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

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**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

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**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

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**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

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**Outline 6.1 AC voltage controllers 6.2 Other AC controllers**

6.3 Thyristor cycloconverters 6.4 Matrix converters

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**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

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**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

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**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)

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**Inductive (Inductor-resistor) load, operation principle**

1 o i VT 2 The phase shift range: a p

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**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.

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**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

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**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

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**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

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**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.

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**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

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**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

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**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

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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

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**6.2.1 Integral cycle control —AC power controller**

M 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.

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**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

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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.

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**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

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**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

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**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

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**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

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**Single-phase thyristor-cycloconverter**

Modes of operation t O u o , i 1 2 3 4 5 P N Rectification Inver sion Blocking

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**Single-phase thyristor-cycloconverter**

Typical waveforms

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**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

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**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)

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**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)

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**6.3.2 Three-phase thyristor-cycloconverter**

The configuration with common input line

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**Three-phase thyristor-cycloconverter**

The configuration with star-connected output

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**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

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**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

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**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

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**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)

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**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

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**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.

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