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Sinusoidal filter


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Sinusoidal filters are an optional filter that can be added to the output of a variable speed drive. In some cases, there may be a filter already built in.


The variable speed drive uses fast switching transistors, (usually FETs or IGBTs) in the output to genearte the three pahse current to the motor. These switchs are rapidly switched and are either turned hard ON or OFF with no in between state. The result is that the sine wave current is generated by using pulse width modulated switching in one form or another to produce the correct current waveform. The voltage wave form however is a series of pulses between the two DC rails in the inverter, and the voltage transitions are very rapid.

This rapid switching causes a voltage wave form with a very high dv/dt (rate of change in voltage). This high dv/dt can cause resonance in the motor windings, or in the cable between the motor and drive, it will cause high radiated energy causing EMC problems as well. High dv/dt waveforms can also cause premature bearing failure as well.


The sinusiodal filter is designed to remove the high frequency switching component from the waveform and convert this output voltage waveform to a smooth sine wave, eliminating dv/dt problems.


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