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 current
natural current

6. In steady – state.
By symmetry，Imax=－Imin =

7. rms load current : .
If the switches are ideal , then .

8. Fig 8-3 Full – bridge inverter using BJTs

9. 8-4 Fourier Series Analysis
With no dc component in the output
In square wave output

10. A quality of the AC output voltage or current.
8-5 Total harmonic distortion (THD)
A quality of the AC output voltage or current.
Assuming no dc component in the output

11. 8-7 Amplitude and harmonic control
By adjusting the interval , the output voltage can be controlled.
Fig.8-4

13. Harmonic n is eliminated if
∵Half – wave symmetry
Harmonic content can also be controlled by adjusting α
Harmonic 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 8-5
A graphical representation of the integration in the Fourier series coefficient.

17. 8-8 The half – bridge Inverter Fig 8-7

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 the
control of the output voltage amplitude can be realized
Disadvantage：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 8-8
S1 and S2 are on when Vsine>Vtri
S3 and S4 are on when Vsine < Vtri

21. S4 is on when Vsine<Vtri (high fre)
Unipolar Switching：
One (First)
Fig 8-9
S1 is on when Vsine>Vtri
S2 is on when -Vsine<Vtri
S3 is on when -Vsine>Vtri
S4 is on when Vsine<Vtri
Another (second) Fig 8-10
S1 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 allowing
for 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 symmetry
For the k-th pulse of the PWM output. (Fig 8-11)

28. Normalized Fourier coefficients ： Table 8-3
Normalized frequency spectrum： Fig 8-12
Normalized Fourier coefficients ： Table 8-3

29. Unipolar switching：(Fig 8-9)
If mf =even integer, some harmonics that were in the spectrum for the bipolar scheme are absent.
(seeing Fig 8-13 and Table 8-5)

30. 8-13 Three-phase inverters
Six-step Inverter：Fig 8-17

35. voltage resulting from the six switching transitions per period.為
Because of the six steps in the output waveform for the line-to-neutral
voltage resulting from the six switching transitions per period.
為+Vdc or 0
(1)+(2)+(3)

36. Where n=1, 6k , k=1、2…
(fundamental)
(harmonics)

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 8-18
S1 is on when
S2 is on when
S3 is on when
S4 is on when
S6 is on when
S5 is on when
Harmonics will be minimized if the carrier fre. is chosed to be
an odd triple multiple of the reference fre. (that is 3,9,15,…times the reference).

39. Table 8-8 Significant amplitude coefficients
For line-to-line voltage,
….. 三相
(參考P.313) …… 單相
Table Significant amplitude coefficients

40. 8-15 Induction motor speed control
Synchronous speed：
S1ip：
ω：electric fre.
P：number of poles
ωr：rotor speed
If 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 8-20
If the DC source is not controllable, a DC-DC converter may be inserted
between the DC source and the inverter. The PWM inverter is also another selection for this application.