Cycloconverter and its all relevaent parts PDF

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Summary

If the inductance value is not very large, the energy stored in the inductance is
able to maintain the load current only upto ωt = β, where π < β < 2π , well before the
next gate pulse and the load current tends to become discontinuous....

Description

1. Definition: Cycloconverter is a type of AC-AC converter which convert AC voltage at fixed frequency to another frequency without any help of any intermediate circuit.

2. Classification: There are mainly three types of Cycloconverter: a. Single phase to single phase Cycloconverter. b. Three phase to single phase Cycloconverter. c. Three phase to three phase Cycloconverter.

3. Topology:

Fig.01. Cycloconverter.

4. Operation principle: i. Single phase to single phase Cycloconverter: A single-phase input to single-phase output cycloconverter is shown in Fig 2, the simplest cycloconverter circuit. The secondary transformer of the power supply for the cycloconverter consists of two separate windings, which can provide 180° displaced input voltages to each of the two thyristor half bridges. As there are two controlled timing pulses for each thyristor half bridge, this topology is referred as a 2-pulse (or 4-pulse in circulating current mode) cycloconverter. The cycloconverter is connected as shown in Figure 2, which is operating without circulating current; the non-conducting thyristors should always be kept off otherwise the input power supply could be shorted via the positive and negative thyristor half bridges. When the load current is positive, the output voltage is only controlled by phase control of thyristors T1 and T3 whilst the other two negative thyristors T2 and T4 are kept off and vice versa when the load current is negative. When the load current changes direction whilst ensuring that the two thyristor half bridges do not conduct at the same time.[1]

Fig.02. Single phase to single phase cycloconverter. ii. Three phase to single phase cycloconverter:

The principle of the three-phase input cycloconverter is identical to that of the single phase input cycloconverter except that there are three input voltages which have 120° shift between each other instead of 180°. From the point of view of reducing the output voltage and current harmonics to a minimum level, the number of pulses of the cycloconverter should be as large as possible. But this is also achieved by increasing the complexity of the circuit, which requires a relatively large number of thyristors in the circuit. The cycloconverter circuits using converter groups of three and six pulses are shown in figure 4 and 5. The cycloconverter circuit of figure 4 shows the single phase output from the three phase supply source. In order to drive the three phase output, three such circuits are connected as shown in fig 5. It is desirable to connect the load neutral to the supply neutral when loads are unbalanced, otherwise load neutral can be left isolated. If the input voltage is of appropriate magnitude each converter group can be supplied directly from the supply source without necessary of supply transformer. If the load is to be connected to form common terminal than input supply for each converter group must be obtained from an independent source.[1]

Fig.03. Three phase to single phase Cycloconverter. iii. Three phase to three phase cycloconverter: Three identical three-phase input to single-phase output, 3-pulse (or 6-pulse in circulating current mode) cycloconverters connected together to supply a three-phase load. For a balanced three phase output, theoretically, there is no need to connect the load neutral to the supply neutral and therefore it is not possible to have zero-sequence current components in the input lines. Another advantage of three-phase output circuits with a floating neutral point or even without a neutral point such as the delta connection is that it provides a better harmonic content in the output line-to-line voltage due to the cancellation of the common mode voltage harmonics between the outputs.

Fig. 04. Three phase to three phase cycloconverter.

5. Circuit diagrams: a. Single phase to single phase Cycloconverter:

Fig.05. Single phase to single phase Cycloconverter circuit diagram. b. Three phase to single phase Cycloconverter:

Fig. 06. Three phase to single phase Cycloconverter circuit diagram. c. Three phase to three phase Cycloconverter:

Fig. 07. Three phase to three phase Cycloconverter circuit diagram.

6. Switching waveforms: a. Single phase to single phase:

Fig 08. Switching waveform of SS cycloconverter. b.Three phase to single phase:

Fig 09. Switching waveform of TS cycloconverter. c. Three phase to three phase:

Fig 10 . Switching waveform of TT cycloconverter. 7. Input and output waveforms (voltage and current) a. Single phase to single phase:

Fig. 11. Input and output waveform of SS cycloconverter. b.Three phase to single phase:

Fig. 11. Input and output waveform of TS cycloconverter. c. Three phase to three phase:

Fig. 12. Input and output waveform of TT cycloconverter. 8. Performance evaluation of each type (step by step derivation) Output RMS voltage ( Vo(RMS) ) : Input voltage , v s=V m sin ωt d (ωt)

√

V o (RMS)=

1 ∫ V m 2 sin2 ωt d (ωt ) π

2 ωt 1−cos ¿ d (ωt) ¿ ¿ Or, 2 π Vm ∫¿ 2π α V o (RMS)=√ ¿ Or,

V o (RMS)=

Or,

V o (RMS)=

Or,

V o (RMS)

Or,

V o (RMS)

√ √ √

V m2 2π

π

[ωt −

sin 2 ωt ] 2 α

V m2 sin 2 π sin 2 α [ ( π−α ) − ] − 2π 2 2

( V [ ( π−α ) + ( sin 2 α ) ] = 2π 2 V sin 2 α 1 [ ( π−α )+( ] = 2 ) √ 2 √π 2 m

m

Output RMS Current:

Vo , RMS R

Io(RMS)=

Vm √2

Io(RMS)=

√

1 sin 2 α [ ( π−α ) + ] π 2 R

√

sin 2 α ℑ 1 ( ] [ π −α ) + √2 π 2

Io(RMS)=

Output Power: Po= Io(RMS)2 R Output Performance: PF =

P o (RMS) Vs Is

PF =

Io( RMS )2 R Vin ( RMS ) Io ( RMS )

PF =

Io ( RMS) ∗R Vin (RMS )

PF =

Vo ( RMS) Vin ( RMS )

PF =

√

1 sin 2 α ] [ ( π −α )+ 2 π

9. Applications: a. Cement mill drives b.Ship propulsion drives

)

c. Rolling mill drives d.Schermie’s drives e.Ore grinding mills f. Mine winders 10. Advantages and disadvantages Advantages: i. Efficiency is very high compared to other converters ii. Four quadrant operation is possible because cyclo-converter is capable of power transfer in both the directions iii. AC power at one frequency is directly converted to a lower frequency in a single conversion iv. If one of the SCR fails, the cyclo-converter operates with a distorted output v. In this converter, power transfer is possible from supply load and vice versa at any power factor vi. Dynamic response is good vii. Smooth low-speed operation Disadvantages: i. Control circuit become complex so difficult to design ii. The output frequency of the cyclo-converter is 1/3th or 1/2th for responsible power output and efficiency iii. Power factor is too much poor at large values of α iv. More distortion at low frequencies 11. Recent improvement (a) Matrix Converter: A matrix converter is defined as a converter with a single stage of conversion. It utilizes bidirectional controlled switch to achieve automatic conversion of power from AC to AC. Matrix converters are characterized by sinusoidal waveforms that show the input and output switching frequencies. The principle of operation of a matrix converter is explained using the following diagram:

Fig.13. Matrix converter

(b) Integral Pulse Modulated (1f-3f) Cycloconverters: The input to these cycloconverters is single-phase high frequency sinusoidal or square waveforms with or without zero voltage gaps. Every half-cycle of the input signal, the control for each phase decides if it needs a positive pulse or a negative pulse using integral pulse modulation. For integral pulse modulation, the command signal and the output phase voltage are integrated and the latter result is subtracted from the former. For a positive difference, a negative pulse is required, and vice versa for the negative difference. For the positive (negative) input half-cycle, if a positive pulse is required, the upper (lower) switch is turned on; otherwise, the lower (upper) switch is turned on. The principle of operation of a high frequency ac link converter is explained using the following diagram:

Fig.14. High frequency ac link converter (1f hf inverter + (1f-3f) Cycloconverter)

12. Conclusions: The cycloconverter circuits are designed and simulated and desired results are obtained. Three phase cycloconverter used for three phase motors to generate supply torque characteristics that matches with demand torque characteristics of particular machine by the use of designing cycloconverter different desired frequency are obtained to equalize the torque demand of machine. This different frequency of cycloconverter is also useful to replace flywheel from the operating machine which reduces the cause of torsional vibration and fatigue damage of machine. A method is proposed in which MATLAB/Simulink, a widely accepted simulation and analysis tool, is used to model the complete cycloconverter drive system. This contribution will report on the results obtained using MATLAB for three phase cycloconverter coupled to an induction motor....