The Cycloconverter has been traditionally used only in very high power drives, usually above one megawatt, where no other type of drive can be used. Examples are cement tube mill drives above 5 MW, the 13 MW German-Dutch wind tunnel fan drive, reversible rolling mill drives and ship propulsion drives. The reasons for this are that the traditional cycloconverter requires a large number of thyristors, at least 36 and usually more for good motor performance, together with a very complex control circuit, and it has some performance limitations, the worst of which is an output frequency limited to about one third the input frequency .
Figure1. Block Diagram of Cycloconverter
The Cycloconverter has four thyristors divided into a positive and negative bank of two thyristors each. When positive current flows in the load, the output voltage is controlled by phase control of the two positive bank thyristors whilst the negative bank thyristors are kept off and vice versa when negative current flows in the load. An idealized output waveform for a sinusoidal load current and a 45 degrees load phase angle is shown in Figure 2. It is important to keep the non conducting thyristor bank off at all times, otherwise the mains could be shorted via the two thyristor banks, resulting in waveform distortion and possible device failure from the shorting current. A major control problem of the Cycloconverter is how to swap between banks in the shortest possible time to avoid distortion whilst ensuring the two banks do not conduct at the same time. A common addition to the power circuit that removes the requirement to keep one bank off is to place a centre tapped inductor called a circulating current inductor between the outputs of the two banks. Both banks can now conduct together without shorting the mains. Also, the circulating current in the inductor keeps both banks operating all the time, resulting in improved output waveforms. This technique is not often used, though, because the circulating current inductor tends to be expensive and bulky and the circulating current reduces the power factor on the input
In a 1- φ Cycloconverter, the output frequency is less than the supply frequency. These converters require natural commutation which is provided by AC supply. During positive half cycle of supply, thyristors P1 and N2 are forward biased. First triggering pulse is applied to P1 and hence it starts conducting.
As the supply goes negative, P1 gets off and in negative half cycle of supply, P2 and N1 are forward biased. P2 is triggered and hence it conducts. In the next cycle of supply, N2 in positive half cycle and N1 in negative half cycle are triggered. Thus, we can observe that here the output frequency is 1/2 times the supply frequency.
The Cycloconverter are classified into three types based on the type of input ac supply applied to the circuit.
The following sections will describe the operation principles of the cycloconverter starting from the simplest one, single-phase to single-phase (1f-1f) cycloconverter.
To understand the operation principles of cycloconverters, the single-phase to single-phase cycloconverter (Fig. 2) should be studied first. This converter consists of back-to-back connection of two full-wave rectifier circuits. Fig 3 shows the operating waveforms for this converter with a resistive load.
The input voltage, vs is an ac voltage at a frequency, fi as shown in Fig. 3a. For easy understanding assume that all the thyristors are fired at α=0°firing angle, i.e. thyristors act like diodes. Note that the firing angles are named as αP for the positive converter and αN for the negative converter.
Consider the operation of the cycloconverter to get one-fourth of the input frequency at the output. For the first two cycles of vs, the positive converter operates supplying current to the load. It rectifies the input voltage; therefore, the load sees 4 positive half cycles as seen in Fig. 3b. In the next two cycles, the negative converter operates supplying current to the load in the reverse direction. The current waveforms are not shown in the figures because the resistive load current will have the same waveform as the voltage but only scaled by the resistance. Note that when one of the converters operates the other one is disabled, so that there is no current circulating between the two rectifiers.
Fig:2 Single Phase to Single Phase Cyclo Converter
Fig:3 Single Phase to Single Phase Cyclo Converter
Thus by varying α, the fundamental output voltage can be controlled.
Constant αoperation gives a crude output waveform with rich harmonic content. The dotted lines in Fig. 3b and c show a square wave. If the square wave can be modified to look more like a sine wave, the harmonics would be reduced. For this reason αis modulated as shown in Fig.3d. Now, the six-stepped dotted line is more like a sinewave with fewer harmonics. The more pulses there are with different α's, the less are the harmonics.
a) Frequency Select =F
b) Frequency Select =F/2
c) Frequency Select =F/3
d) Frequency Select =F/4
There are two kinds of three-phase to single-phase (3Φ-1Φ) cycloconverters: 3φ-1φhalf-wave cycloconverter (Fig. 4) and 3Φ-1Φbridge cycloconverter (Fig. 5). Like the 1Φ-1Φcase, the 3Φ-1Φ cycloconverter applies rectified voltage to the load. Both positive and negative converters can generate voltages at either polarity, but the positive converter can only supply positive current and the negative converter can only supply negative current. Thus, the cycloconverter can operate in four quadrants: (+v, +i) and (-v, -i) rectification modes and (+v, -i) and (-v, +i) inversion modes. The modulation of the output voltage and the fundamental output voltage are shown in Fig. 6. Note that αis sinusoidally modulated over the cycle to generate a harmonically optimum output voltage.
The polarity of the current determines if the positive or negative converter should be supplying power to the load. Conventionally, the firing angle for the positive converter is named α P, and that of the negative converter is named α N. When the polarity of the current changes, the converter previously supplying the current is disabled and the other one is enabled. The load always requires the fundamental voltage to be continuous. Therefore, during the current polarity reversal, the average voltage supplied by both of the converters should be equal. Otherwise, switching from one converter to the other one would cause an undesirable voltage jump. To prevent this problem, the converters are forced to produce the same average voltage at all times.
If the outputs of 3 3Φ-1Φ converters of the same kind are connected in wye or delta and if the output voltages are 2∏/3 radians phase shifted from each other, the resulting converter is a threephase to three-phase (3Φ-3Φ) cycloconverter. The resulting cycloconverters are shown in Figs. 7 and 8 with wye connections. If the three converters connected are half-wave converters, then the new converter is called a 3Φ-3Φhalf-wave cycloconverter. If instead, bridge converters are used, then the result is a 3Φ-3Φbridge cycloconverter. 3Φ-3Φhalf-wave cycloconverter is also called a 3-pulse cycloconverter or an 18-thyristor cycloconverter. On the other hand, the 3Φ-3Φbridge cycloconverter is also called a 6-pulse cycloconverter or a 36-thyristor cycloconverter. The operation of each phase is explained in the previous section.
Fig. 7 3Φ-3Φhalf-wave cycloconverter
Fig. 8 3Φ-3Φbridge cycloconverter
The three-phase cycloconverters are mainly used in ac machine drive systems running three phase synchronous and induction machines. They are more advantageous when used with a synchronous machine due to their output power factor characteristics. A cycloconverter can supply lagging, leading, or unity power factor loads while its input is always lagging. A synchronous machine can draw any power factor current from the converter. This characteristic operation matches the cycloconverter to the synchronous machine. On the other hand, induction machines can only draw lagging current, so the cycloconverter does not have an edge compared to the other converters in this aspect for running an induction machine. However, cycloconverters are used in Scherbius drives for speed control purposes driving wound rotor induction motors.
Cycloconverters produce harmonic rich output voltages, which will be discussed in the following sections. When cycloconverters are used to run an ac machine, the leakage inductance of the machine filters most of the higher frequency harmonics and reduces the magnitudes of the lower order harmonics.
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