How do filters affect the efficiency and reliability of AC drives?

AC drives, also known as variable frequency drives (VFDs), are devices that control the speed and torque of electric motors by varying the frequency and voltage of the input power. They are widely used in industrial applications to save energy, improve performance, and reduce wear and tear. However, AC drives also generate harmonics, which are distortions in the current and voltage waveforms that can cause interference, overheating, and damage to the drive and other equipment. To mitigate the harmful effects of harmonics, filters are often installed in the AC drive system. But how do filters affect the efficiency and reliability of AC drives? In this article, we will explain the types, functions, and benefits of filters, as well as some tips on how to select and install them properly.

1 Types of filters

There are two main types of filters for AC drives: passive and active. Passive filters are composed of capacitors, inductors, and resistors that create a low-impedance path for harmonic currents to flow to the ground, while blocking the fundamental frequency. Passive filters are simple, cheap, and robust, but they have some drawbacks, such as fixed tuning, resonance risk, and large size. Active filters are electronic devices that inject opposite-phase currents to cancel out the harmonic currents generated by the AC drive. Active filters are more flexible, adaptive, and efficient, but they are also more expensive, complex, and sensitive to faults.

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Jose L.

Sr. Power Electronics Engineer (Backup energy and Power quality markets)
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In my experience efficiency increases when you apply a passive filter because decreases harmonic components, increasing PF and protects the VFD itself from the harmonic current spikes. For me the best cost-effective solution technically and economically speaking.

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2 Functions of filters

Filters are essential to reduce harmonic distortion in AC drive systems, which can enhance power quality, efficiency, and reliability. However, harmonic distortion can cause increased losses and heating in transformers, cables, motors, and capacitors, as well as malfunctioning or false tripping of protection devices. Additionally, it can create interference and noise in communication and control systems, such as sensors, PLCs, and SCADA. Furthermore, overvoltage and overcurrent in the DC bus of the AC drive can damage the drive components. By using filters, these problems can be minimized or avoided for better performance and longer service life of the AC drive and other equipment.

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3 Benefits of filters

Using filters for AC drives can offer a variety of benefits, depending on the type, size, and configuration of the filter, as well as the characteristics of the AC drive system and the load. These can include reduced energy consumption and utility bills due to lowered power factor and current demand, increased motor efficiency and torque due to reduced voltage and current harmonics, extended equipment lifespan and reduced downtime due to protection from overheating, overloading, and harmonic distortion-induced failures, and enhanced system stability and compatibility due to prevention of resonance, interference, and noise.

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4 Tips on filter selection

When selecting a filter for an AC drive system, there are several factors to take into account. It’s important to consider the harmonic level and spectrum of the AC drive system, which can be calculated using tools such as harmonic analyzers or software. Additionally, the filter performance and specifications such as harmonic attenuation, power rating, impedance, and frequency range should be considered. Furthermore, the filter installation and maintenance requirements such as space, wiring, cooling, and grounding of the filter should be taken into account. Lastly, it’s important to do a cost and benefits analysis of the filter which should compare the initial and operating costs with the expected savings and improvements in the AC drive system.

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5 Tips on filter installation

When installing a filter for an AC drive system, there are some best practices to follow. It is important to adhere to the manufacturer's instructions and recommendations for the filter installation and operation. Additionally, choose a suitable location for the filter, preferably close to the AC drive or the load, in order to reduce harmonic propagation and losses in the cables. Furthermore, make sure proper grounding and shielding of the filter and the AC drive system is completed in order to avoid potential faults and safety hazards. Lastly, monitor and adjust the filter settings and parameters regularly to guarantee optimal performance and compatibility with the AC drive system and the load.

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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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Chris Vogel

Principal at C.F.M.E.
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Another option that seems to never get brought up is using active front ends. An active front end is basically the beginning of the inverter. It converts AC to DC. You can oversize these or use several and parallel them together. This increases redundancy allows for the distribution of a DC network versus AC, so distance harmonics wiring all of that has a significantly reduced cost. Active front ends as their name implies. Do all your filtering deal with all your harmonics and allow for power regeneration when you're slowing down? Also, by using bus bar versus copper conductors you can add VFDs very easily throughout the facility and if you need more KVA you can just add another front end as long as you size your DC bus correctly.

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Abel Briones Ramirez
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Also a regenerative inverter can be used with an Active Front End an a isolation transformer, a di/dt filter can be used at the output to avoid noise emisions and also the capability of have a larger wiring to the motor and avoid reflected wave problems.

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How do filters affect the efficiency and reliability of AC drives?

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1 Types of filters

2 Functions of filters

3 Benefits of filters

4 Tips on filter selection

5 Tips on filter installation

6 Here’s what else to consider

Variable Frequency Drives

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