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Harmonics Analysis in Control System

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

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  1. Harmonics Analysis in Control System Limei Zhao 06/06/03

  2. Origin of Noise • DSP part • Fast switching frequency • Voltage Source Inverter • Thermal noise • Dv/Dt effects • LPF • Thermal loss • Motor • EM interaction • Acoustic interaction

  3. PWM Signal from DSP

  4. Signal after VSI before LPF

  5. Signal after LPF before Motor

  6. 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

  7. 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

  8. 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

  9. 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

  10. 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)

  11. 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

  12. 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

  13. Current & Voltage to Motor Red is phase to phase voltage ; Green is phase current @ about 60KRPM with LPF(L=150uH)

  14. Signals power harmonics analysis

  15. 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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