10-04-2017, 08:29 PM
Cyclo-converter
A cycloconverter or a cycloinverter converts an AC waveform, such as the mains supply, to another AC waveform of a lower frequency, synthesizing the output waveform from segments of the AC supply without an intermediate direct-current link.
Cycloconverter
Cycloconverter is frequency changer that converts AC power at one input frequency to AC output power at a different frequency.
Variable frequency output can be obtained by AC to DC to AC using phase-controlled converters (rectifier followed by inverter). This is two stage frequency conversion.
Single-stage frequency conversion is called cycloconversion
Cycloconversion can be anyone of following:
1- conversion of variable frequency to fixed one (aircraft or shipboard power supplies or wind generators).
2- conversion of fixed frequency to a variable one (AC motor speed control)
Cycloconverters can be step-up or step-down and for efficient operation, cycloconverters are usually operated in frequency range of zero to less than one-third the source frequency.
Operation Principles:
The operation principle of the Single-phase to Single-phase (1 -1 ) Cycloconverter
Single-phase (1 -1 ) Cycloconverter
This converter consists of back-to-back connection of two full-wave rectifier circuits.below figure shows the operating waveforms for this converter with a resistive load.
The input voltage, vs is an ac voltage at a frequency, For easy understanding assume that all the thyristors are fired at a=0 firing angle, i.e. thyristors act like
diodes. Note that the firing angles are named as P for the positive converter and N 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 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.
Three-Phase to Single-Phase (3 -1 ) Cycloconverter:
There are two kinds of three-phase to single-phase (3 -1 ) cycloconverters: 3 -1 half-wave cycloconverter and 3 -1 bridge cycloconverter. Like the 1 -1 case, the 3 -1 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. Note that a is sinusoidally modulated over the cycle to generate a harmonically optimum output voltage.
Advantages and Disadvantages of Cyclo-converter
Advantages
1. In a cyclo-converter, ac power at one frequency is converted directly to a lower frequency in a single conversion stage.
2. Cyclo-converter functions by means of phase commutation, without auxiliary forced commutation circuits. The power circuit is more compact, eliminating circuit losses associated with forced commutation.
3. Cyclo-converter is inherently capable of power transfer in either direction betwee source and load. It can supply power to loads at any power factor, and is also capable of regeneration over the complete speed range, down to standstill. This feature makes it preferable for large reversing drives requiring rapid acceleration and deceleration, thus suited for metal rolling application.
4. Commutation failure causes a short circuit of ac supply. But, if an individual fuse blows off, a complete shutdown is not necessary, and cyclo-converter continues to function with somewhat distorted waveforms. A balanced load is presented to the ac supply with unbalanced output conditions.
5. Cyclo-converter delivers a high quality sinusoidal waveform at low outputfre-quencies, since it is fabricated from a large number of segments of the supply waveform. This is often preferable for very low speed applications.
6. Cyclo-converter is extremely attractive for large power, low speed drives.
Disadvantages
1. Large number of thyristors is required in a cyclo-converter, and its control circuitry becomes more complex. It is not justified to use it for small installations, but is economical for units above 20 kVA.
2. For reasonable power output and efficiency, the output frequency is limited to one-third of the input frequency.
3. The power factor is low particularly at reduced output voltages, as phase control is used with high firing delay angle.