1. Power Electronics
Chapter AC to AC Converters ( AC Controllers and Frequency Converters )

2. Classification of AC to AC converters
Electronics
Same frequency
variable magnitude
AC power
AC power
Variable
frequency
AC power
Power
AC controllers
Frequency converters
(Cycloconverters)
AC to AC converters 2

3. Classification of AC controllers
Phase control: AC voltage controller
(Delay angle control)
Integral cycle control: AC power controller
AC controller
PWM control: AC chopper
(Chopping control)
On/off switch: electronic AC switch
PWM: Pulse Width Modulation

4. Classification of frequency converters
Electronics
Phase control: thyristor cycloconverter
(Delay angle control)
Frequency converter
(Cycloconverter)
PWM control: matrix converter
(Chopping control)
Power
Cycloconverter is sometimes referred to
in a broader sense—any ordinary AC to AC converter
in a narrower sense—thyristor cycloconverter 4

5. Outline 6.1 AC voltage controllers 6.2 Other AC controllers
6.3 Thyristor cycloconverters
6.4 Matrix converters

6. 6.1 AC voltage controllers
6.1.1 Single-phase AC voltage controller
6.1.2 Three-phase AC voltage controller
Applications
Lighting control
Soft-start of asynchronous motors
Adjustable speed drive of asynchronous motors
Reactive power control

7. 6.1.1 Single-phase AC voltage controller
Resistive load O u 1 o i VT w t
Electronics R u 1 o i VT 2
Power
The phase shift range (operation range of phase delay angle):
0  a  p 7

8. Resistive load, quantitative analysis
RMS value of output voltage
RMS value of output current
RMS value of thyristor current
Power factor of the circuit
(6-1)
(6-2)
(6-3)
(6-4)

9. Inductive (Inductor-resistor) load, operation principle1 o i VT 2
The phase shift range:
  a  p

10. Inductive load, quantitative analysis
Differential equation
Solution
Considering io=0 when wt=a+q
We have
(6-5)
(6-6)
(6-7)
The RMS value of output voltage, output current, and thyristor current can then be calculated.

11. Inductive load, when a < 
The circuit can still work.
The load current will be continuous just like the thyristors are short-circuit, and the thyristors can no longer control the magnitude of output voltage.
The start-up transient will be the same as the transient when a RL load is connected to an AC source at wt =a (a < ).
Start-up transient

12. Electronics Power Harmonic analysis
There is no DC component and even order harmonics in the current.
The current waveform is half-wave symmetric.
The higher the number of harmonic ordinate, the lower the harmonic content.
a = 90 is when harmonics is the most severe.
The situation for the inductive load is similar to that for the resistive load except that the corresponding harmonic content is lower and is even lower as  is increasing. 60
120
180
Fundamental 3 5 7 a /
( °) I n * % 20 40 80
100
Electronics
Power
Current harmonics
for the resistive load 12

13. 6.1.2 Three-phase AC voltage controller
Classification of three-phase circuits
Y connection
Line-controlled ∆ connection
Branch-controlled ∆ connection
Neutral-point-controlled ∆ connection

14. 3-phase 3-wire Y connection AC voltage controller' a b c u i U
a0' VT 5 3 6 4 2 1
For a time instant, there are 2 possible conduction states:
Each phase has a thyristor conducting. Load voltages are the same as the source voltages.
There are only 2 thyristors conducting, each from a phase. The load voltages of the two conducting phases are half of the corresponding line to line voltage, while the load voltage of the other phase is 0.

15. 3-phase 3-wire Y connection AC voltage controller
Resistive load, 0  a < 60 a 4 p 3 2 5 u
ao' ab ac t 1 VT 6

16. 3-phase 3-wire Y connection AC voltage controller
Resistive load, 60  a < 90 a p 4 3 2 5 u
ao' ab ac t 1 VT 6

17. 3-phase 3-wire Y connection AC voltage controller
Resistive load, 90  a < 150 a p 4 3 2 5 u
ao' ab ac VT 1 6

18. 6.2 Other AC controllers
6.2.1 Integral cycle control—AC power controller
6.2.2 Electronic AC switch
6.2.3 Chopping control—AC chopper

19. 6.2.1 Integral cycle control —AC power controllerM
Line period
Control period =
*Line period 2 4 O
Conduction
angle N 3 u o 1 , i w t U R u 1 o i VT 2
Circuit topologies are the same as AC voltage controllers. Only the control method is different.
Load voltage and current are both sinusoidal when thyristors are conducting.

20. Spectrum of the current in AC power controller
There is NO harmonics in the ordinary sense.
There is harmonics as to the control frequency. As to the line frequency, these components become fractional harmonics.
Harmonic order as to control frequency
Harmonic order as to line frequency 5 1 2 3 4 12 14 6 10 8
0.6
0.5
0.4
0.3
0.2
0.1 I n / 0m

21. 6.2.2 Electronic AC switch
Circuit topologies are the same as AC voltage controllers. But the back-to-back thyristors are just used like a switch to turn the equipment on or off.

22. 6.2.3 Chopping control—AC chopper
Principle of chopping control
The mean output voltage over one switching cycle is proportional to the duty cycle in that period. This is also called Pulse Width Modulation (PWM).
Advantages
Much better output waveforms, much lower harmonics
For resistive load, the displacement factor is always 1.
Waveforms when the load is pure resistor

23. AC chopper Modes of operation
　>0, io>0: V1 charging, V3 freewheeling
　>0, io<0: V4 charging, V2 freewheeling
　<0, io>0: V3 charging, V1 freewheeling
　<0, io<0: V2 charging, V4 freewheeling

24. 6.3 Thyristor cycloconverters (Thyristor AC to AC frequency converter)
Another name—direct frequency converter (as compared to AC-DC-AC frequency converter which is discussed in Chapter 8)
Can be classified into single-phase and three-phase according to the number of phases at output
6.3.1 Single-phase thyristor-cycloconverter
6.3.2 Three-phase thyristor-cycloconverter

25. 6.3.1 Single-phase thyristor-cycloconverter
Circuit configuration and operation principle P N u Z o O u o a P =0 = p 2 w t
Output voltage
Average output voltage

26. Single-phase thyristor-cycloconverter
Modes of operation t O u o , i 1 2 3 4 5 P N
Rectification
Inver
sion
Blocking

27. Single-phase thyristor-cycloconverter
Typical waveforms

28. Modulation methods for firing delay angle
Calculation method
For the rectifier circuit
For the cycloconverter output
Equating (6-15) and (6-16)
Therefore
Cosine wave-crossing method
(6-15)
(6-16)
(6-17)
(6-18)
Principle of cosine wave-crossing method

29. Calculated results for firing delay angle=
0.1 a / ( )
Output voltage phase angle w t
120
150
180 30 60 90
0.2
0.3
0.8
0.9
1.0 p 2 3
Output voltage ratio (Modulation factor)

30. Input and output characteristics
Maximum output frequency: 1/3 or 1/2 of the input frequency if using 6-pulse rectifiers
Input power factor
Harmonics in the output voltage and input current are very complicated, and both related to input frequency and output frequency.
0.8
0.6
0.4
0.2 g =
1.0
Input displacement factor
Load power factor (lagging)
Load power factor (leading)

31. 6.3.2 Three-phase thyristor-cycloconverter
The configuration with common input line

32. Three-phase thyristor-cycloconverter
The configuration with star-connected output

33. Three-phase thyristor-cycloconverter
Typical waveforms
Output voltage
200 t / ms
Input current with
Single-phase output
200 t / ms
Input current with
3-phase output
200 t / ms

34. Input and output characteristics
The maximum output frequency and the harmonics in the output voltage are the same as in single-phase circuit.
Input power factor is a little higher than single-phase circuit.
Harmonics in the input current is a little lower than the single-phase circuit due to the cancellation of some harmonics among the 3 phases.
To improve the input power factor:
Use DC bias or 3k order component bias on each of the 3 output phase voltages

35. Features and applications
Direct frequency conversion—high efficiency
Bidirectional energy flow, easy to realize 4-quadrant operation
Very complicated—too many power semiconductor devices
Low output frequency
Low input power factor and bad input current waveform
Applications
High power low speed AC motor drive

36. 6.4 Matrix converter Circuit configuration a b c u v w S S a) b) Input
Output a b c u v w S 1 2 3 S ij a) b)

37. Matrix converter Usable input voltage a) Single-phase input voltage
b) Use 3 phase voltages to construct output voltage
c) Use 3 line-line voltages to construct output voltage

38. Features Direct frequency conversion—high efficiency
Can realize good input and output waveforms, low harmonics, and nearly unity displacement factor
Bidirectional energy flow, easy to realize 4-quadrant operation
Output frequency is not limited by input frequency
No need for bulk capacitor (as compared to indirect frequency converter)
Very complicated—too many power semiconductor devices
Output voltage magnitude is a little lower as compared to indirect frequency converter.