###### Abstract  
In variable speed applications, the cycloconverter-based AC to AC power conversion technique has gained more attention among researchers and academics than the traditional rectifier-inverter-based AC to AC power conversion process. The conventional rectifier-inverter-based AC to AC power conversion process has several disadvantages. It uses multi-power stages that increase the converter power conversion losses and increase the cost, volume, and weight of power losses. Besides high conduction and switching losses, the electromagnetic interference problems also accompany the above issues. In this regard, this paper proposes a novel step-down Triac based cycloconverter for variable speed control applications. The proposed topology uses only five Triac devices for one-third and one-fourth frequency conversion of 50 Hz with reduced total harmonics distortion without using any pulse width modulation techniques. The proposed model is designed in the MATLAB/SIMULINK environment. The simulation results show that around 18.85% and 23.67% of total harmonics distortions are reduced in the proposed converter for one-third and one-fourth frequency conversion of 50 Hz, respectively. Two physical experiments are carried out to prove the validity of the simulation results. cycloconverter; AC to AC power conversion; variable speed; switching loss; conduction loss; pulse width modulation +
Footnote †: journal: Applied  
the authority of railway transportation in Germany first converted electricity from 50 Hz to 16-2/3 Hz in railways with arc rectifiers [5]. In 1950, a new cycloconverter was built with a varying speed and constant frequency for aircraft purposes using thyristors [6]. Following 1970, they were theoretically and practically analyzed [7]. Cycloconverters are normally phase controlled and traditionally use SCR because they are easy to switch phases with. The cycloconverter was developed in 1980 by replacing the thyristors with the soft switches [8]. Nowadays, it has gained the concern of the researchers as well as academics for performing as a variable speed controller in the rolling steel mill, cement industry applications, ship propellers, and SAG mill because of its low cost and complexity [9]. [PERSON] [10], [PERSON] [11], [PERSON] [12], [PERSON] [13] and [PERSON] [14] presented several new design topologies and applications for cycloconverters. They also launched different techniques related to static frequency changes. The improvement in design of cycloconverters was described following research and development in microcomputers due to their fast operating time, high speed performance, and reduced total power requirement. Figure 1 shows some conventional single-phase step-down cycloconverters for the conversion of the frequency to one-third and one-fourth its value [9; 10; 11; 12]. Figure 2 displays the output of conventional cycloconverters which have higher total harmonics distortion. However, these cycloconverters were concluded from previous studies to have the following limitations:  
* They cannot perform multi-frequency power conversion;
* The output voltage gives from total harmonics distortions (THDs) problems;
* They still use large semiconductor devices, although they use fewer switching components than the VFD technique. Figure 1: Conventional cycloconverters (**a**) [PERSON] [10], (**b**) [PERSON] [11] (**c**) [PERSON] [12] (1/3) and (1/4) frequency conversion of 50 Hz. To reduce harmonics, several modulation techniques and conventional filters are used [15; 16]. Nevertheless, it is not possible to reduce the harmonics contents by using conventional filters [17]. The tuned filter can be the solution to these problems. However, it makes the controlling complex. Therefore, a suitable modulation technique can be the best choice for reducing THDs. The various high frequency modulation techniques such as sinusoidal, space vector, trapezoidal, and delta modulation techniques are used to reduce THDs and improve the power quality [18; 19; 20; 21].