8-1 School of Electrical Systems Engineering ABD RAHIM 2008 EET421 Power Electronic Drives - DC to AC converter / Inverter Abdul Rahim Abdul Razak

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8-3 Summary dc-to-ac converters are known as inverters The function of an inverter is to change the dc input voltage to an ac output voltage of desired magnitude and frequency The output voltage waveforms of ideal inverters should be sinusoidal However, the output of practical inverters contains harmonics For high power applications, low distorted sinusoidal waveforms are required Harmonic contents could be minimized by the use of high-speed semiconductor switching techniques Inverters are widely used in industrial applications - motor drives, UPS, induction heating, standby power supplies, etc. - input may be a battery, fuel cell, solar cell, or there dc source dc-to-ac inverters can make smooth transition into the rectification mode, where the flow of power reverses from the ac side to the dc side Two types of inverters: single-phase inverters and three-phase inverters School of Electrical Systems Engineering ABD RAHIM 2008

8-4 Switch-Mode DC-AC Inverters Applications: ac motor drives Uninterruptible ac power supplies Where a sinusoidal ac output is required whose magnitude and frequency both have to be controlled Terminal voltage is adjustable in its magnitude and frequency School of Electrical Systems Engineering ABD RAHIM 2008

8-5 Switch-Mode DC-AC Inverter: Bi-directional power flow

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8-20 School of Electrical Systems Engineering ABD RAHIM 2008 Voltage control of 1-phase inverter -The needs to have a controllable output voltage : 1)To cope with variations of DC input voltage 2)For inverter voltage regulation 3)For constant volts/frequency control requirement -Most efficient techniques is by incorporating PWM control within the inverter : a)Single pulse width modulation - PWM b)Multiple pulse width modulation - MPWM c)Sinusoidal pulse width modulation - SPWM d)Modified sinusiodal pulse width modulation - MSPWM e)Phase displacement control

8-21 School of Electrical Systems Engineering ABD RAHIM 2008 a)Single pulse width modulation - PWM Modulation index: M=Ar/Ac Varying Ar from 0 to Ac, will increase the δ thus the output will vary from 0 to Vs

8-22 School of Electrical Systems Engineering ABD RAHIM 2008 a)Single pulse width modulation - PWM Modulation index: M=Ar/Ac Varying Ar from 0 to Ac, will increase the δ thus the output will vary from 0 to Vs DF increased badly significantly at low output voltage 3rd harmonic more dominant on single PWM

8-23 School of Electrical Systems Engineering ABD RAHIM 2008 b) Multiple pulse width modulation - MPWM f o will set the output frequency while f c will determines the pulses per half cycle,p. f c / f o = m f, frequency modulation ratio

8-24 School of Electrical Systems Engineering ABD RAHIM 2008 b) Multiple pulse width modulation - MPWM f o will set the output frequency while f c will determines the pulses per half cycle,p. f c / f o = m f, frequency modulation ratio -By using several pulses, (p=5) The harmonic content are reduced compared to single PWM - the DF is also reduced significantly. - but switching loss would increased. - if p increased, the amplitude of lower order harmonics would be lowered but it will increase the high-orders harmonics

8-25 School of Electrical Systems Engineering ABD RAHIM 2008 c) Sinusoidal pulse width modulation - SPWM -Pulse width varied proportional to the amplitude of sinewave ref signal. - the gating signal generated by comparing the triangular carrier wave and sinusoidal ref signal.

8-26 School of Electrical Systems Engineering ABD RAHIM 2008 Sinusoidal pulse width modulation - SPWM Modulation index: M=Ar/Ac Varying Ar from 0 to Ac, will increase the δ thus the output will vary from 0 to Vs -DF is less compared to MPWM - eliminates the lower order harmonics of 2p-1, for this p=5, lowest order harmonics is 9 th. - the harmonics were pushed to the high frequency range of f c. So it would be much easier for low-pass filtering process.

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8-28 School of Electrical Systems Engineering ABD RAHIM 2008 Overmodulation leads to squarewave operation and add harmonics to the system

8-29 School of Electrical Systems Engineering ABD RAHIM 2008 d) Modified sinusiodal pulse width modulation - MSPWM - instead of 100% pulses, carrier wave is applied on 1 st and last 60 degree of half cycle. - advantages : a) Fundamental component increased b) Harmonics characteristics are improved. c) Reduce switching loss.

8-30 School of Electrical Systems Engineering ABD RAHIM 2008 d) Modified sinusiodal pulse width modulation - MSPWM advantages : a) Fundamental component increased b) Harmonics characteristics are improved. c) Reduce switching loss.

8-31 School of Electrical Systems Engineering ABD RAHIM 2008 e) Phase displacement control - multiple inverter are used, output is taken from summation of output voltage of the individual inverter. - 2 half bridge inverter output Output with 180 degree displacement. Output with β degree displacement

8-32 Three-Phase Inverter Used to supply three-phase loads Three single-phase inverters could be used with 120 degree displacement or shifting between phases, however, 12 switches are necessary Consists of three legs, one for each phase One of the two switches in a leg is always ON at any instant Output of each leg depends on V d and the switching status

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8-34 School of Electrical Systems Engineering ABD RAHIM 2008 Current source inverters - a large inductor component inserted at the input -input behaves like a current source -The output current will maintained constant at any loads but output voltage forced to change -Diodes in series are required to block the reverse voltage on the transistors.