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 + 3 rd 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 t Domainω Domain

Generally, 3 rd harmonics is thought unharmful to three phase systems But the results show that 3 rd 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 THD Ih6p,% % H THD Ih12p,% %

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(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 FilterLC 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 3 rd ) 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 5 th and 7 th harmonics 3-phase machines can block 3 rd 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 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 Unstable Insulation stress Especially for new motor’s much high current Load disruption