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Harmonic distortion of the AC power line in HVAC systems
Harmonic distortion of the AC power line in HVAC systems (photo credit: ke-next.de)

Causes of power line distortion

Most adjustable frequency drives operate by using a bridge rectifier to convert the incoming AC voltage to DC voltage (see figure 1). An inverter in the drive then converts the DC voltage into a precise output voltage and frequency to control the speed of the motor.

General block diagram of an adjustable frequency drive
Figure 1 – General block diagram of an adjustable frequency drive

Drives today use a diode bridge rectifier to convert the AC line power into a fixed‐voltage DC bus (see figure 2). A DC bus capacitor bank is then used to filter out the AC ripple.

Diode bridge rectifier on a PWM drive
Figure 2 – Diode bridge rectifier on a PWM drive

While this results in a very efficient drive, it can cause disturbances on the AC power line due to the way the drive draws AC current.

Current cannot flow from the rectifier into the DC bus until the input voltage is greater than the DC bus voltage. As shown in figure 3, this only happens for a very short period of time for each phase.

Input with high peak current
Figure 3 – Input with high peak current

This causes a non‐sinusoidal current flow created by the input stage of the drive. In order to transfer the energy required by the motor in such a short period of time, the peak current must be high.

Title: Harmonic Distortion of the AC Power Line – Danfoss
Format: PDF
Size: 365 KB
Pages: 7
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Harmonic Distortion of the AC Power Line
Harmonic Distortion of the AC Power Line – Danfoss

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2 Comments


  1. Pavel Chernikov
    May 26, 2014

    Detail description see IEEE Std 519-1992 section 4


  2. mark lodovice
    May 25, 2014

    thanks for the news it is very useful to me

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