effect a low-pass filter, onto the output side of the power supply. Obviously this is an additional cost but it is partially offset by removing the need for screened cables. The screening is required to reduce the high-frequency capacitive coupling between the cable and ground.

Often, when a sinusoidal filter is not used, the switching frequency is set high to reduce the perceptible audible noise generated by the load, such as in the case of a fan, for instance. However, this can cause higher losses due to the high-frequency currents in conductors and the load caused by the skin effect and higher eddy-currents.

REO has developed a range of sinusoidal filters (3 x 500V/8A-2000A) which address these problems and also takes into consideration some of the practical aspects of using such components. Conventional sinusoidal filters comprise a laminated core with coils on bobbins fitted to the limbs to create a choke and capacitors connected to ground. However, because the windings are impregnated with hot varnish, under vacuum the plastic becomes brittle and can be easily damaged in transit. To overcome this problem, REO has introduced a new range of sinusoidal filters that are fully encapsulated. Being IP64 rated, they are suitable for use in dusty and damp conditions, without the need for a special enclosure.

Another novel feature of REO sinusoidal filters is the interchangeable terminal system which allows the standard filter body to be adapted to a variety of termination methods. This makes it is easier to hold a wide range of stock components and to respond quickly to demand – another key requirement of the drives market. The REO termination system also simplifies the EMC connection so that the connection between the drive and filter is both correct according to best practice and effective.

In line with the latest trends in the drive industry, REO has now developed a range of water-cooled accessories including sinusoidal filters. These are approximately one third the volume of air-cooled components.

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