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1、Power Electronics,Chapter 6 AC to AC Converters ( AC Controllers and Frequency Converters ),Classification of AC to AC converters,Power,Electronics,2,Classification of AC controllers,AC controller,Phase control: AC voltage controller (Delay angle control),Integral cycle control: AC power controller,
2、PWM control: AC chopper (Chopping control),On/off switch: electronic AC switch,PWM: Pulse Width Modulation,Classification of frequency converters,Frequency converter (Cycloconverter),Phase control: thyristor cycloconverter (Delay angle control),PWM control: matrix converter (Chopping control),Power,
3、Electronics,Cycloconverter is sometimes referred to in a broader senseany ordinary AC to AC converter in a narrower sensethyristor cycloconverter,4,Outline,6.1 AC voltage controllers 6.2 Other AC controllers 6.3 Thyristor cycloconverters 6.4 Matrix converters,6.1 AC voltage controllers,6.1.1 Single-
4、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,6.1.1 Single-phase AC voltage controller,The phase shift range (operation range of phase delay angl
5、e): 0 a p,Resistive load,Power,Electronics,7,RMS value of output voltage RMS value of output current RMS value of thyristor current Power factor of the circuit,Resistive load, quantitative analysis,(6-1),(6-2),(6-3),(6-4),Inductive (Inductor-resistor) load, operation principle,The phase shift range:
6、 a p,Differential equation Solution Considering io=0 when wt=a+q We have,Inductive load, quantitative analysis,The RMS value of output voltage, output current, and thyristor current can then be calculated.,(6-5),(6-6),(6-7),Inductive load, when a ,The circuit can still work. The load current will be
7、 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,Harmonic analysis,There is no DC co
8、mponent 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
9、 that the corresponding harmonic content is lower and is even lower as is increasing.,Power,Electronics,Current harmonics for the resistive load,12,6.1.2 Three-phase AC voltage controller,Classification of three-phase circuits,Y connection,Line-controlled connection,Branch-controlled connection,Neut
10、ral-point-controlled connection,3-phase 3-wire Y connection AC voltage controller,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
11、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.,3-phase 3-wire Y connection AC voltage controller,Resistive load, 0 a 60,3-phase 3-wire Y connection AC voltage controller,Resistive load, 60 a 90,3-phase 3-wire
12、Y connection AC voltage controller,Resistive load, 90 a 150,6.2 Other AC controllers,6.2.1 Integral cycle controlAC power controller 6.2.2 Electronic AC switch 6.2.3 Chopping controlAC chopper,6.2.1 Integral cycle control AC power controller,Circuit topologies are the same as AC voltage controllers.
13、 Only the control method is different. Load voltage and current are both sinusoidal when thyristors are conducting.,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 b
14、ecome fractional harmonics.,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.,6.2.3 Chopping controlAC chopper,Principle of chopping control The mean output voltage over o
15、ne 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,AC chopper,Modes of o
16、peration 0, io0: V1 charging, V3 freewheeling 0, io0: V3 charging, V1 freewheeling 0, io0: V2 charging, V4 freewheeling,6.3 Thyristor cycloconverters (Thyristor AC to AC frequency converter),Another namedirect 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-pha
