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# Harmonics Analysis in Control System - PowerPoint PPT Presentation

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

Limei Zhao

06/06/03

• DSP part

• Fast switching frequency

• Voltage Source Inverter

• Thermal noise

• Dv/Dt effects

• LPF

• Thermal loss

• Motor

• EM interaction

• Acoustic interaction

t Domain

ω Domain

• Electrical frequency harmonics

• Space vector pwm algorithm

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

• 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 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

• 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

Over voltage is decreased

Switching losses are decreased

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

Simple and

Reliable

Inexpensive

Limited effectiveness

Line drop

Simple and reliable

For tune frequency: THDI<5%

Increase in the input power factor due to capacitive current

Limited spectrum

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

L Filter

LC Filter

• 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

Red is phase to phase voltage ; Green is phase current

• Thermal stress

• With strong cool air, motor current are obviously better

• Heat sink

• Unstable

• Insulation stress

• Especially for new motor’s much high current