## 1 Introduction  
The need to develop environmentally friendly technology comes from two critical issues: climate change; and the need to limit the use of non-renewable energy sources. As the use of coal impacts pollution levels in the air, it also affects climate change. Both issues are related. If the use of fossil energy can be reduced, it will also impact the reduction of pollution. Thus, the development of environmentally friendly and sustainable technology needs to be undertaken by utilizing renewable energy sources, such as power plants. Small-scale wind turbines (SWT) can produce electricity below 100 kW [1]. Researchers have carried out development work on SWT designs. Various designs have been adapted to the needs of the areas where they are installed. This type of wind turbine has a simple mechanical structure compared to large-scale wind turbines. The structure entails a lower purchase price, is lightweight, has fewer space constraints and is easy to mount [2]. Furthermore, some wind turbine manufacturers have also produced multiple types of SWTs. The ideasfor developing SWTs are as patented by [PERSON] [3] and [PERSON] [4]; the aim is to increase the power supply by employing two rotors. Both patents show their designs equipped with mechanical planetary gear trains. For [PERSON]'s design, two counter-rotating propellers are connected to the ring gear and carrier, respectively, of a 2-degrees-of-freedom (2-DOF) basic planetary gear train to generate the largest angular speed of the sun gear connected to the generator. The efficiency of the dual rotor wind turbine is better than that of single construction [4]. Other than that, a smaller generator can be installed into the dual rotor wind turbine compared to the single one [4]. Another researcher examined a counter-rotating wind turbine which can increase power production by up to 25% [5]. Compared to existing mechanical gearboxes used for wind turbines, magnetic gear mechanisms offer new opportunities to build new-concept wind turbines. Magnetic gears are used to replace mechanical gears because the gearbox generates turbine stop hours as 42% [6]. Furthermore, the gearbox also causes power losses in a wind system (composed of a generator, converter, and gearbox). This power loss occurs because a machine generates shallow power for wind speeds of less than 4 m/s, such as off-the-shelf wind systems [7]. The authors chose to design a magnetic gear mechanism because it has the advantage of transmitting power without physical contact. The other benefits are less vibration and noise, no need for lubricant and maintenance, and unique characteristics of torque-overload protection [8; 9]. Unlike mechanical gears, magnetic gears will not fail in the overload condition of transmitted torque [10]. The use of NdFeB permanent magnet material can provide high energy density, durable magnetization, and high conductivity [11]. The magnetic gear topology widely discussed is the coaxial magnetic gear train (CMGT) because of its compact size, high torque density, and manufacturing possibilities. Despite those advantages, CMGT has some drawbacks. The mechanical complexity of the magnetic gears is one of the drawbacks of CMGT, especially for the complex mechanical structure of the carrier. The mechanical construction is challenging because the components are concentric and move in different angular velocities. The rotor components will slip when the torque overload occurs and cannot transmit power, which may have security concerns if used in the transmission systems of mobile vehicles. However, this disadvantage becomes an advantage of magnetic gears used in the transmission systems of wind turbines, because it can protect the mechanism from overload failure. Another disadvantage of CMGT is that the permanent magnet will demagnetize when the overheat occurs. Despite these disadvantages, a the magnetic gear mechanism has been successfully developed in the continuous variable transmission; the prototype uses a 2-DOF CMGT [12].