The Basics of Passive Harmonic Filters

February 15, 2021


In order to understand the basics of passive harmonic filters, we first need to look at how motor drive systems work. Motors that operate on alternating current need special power supplies that deliver power to start and stop the motor smoothly. For example, a motor driving a large fan used in an HVAC system would put undue stress on the mechanical parts of the fan if it were to transition from full “off” to full “on” in a short period of time. Instead, the power supply needs to start and stop the fan motor gradually. Likewise, once a motor is running, many applications require adjusting its speed and torque to accommodate different mechanical loads. Variable frequency drives (VFDs) provide a power supply that can offer gradual start/stop and variable speed/torque. By adjusting the frequency of current sent to a motor, a VFD controls its speed.

Variable Frequency Drive (VFD):

AC motors operate at their peak efficiency when they run on clean sinusoidal power. VFD’s, however, produce modulated square waves. The pulse width of these square waves are variable. This is known as PWM—pulse width modulation. VFDs can also control the amplitude of each square wave.

Powering a motor with square waves might, at first, sound like a poor idea. However, VFD’s maintain such fine control over the width and amplitude of the square waves that they take on the appearance (as viewed on an oscilloscope) of fairly good approximations of a sine wave. Yet even the most sophisticated VFD’s fail to match the purity of a true sine wave. These variable frequency drives produce distortion or “noise” on both the input and output of the drive. That’s where passive harmonics filters come in.

The Benefits of Passive Harmonic Filters:

Installing a passive harmonic filter between the utility gird and the VFD, protects the grid from drive produced harmonic distortion. The VFD and passive harmonic filter combination reduce overall input harmonic distortion, helping you maintain IEEE-519 compliance. Operating a motor drive system without a harmonic filter can cause several problems:

  • If you are generator powered, a high level of reflected harmonics can cause unwanted nuisance tripping.
  • Elevated levels of harmonic distortion force you to derate the size of your generator in order to compensate. Generator derates can be as high as 3 times the desired power rating.
  • The motor drive system could be sending significant harmonic distortion back to the utility grid, effecting IEEE-519 compliance.
  • Harmonic distortion can effect other equipment connected to the same grid system.

CTM GridHawk filters solve all of these problems while reducing capitol investment (CAPEX), operating cost (OPEX), and increasing over all system efficiency. In all, CTM has filters designed for max current loads between 80 and 950 amps, providing a wide selection of products and price points.

For more information on passive harmonic filter basics, contact CTM today.

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