Learn what passive filters are, how they reduce harmonics in AC drive systems, and what are their advantages and disadvantages.

This paragraph starts to mix VFD “input side” with “output side” characteristics. Typically 3-level active front ends are referenced for harmonic mitigation of the input side of the VFD. But this paragraph starts talking about 3-level output side harmonics, and while it was a true statement going out to the motor… everything up to this paragraph had me thinking this article was about input side harmonics only. In addition, the sinusoidal filters are output side filters only and are unrelated to input harmonics. Because this article is intended as a 101 level introduction, it is not wise to introduce output side characteristics without making a much clearer distinction.

Many passive filter designs result in a leading power factor at partial load. That leading power factor can be significant enough to cause issues with other equipment, such as back-up generators. The remedy is to have a contractor drop out the capacitor in the passive filter, when the filter is at partial load. However, once the capacitor is removed from the filter circuit, the harmonic mitigation is much less effective. The article references a large inrush… I have never heard of a functioning filter resulting in a large inrush tripping circuit breakers or damaging the drive.

What are the benefits and drawbacks of passive filters for AC drives?

If you use AC drives to control the speed and torque of your motors, you may have encountered some issues with harmonics. Harmonics are distortions in the voltage and current waveform that can cause overheating, noise, vibration, and reduced efficiency of your equipment. One way to reduce harmonics is to install filters, which are devices that block or divert unwanted frequencies. There are two main types of filters: active and passive. In this article, we will focus on the benefits and drawbacks of passive filters for AC drives.

1 What are passive filters?

Passive filters are made of simple components such as resistors, capacitors, and inductors. They do not require any external power source or control circuit. They are designed to resonate at specific frequencies, creating a low impedance path for the harmonics to flow. Passive filters can be classified into shunt, series, or hybrid filters, depending on how they are connected to the AC drive and the load.

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2 What are the benefits of passive filters?

Passive filters are a cheaper and easier alternative to active filters, which are more complex and expensive devices that use power electronics and feedback loops. Passive filters offer many advantages, such as high reliability and durability due to their lack of moving parts or sensitive components. Furthermore, they can improve the power factor and reduce the reactive power demand of the AC drive and the load, as well as protect them from voltage spikes and surges.

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3 What are the drawbacks of passive filters?

When considering passive filters for an AC drive application, there are a few limitations and challenges that must be taken into account. Resonance problems with other elements in the system, such as transformers, cables, and capacitors, can amplify the harmonics and damage the equipment. Additionally, changes in the load or supply frequency can reduce the effectiveness or create unwanted harmonics. Passive filters can also consume a lot of reactive power, leading to increased losses and reduced efficiency. Furthermore, they can create a large inrush current when switched on, potentially tripping circuit breakers or damaging the AC drive.

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Tim Skell

Global HVACR Application Engineering Manager (VFDs) | ABB Inc. | Leading an expert team to help solve your toughest HVAC challenges
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Many passive filter designs result in a leading power factor at partial load. That leading power factor can be significant enough to cause issues with other equipment, such as back-up generators. The remedy is to have a contractor drop out the capacitor in the passive filter, when the filter is at partial load. However, once the capacitor is removed from the filter circuit, the harmonic mitigation is much less effective. The article references a large inrush… I have never heard of a functioning filter resulting in a large inrush tripping circuit breakers or damaging the drive.

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4 How to select and install passive filters?

If you decide to use passive filters for your AC drive application, you need to follow some guidelines in order to select and install them correctly. This includes analyzing the harmonic spectrum and power quality of your system with tools like harmonic analyzers or software simulations, choosing a filter that matches your AC drive and load characteristics, such as voltage, current, frequency, power factor, and harmonic distortion, locating the filter as close as possible to the AC drive or load to reduce harmonic propagation and interference, avoiding parallel or series connections with other capacitors or reactors to prevent resonance issues, and monitoring and adjusting the filter periodically to maintain its optimal performance.

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5 What are some alternatives to passive filters?

Passive filters are not the only solution for harmonic mitigation in AC drive applications. Depending on your needs and budget, there are other options to consider, such as active filters, which are more flexible and adaptable devices that can compensate for multiple harmonics and load variations, but require more space and maintenance than passive filters. Multilevel inverters are another option, as they use multiple voltage levels to create a smoother output waveform with lower harmonics; however, they are more complex and costly than conventional AC drives. Lastly, sinusoidal filters use a combination of reactors and capacitors to create a sinusoidal output waveform, yet have a higher voltage drop and power loss than other passive filters.

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Tim Skell

Global HVACR Application Engineering Manager (VFDs) | ABB Inc. | Leading an expert team to help solve your toughest HVAC challenges
Report contribution
This paragraph starts to mix VFD “input side” with “output side” characteristics. Typically 3-level active front ends are referenced for harmonic mitigation of the input side of the VFD. But this paragraph starts talking about 3-level output side harmonics, and while it was a true statement going out to the motor… everything up to this paragraph had me thinking this article was about input side harmonics only. In addition, the sinusoidal filters are output side filters only and are unrelated to input harmonics. Because this article is intended as a 101 level introduction, it is not wise to introduce output side characteristics without making a much clearer distinction.

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6 Here’s what else to consider

This is a space to share examples, stories, or insights that don’t fit into any of the previous sections. What else would you like to add?

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Xavier Montesdeoca

Field Service Specialist at Schlumberger
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A good point is to know the difference of the THD that produce the VSDs going back to the Input Power supply and the THD going forward to the Load, and the particular treatment that solve or mitigate each situation.

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What are the benefits and drawbacks of passive filters for AC drives?

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1 What are passive filters?

2 What are the benefits of passive filters?

3 What are the drawbacks of passive filters?

4 How to select and install passive filters?

5 What are some alternatives to passive filters?

6 Here’s what else to consider

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