Abstract
The majority of South Africa’s electricity is generated from coal-fired power stations. After expansion through the turbine stage the heated steam is condensed by air-cooled condensors (ACCs). In water scarce areas air-cooled condensers are used to reject heat by means of convection through an array of fans that force air over the heated steam tubes positioned in a A-frame structure above the fan. The ACC system consists of an induction motor, two-stage gear reducer and an axial fan. For this research, the rotational velocity of the gearbox’s output shaft is measured as well as the bending and torsional strain experienced during different stages of operation. A proximity sensor and various sets of strain gauges are used to record the measurements respectively. Testing was conducted at three different sites that utilize different control methods for the induction motors. The purpose of testing at different sites was to evaluate the effectiveness of each start-up control system and its application inside an ACC. The results obtained in each of the three cases are quite clear that any motor controller is beneficial to the overall system simply due to the reduced inrush current and torque peaks.
Highlights
South Africa has large coal reserves situated in the North-Eastern regions of the country
The induction motors used in air-cooled condensors (ACCs) typically have a rated output of ∼250 kW
Within the first 5 s of start-up, the torque peaks exerted on the gearbox by the induction motor can reach up to four times the rated torque of the gearbox, which over a prolonged period of frequent start-ups can damage the gearbox and lead to subsequent failure thereof
Summary
South Africa has large coal reserves situated in the North-Eastern regions of the country. It is because of this reason that the majority of the country’s electricity is generated by coalfired power stations in those areas These regions are considered water scarce and cannot sustain the traditional wet cooling tower power plants. The method of starting an induction motor is important to consider as this can significantly reduce the start-up torque peak [7]. The most common areas of gearbox failure are found to be the bearings and gear tooth fatigue damage [4] Measurements on both shafts of the gearbox was taken by [1] to relate the input torsion and bending strain to the output side of the gearbox. Each test site is installed with one of the aforementioned motor starting controllers, a two-stage gearbox (∼ 10 : 1) and a axial fan with a diameter of ∼10.36 m
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