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Harmonics Analysis in Control SystemPowerPoint Presentation

Harmonics Analysis in Control System

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Presentation Transcript

Origin of Noise

- DSP part
- Fast switching frequency

- Voltage Source Inverter
- Thermal noise
- Dv/Dt effects

- LPF
- Thermal loss

- Motor
- EM interaction
- Acoustic interaction

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)

- Space vector pwm algorithm
- Switching Frequency Harmonics
- Switching function s(m,c)
- PWM
- Pulse of same amplitude but different width

- Switching function s(m,c)

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 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

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

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)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 FilterL 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)

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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