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

Signal after LPF before Motor

DSP Part Harmonics

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

t Domain ω Domain

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 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

h 1 5 7 11 13 17 19 THD

Ih6p,% 100 20.0 14.3 9.1 7.7 5.9 5.3 28.45%

H 1 5 7 11 13 17 19 23 25 29 31 35 37 THD

Ih12p,% 100 1.8 1.6 6.6 5.4 0.33 0.3 1.5 1.3 0.25 0.20 0.8 0.4 9.14%

LPF Part

• Power dissipation on filter passive components

2

2 fCrms

phVph

CP

2

2 fLrmsI

LP

• 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(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

L Filter LC Filter

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

Red is phase to phase voltage ; Green is phase current @ about 60KRPM with LPF(L=150uH)

Signals power harmonics analysis

The Effects of Harmonic Distortion

Thermal stress With strong cool air, motor current are

obviously better Heat sink

UnstableInsulation stress Especially for new motor’s much high current

Load disruption