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Harmonics Analysis in Control System. Limei Zhao 06/06/03. Origin of Noise. DSP part Fast switching frequency Voltage Source Inverter Thermal noise Dv/Dt effects LPF Thermal loss Motor EM interaction Acoustic interaction. PWM Signal from DSP. Signal after VSI before LPF.

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Harmonics analysis in control system

Harmonics Analysis in Control System

Limei Zhao

06/06/03


Origin of noise
Origin of Noise

  • DSP part

    • Fast switching frequency

  • Voltage Source Inverter

    • Thermal noise

    • Dv/Dt effects

  • LPF

    • Thermal loss

  • Motor

    • EM interaction

    • Acoustic interaction





Dsp part harmonics
DSP Part Harmonics

t Domain

ω Domain

  • Electrical frequency harmonics

    • Space vector pwm algorithm

      • Modulated Signal m(t)= sine wave + 3rd Harmonic

    • Non Linear Load

      • VSI

      • PMSM (oscillations but assume none)

  • Switching Frequency Harmonics

    • Switching function s(m,c)

      • PWM

      • Pulse of same amplitude but different width

1 2 3


  • Generally, 3rd harmonics is thought unharmful to three phase systems

  • But the results show that 3rd harmonic still need to be considered when designing a filter

The signal is before VSI


Vsi harmonics analysis
VSI Harmonics Analysis

  • VSI generated harmonic are of following orders

    n = p* k ± 1, where k = 1, 2, 3, …; p is the number of pulse

  • The harmonic currents rms value for 6-pulse

    Hn = H1/n


Lpf part
LPF Part

  • Power dissipation on filter passive components

  • It is important to find balance value of L or C to get both

  • good LPF and less power dissipation


Some useful filter 1 dv dt filter soft switching

Prolong rise time of inverter output voltage pulses

Over voltage is decreased

Switching losses are decreased

Some Useful Filter(1) --- Dv/Dt Filter (Soft Switching)


Some useful filter 2 passive sinusoidal filter

Advantages

Simple and

Reliable

Inexpensive

Disadvantages

Limited effectiveness

Line drop

Advantages

Simple and reliable

For tune frequency: THDI<5%

Increase in the input power factor due to capacitive current

Disadvantages

Limited spectrum

Some Useful Filter(2)--- Passive Sinusoidal Filter

L Filter

LC Filter


Motor part
Motor Part

  • Power transfer are sources of harmonics

    • Because Magnetic materials are operated very close and in non-linear region

    • The transfer magnetising current will be non-sinusoidal and containing harmonics (mainly 3rd ) even if the applied voltage are sinusoidal

  • Rotating machines are sources of harmonics

    • The winding can never be exactly sinusoidally distributed so that MMF is distroted

    • Coil spanning in 3-phase machine can be used to reduce 5th and 7th harmonics

    • 3-phase machines can block 3rd harmonic current at some extent

    • Harmonics produced are considered negligible compared to those produced by other sources


Current voltage to motor
Current & Voltage to Motor

Red is phase to phase voltage ; Green is phase current

@ about 60KRPM with LPF(L=150uH)



The effects of harmonic distortion
The Effects of Harmonic Distortion

  • Thermal stress

    • With strong cool air, motor current are obviously better

    • Heat sink

  • Unstable

  • Insulation stress

    • Especially for new motor’s much high current

  • Load disruption


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