CHAPTER 2INTRODUCTION TO CYCLOCONVERTERS2.1 BASIC CONCEPT CYCLOCONVERTERSA device which converts input power at one frequency to output power at a different frequency with one stage conversion is called a cycloconverters. A cycloconverters is thus one stage frequency changer. Basically, cycloconverters are of two types, namely:(i) Step down cycloconverters and(ii) Step up cycloconverters.A Cycloconverter is a means of changing the frequency of alternating power using controlled rectifiers which are AC line commutated. The Cycloconverter is thus an alternative to the frequency changing system using two single conversions fixed AC to variable AC. A Cycloconverter is a direct Frequency changer that converts AC power at one frequency to AC power at another frequency by AC-AC conversion, without an intermediate conversion link. The majority of Cycloconverter are naturally controlled to commutate and the maximum output frequency is limited to a value that is only a fraction of the source frequency. As a result the major applications of Cycloconverter are low speed. AC motor drives in the range up to 15,000kw with frequencies from 0 to 20 Hz. The cyclo-converter is generally used for the speed control of motors, constant frequency power supplies, controllable reactive power supply for an ac system and induction heating system, VSCF (variable frequency input and constant frequency output) generation which is used in aircraft power suppliers, mobile power supplies and space vehicles. Since in induction and synchronous motors the speed is directly proportional to the supply frequency, the speed can be effectively controlled by controlling the input frequency. Generally the cyclo-converter lies in the category of static frequency changer. Both three phase and single phase cyclo-converters are used in different applications. The output frequency of a three-phase cyclo-converter must be less than about one-third to one-half of the input frequency. The cyclo-converter requires high power switching mechanism which is generally performed by thyristors, MosFETs and TRIACs in order to get alternating output of variable frequency. 2.2 OPERATION OF SINGLE PHASE CYCLO-CONVERTER: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 =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 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 2.1: Cyclo Converter

Now consider the operation of the cycloconverter to get one-fourth of the input frequency at the output as shown below

Fig: 2.2 output waveform of F/4 pulsesNow consider the operation of the cycloconverter to get one-half of the input frequency at the output as shown below

Fig: 2.3 output waveform of F/2 pulses2.3 APPLICATIONS OF CYCLOCONVERTERWith the advent of high power thyristors, cycloconverters are again becoming popular. At present, the applications of cycloconverters include the following: Speed control of high power ac drives Induction heating Cement mill drives Ship propulsion drives Rolling mill drives Scherbius drives Ore grinding mills Mine winders Static Var compensation For converting variable speed alternator voltage to constant frequency output voltage for use as power supply in aircraft or shipboards.The general use of cycloconverters is to provide either a variable frequency power from a fixed input frequency power or a fixed frequency power from a variable input frequency power.

12