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AC Filter Capacitors for PWM Inverter

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The overall variable frequency drive (VFD) system relies on capacitors to control an AC induction motor properly. The VFD system includes an input sine filter, AC/DC converter, DC link, DC/AC inverter, and AC inverter output filer.

Within the VFD are DC bus capacitors, located between the output of the AC/DC converter and the input to the DC/AC inverter. These high-energy capacitors store energy, smooth rectifier ripple currents, and provide “ride – through capability” required to lower the total harmonic distortion (THD) from the drive’s rectifier.

At the VFD’s output, transient voltage spikes occur from fast-switching transistors and long cable runs to the motor. AC filter capacitors on the inverter output serve to absorb transients and filter the harmonic current spectrums. Broadband harmonic current spectrums result from the DC chopping for pulse width modulation (PWM) output to the motor. These harmonic currents (usually the odd 3rd to 11th harmonics are the highest currents) are superimposed on the carrier or fundamental wave at 50/60Hz.

Electronic Concepts offers several UL Recognized AC filter capacitors specifically designed for inverter outputs including:

Good engineering practices noteworthy at the system design level include:

For more information on this and other ECI capacitor products click on the link below:

Original: J. Bond – 2002
Revision A: J. Bond – Dec. 22, 2014