Variable Frequency Drives Harmonics Overview

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Variable Frequency DrivesHarmonics Overview
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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.

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Harmonics Overview

• Harmonics operate at a frequency that 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.

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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.

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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.

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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

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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.

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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

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Poor Power. . .
Harmonics Overview

• 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.

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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 Three
V

Phase Power
C
C

Current
-
D C Link
Rectifier
Inverter
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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
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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.
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SED2 Typical Waveform
SED2 11kW Line Current Waveform
I RMS 53A Power 12kW (x3) If current is
sinusoidal Only 52A is required for the same
power.
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DC Link Design Results
15HP Typical VFD Line Current Harmonics Spectrum
15 HP SED2 Line Current Harmonics Spectrum
Meter deflection
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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

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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

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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