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Variable Frequency Drives Harmonics Overview. Harmonics Overview. What causes harmonics ? Non-linear load devices create harmonics when they convert ac to dc, dc to dc, dc to ac, and ac to ac. This can produce high levels of harmonic voltage and current distortion. . Harmonics Overview.

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harmonics overview
Harmonics Overview

What causes harmonics ?

Non-linear load devices create harmonics when they convert ac to dc, dc to dc, dc to ac, and ac to ac.

This can produce high levels of harmonic voltage and current distortion.

harmonics overview3
Harmonics Overview
  • Harmonics operate at a frequencythat is a multiple of the fundamental
  • Harmonics operate at a magnitude that is inversely proportional to the magnitude of the fundamental
  • In other words... the higher the harmonic order, the less effect it has on the waveform.
  • In most cases, harmonic analysis is limited to the 13th harmonic.
harmonics overview4


60 times

every second


166 msec

1 cycle

Harmonics Overview
  • Frequency is defined as the number of times a given waveform repeats itself every second.
  • Example: the typical household line current impresses 115 volts AC, 60 times per second.
harmonics overview5

Fundamental = 60Hz

5th Harmonic = 300Hz

Harmonics Overview
  • Harmonic currents operate at the same time as the fundamental, but at faster rates
  • Harmonic currents are additive, producing a “distorted” sine wave.

Harmonics Overview

  • Harmonic Currents are currents that flow at the same time as a fundamental current, 60Hz frequency, but at multiples of the fundamental.
  • For a typical 6 pulse inverter, these multiples are:
    • Fundamental = 60 Hz
    • 5th Harmonic = 60 x 5 = 300 Hz
    • 7th Harmonic = 60 x 7 = 420 Hz
    • 11th Harmonic = 60 x 11 = 660 Hz
    • 13th Harmonic = 60 x 13 = 780 Hz
    • 17th Harmonic = 60 x 17 = 1,020 Hz
harmonics overview7


300 times

every second


30 msec

1 cycle

Harmonics Overview
  • A harmonic current operates in “harmony” with the fundamental but faster; i.e. the 5th harmonic of the 60 Hz fundamental repeats itself 300 (60 x 5) times per second.
harmonics overview8
Harmonics Overview

Harmonics have several harmful side effects:

  • Can interfere with electronic equipment and people
    • through radiated (RFI) interference through the air
    • emitted (EMI) interference over power and control wires.
  • Can cause an increase in supplied power (KVA) over what is really needed (kW or HP)
    • Entire AC supply line must be sized accordingly
    • End user transformers, switchgear, wires, contactors, and fuses have to be bigger.
    • The power company generator
  • Can cause Poor Power
poor power

Harmonics Overview

Poor Power. . .

Reduces the Load Carrying Capacity of Your Electrical System!

  • Transformers & wiring are sized by current capacity. If additional current is required due to low power factor, capacity for work producing current is displaced. Everything in your building is oversized to make up for poor power factor.
harmonics dc capacitance link
Harmonics – DC Capacitance Link
  • A DC capacitance link provides voltage to the inverter between the peaks of the mains voltage waveform. Current only flows from the mains into the DC link when the mains voltage exceeds that of the capacitor. Current only flows when the mains voltage is near its peak, as shown.
  • The larger the DC link capacitor, the shorter the period of current flow, and the higher the peak current.

Single or




Phase Power





D C Link



harmonic current wave form
Harmonic - Current Wave Form

Ideal current wave but not attainable with a 6-pulse drive

What the current wave form looks like with a 6-pulse drive

Approaching a perfect current wave

typical drive input current waveform
Typical Drive Input Current Waveform

MM4 45kW Line Current Waveform (low impedance mains)

I RMS = 75A

Power = 12kW (x3)

If current is sinusoidal

Only 52A is required

for the Same power.

sed2 typical waveform
SED2 Typical Waveform

SED2 11kW Line Current Waveform

I RMS = 53A

Power = 12kW (x3)

If current is sinusoidal

Only52Ais required

for the same power.

dc link design results
DC Link Design Results

15HP Typical VFD

Line Current Harmonics Spectrum

15 HP SED2

Line Current Harmonics Spectrum

Meter deflection

harmonics competitive analysis
Harmonics Competitive Analysis

How do other VFD manufacturers address Harmonics?

  • Some build in DC link chokes
    • This adds cost, weight and current losses
  • Some offer line reactors
    • Rarely built-in to VFD - often offered as a separate option
    • Commonly mounted in bypass cabinet, or supplied in its own enclosure
    • Adds cost, weight and can cause nuisance tripping of drive on undervoltage
  • Some build special 12 and 18 pulse drives
    • Extremely expense and not practical for most application under 100HP
  • Some just ignore the problem
harmonics the sed2 advantage
Harmonics - The SED2 Advantage
  • SED2 is specifically designed for HVAC applications
    • Fans and pumps with variable torque load characteristics
  • Sophisticated Siemens control technology enables the SED2 to significantly reduce current harmonics by dramatically reducing the size (capacitance) of the DC link.
  • In lieu of large (high uF) electrolytic capacitors used in most VFDs, SED2 uses much lower value (low uF) plastic capacitors.
  • Typical 15Hp VFD: DC link capacitance of 1000uF
  • SED2 15HP VFD: DC link capacitance of 20uF
siemens sed2 vfd advantages
Siemens SED2 VFD Advantages
  • Low harmonics design
  • Small enclosure size
  • Many standard features
  • Reduces operating cost
  • Low power losses/high efficiency
  • Minimal effect on customer power
  • Wide power range – from ½ to 125 HP