Presentation on theme: "Ch8 Inverters (converting DC to AC)"— Presentation transcript:
1 Ch8 Inverters (converting DC to AC) 8-1 Introduction․Converting DC to AC․Applications: adjustable-speed AC motor drives.Uninterruptible power supply (UPS).AC appliances run from an automobile battery.
2 8-2 The full-bridge converter Fig 8-1 : S1 and S4 should not be closed at the same time, nor should S2 and S3, otherwise, a short circuit would exist across the dc source
4 8-3 The square-wave Inverter Fig 8-2. (Waveform) An inductive load presents some considerations in designing the switches in full – bridge circuit because the switch current must be bidirectional.forced currentnatural current
13 Harmonic n is eliminated if ∵Half – wave symmetryHarmonic content can also be controlled by adjusting αHarmonic n is eliminated if
14 Amplitude control and harmonic reduction may not be compatible Amplitude control and harmonic reduction may not be compatible. To control both amplitude and harmonic, it is necessary to have control over the dc input voltage.Fig 8-5A graphical representation of the integration in the Fourier series coefficient.
18 A square-wave output or bipolar pulse-width-modulated (PWM) output. The voltage across an open switch is twice the load voltage
19 8-9 pulse-width-modulated (PWM) output Advantage：Reduced filter requirements to decrease harmonics and thecontrol of the output voltage amplitude can be realizedDisadvantage：more complex control circuits for switches and increased losses due to more frequent switching.Sinusoidal PWM requires:(1). a reference (modulating or control) signal-sinusoidal.(2). a carrier (triangular wave) signal that controls the switching fre.Bipolar switching: Fig 8-8S1 and S2 are on when Vsine>VtriS3 and S4 are on when Vsine < Vtri
21 S4 is on when Vsine<Vtri (high fre) Unipolar Switching：One (First)Fig 8-9S1 is on when Vsine>VtriS2 is on when -Vsine<VtriS3 is on when -Vsine>VtriS4 is on when Vsine<VtriAnother (second) Fig 8-10S1 is on when Vsine>Vtri (high fre)S4 is on when Vsine<Vtri (high fre)S2 is on when Vsine> 0 (low fre)S3 is on when Vsine< 0 (low fre)
25 8-10 PWM definitions and considerations. Frequency modulation ratio：The Fourier series of the PWM output voltage has a fundamental fre. which is the same as that of the reference signal. Harmonic frequencies exist at and around multiple of the switching fre. A simple low-pass filter can be effective in removing those (harmonics). Higher losses in switches(2) Amplitude modulation ratio：If , then , (linearly).If , the amplitude of the output increases with ,but not linearly.(3). Swithes：carrying current in either direction. Feedback.diode allowingfor switching times in the control.(4). Reference voltage：sinusoidal, non-sinusoidal
26 8-11 PWM harmonics Bipolar switching：(Fig 8-8) If mf =odd integer, the PWM output then exhibits odd symmetryFor the k-th pulse of the PWM output. (Fig 8-11)
35 voltage resulting from the six switching transitions per period. 為Because of the six steps in the output waveform for the line-to-neutralvoltage resulting from the six switching transitions per period.為+Vdc or 0(1)+(2)+(3)
37 PWM three-phase Inverters Fig 8-18 for line-to-line and line-to-neutral voltages.is load dependent and is smaller for a R-L load.Output fre. can be controlled by changing the switching fre..Output voltage can be controlled by adjusting the DC input voltage.PWM three-phase Inverters Fig 8-18S1 is on whenS2 is on whenS3 is on whenS4 is on whenS6 is on whenS5 is on whenHarmonics will be minimized if the carrier fre. is chosed to bean odd triple multiple of the reference fre. (that is 3,9,15,…times the reference).
40 8-15 Induction motor speed control Synchronous speed：S1ip：ω：electric fre.P：number of polesωr：rotor speedIf the applied electrical fre. is changed, the motor speed will change proportionally.To avoid the magnetic flux in the air gap saturated, constant should be held .Fig 8-19
42 The six-step inverter can be used for this application if the dc input is adjustable Fig 8-20If the DC source is not controllable, a DC-DC converter may be insertedbetween the DC source and the inverter. The PWM inverter is also another selection for this application.