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
