I often receive questions from readers and customers regarding a wide variety of electrical/electronics concepts. The next few columns will address some of these, and hopefully provide some additional insight into both the mundane and esoteric issues associated with variable speed drive technology.
Question: What is a suppression capacitor, and is it the same as a suppression reactor?
Answer: In the drives world, suppression filters (which include capacitors) are used to reduce EMI and harmonics by shunting high frequencies to ground, since they present a much lower impedance to higher frequencies than to lower ones. A capacitor is typically inserted parallel to the load path (often as part of a typical 2- or 3-element low-pass filter) on the output side of the drive; low frequency current sees the cap as a high impedance and bypasses it, choosing instead to go through the load (i.e. the drive output section or motor, depending on where the filter is located). But high frequency (noisy/high harmonic) current sees the cap as the lower “resistance” and runs through it to the return path, rather than through the load. Most often in my experience, the term “suppression capacitor” is used in low power, IC-type electronic applications and not so commonly in drive applications (unless used by drive engineers in circuit design).
The term “suppression reactor” is used most often in power quality applications to indicate a multi-element filter used to reduce harmonics resulting during power factor correction. This reactor shifts the PFC capacitor bank’s resonant frequency cancelling out some of the noise. However, in drive applications I have seen Mitsubishi refer to a dV/dt filter (at the drive output) as a “suppression reactor”. Also, 3rd party noise/emission control suppliers such as Trans-coil and MTE also refer to both their line-side filters and their dV/dt filters as “reactors”. Line-side reactors increase the impedance the drive sees by inserting inductance into the line, and thereby (1) protect the drive from transient surges, and (2) reduce drive-induced line-side harmonics caused by commutation notching, which occurs as current is switched from one rectifier to another – because rectifiers cannot switch on or off instantaneously, there is a brief period twice each cycle where two rectifiers are effectively “shorted” (one is turning off while the next is turning on), which sends a voltage spike back into the line.
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