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Protecting valuable assets

01 February 2008

In some duties, inverter-driven motors may either be too expensive or too inaccessible to contemplate early replacement. A novel filter product from REO now provides a high level of protection for such equipment

Frequency inverters, which normally have switching frequencies of 2 to 20kHz, produce considerable common-mode (asymmetrical) noise. The fast switching of the dc voltage causes voltage peaks in motor cables and the longer the length of cable the higher the amplitude of the voltage peaks. The resulting fast rising voltage (dV/dt) comprises many harmonic frequencies and the faster the rise time the wider the resulting harmonic frequency spectrum (above 150 kHz) and the worse the ripple is on the voltage sine wave measured between line and earth.

This disturbance is a result of common-mode interference caused by parasitic capacitance between various metal parts of the motor and earth and causes undesirable pulsed currents to flow. A new filter from REO provides a means of allowing the common-mode interference to flow back to the dc link instead of through capacitive coupling to earth. This technique is mainly used to protect motors that are used in critical processes or where a replacement is too expensive, but it can also be used when leakage currents in parallel connected circuits cause interference.

Fast inverter switching and long cable runs between the inverter and the motor can cause premature failure of the latter - principally bearing damage or insulation failure or other unacceptable phenomena such as power losses, high acoustic noise levels and parasitic earth currents. The conventional solution is to fit a low-pass filter at the output terminals of the frequency inverter, which reduces the differential mode interference (symmetrical) to a smooth sinusoidal waveform with a ripple content of less than 5%.

However, there are some instances where this method is inadequate, particularly in applications where there are common mode (asymmetric), high-frequency disturbances and the reliability of the motor is of utmost importance, or motor replacement is an expensive option. Common-mode interference can cause capacitance coupling through the stator, rotor, bearings, frame and connected equipment which will result in pulsed current circulation.

One solution to this problem involves the combined use of a conventional differential mode and a common-mode output filter. This eliminates bearing damage caused by heat and pitting as a result of circulating, pulsing currents, and introduces other benefits, including unlimited cable lengths between inverter and motor, higher permissible switching frequencies (> 8kHz), and reduced audible noise.

Moreover, the reduction in cable and eddy current losses means that a smaller (and therefore less expensive) drive can be used, and no additional filtering on the mains supply side is required if the inverter has its own filter. Radiated interference from cables is reduced to a minimum, so less expensive unshielded motor cable can be used.

The filter is connected back to the dc link, a configuration that provides a low impedance path to source for parasitic currents. The conventional sinusoidal filter does not remove the common mode disturbance but REO’s Sinus Filter Plus ++ does. It also reduces the RFI on the input side.

A typical application for the Sinus Filter Plus++ is for protecting the bearings of an underground pump motor used in a combined geothermal/solar panel heating system. The cable run from the frequency inverter to the pump is necessarily very long and the water column provides a low impedance path back to ground, which encourages current pulsing through the motor bearings. To replace or repair such a pump would also prove a very costly exercise. The filter is connected to the output terminals of the frequency inverter to reduce the damaging common-mode disturbances; the general EMC performance of the equipment is also greatly improved.

Bearing failure isn’t the only problem that can be solved by applying Sinus Filter Plus++. Water abstraction bore holes are often sited in remote locations; in the middle of a forest for example, and the cable run to the pump motor is usually over a long distance. The high-frequency common-mode disturbances flow back along the path of lowest impedance, to earth through the water pipe as shown in the diagram.

One of the conditions of the abstraction license is that an accurate record is maintained of the water pumped out of the bore hole. However, the high-frequency interference causes the metering equipment to give inaccurate measurements - a problem that can be solved by fitting the REO filter. In another application where there are several motors connected in parallel to a single frequency drive - for example, several ventilation fans driven from one inverter. The filter can counter the effects of a very long cable run where there is also a good chance of capacitive coupling between cables.

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