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Common faults of static phase shifters – how should one respond when a phase shifter malfunctions? A static phase shifter is a local reactive power compensation device designed and manufactured specifically for large and medium-sized wound-rotor asynchronous motors, and it is connected to the motor rotor circuit. After the asynchronous motor starts operating normally, the static phase-shifter collects signals of the motor rotor current; these signals are processed by the microprocessor CPU, which then sends out trigger signals to the thyristors. An AC-AC converter composed of thyristors converts the power supply at line frequency into an additional potential with the same frequency as the motor rotor current, which is applied to the motor rotor circuit. This changes the phase relationship between the rotor current and the rotor voltage; through the magnetic field, it further alters the phase relationship between the motor stator current and the stator voltage, reducing the power factor angle and improving the motor’s power factor. The stator current decreases, which reduces the motor’s own copper losses and temperature rise, improves the motor’s overload capacity, and thus enhances the motor’s operating conditions. Since their introduction in the 1990s, static phase shifters have been widely used for reactive power compensation in various wound-rotor asynchronous motors in industrial and mining enterprises, yielding significant economic and social benefits. At the same time, some problems have arisen to varying degrees during use. Xiangyang Yuanchuang Industrial Control Co., Ltd. would like to summarize here the experience we have accumulated over the years regarding the use and modification of static phase shifters. 1. Common faults of static phase-shifters: 1.1 Thyristor breakdown can cause, when the static phase-shifter is activated, the low-voltage air circuit breaker used for protecting the primary winding of the phase-shifting main transformer to trip due to overcurrent, or the fuses to blow. Alternatively, although the phase-shifter can be activated, the current of the main motor decreases to a lesser extent than normal and there is significant current fluctuation; the analog power factor meter gives unstable readings with large fluctuations in the pointer, and the main circuit and reactors of the static phase-shifter heat up severely in a very short time. At this point, the faulty thyristor needs to be replaced. A malfunction in the 1.2 cooling fan results in poor cooling conditions for the thyristors in the static phase shifter as well as for the key components located in the radiator chamber; this leads to excessive heating of the thyristors and the radiator, accelerating their oxidation and aging process and thus **reducing the service life of the thyristors**. 1.3 Faults in the control board, trigger board, etc., result in the inability to properly activate the static phase shifter; or, when it is activated, the current of the main motor decreases to a lesser extent than normal and there is greater current fluctuation. The analog power factor meter gives unstable readings, with large swings in the pointer, and severe circulating current in the main circuit of the phase shifter. 2. Main improvement and modification measures: Based on the analysis of its common faults and causes mentioned above, we have adopted the following main improvement and modification measures: 3.1 We strictly control the quality of spare parts, such as thyristor devices and cooling fans; high-quality products from reputable manufacturers are used, and careful attention is paid to the details during replacement in accordance with the product instructions. 3.2 The control circuit of the static phase shifter has been modified; it no longer relies solely on the closing signal of the contactor that shorts the rotor circuit during the start-up of the wound-rotor motor as the condition for enabling operation. Instead, a time delay and current detection interlock have been added, allowing the phase shifter to be operated only after the motor has truly started up and the current is within normal levels. 3.3 By replacing all static phase-advance devices that use shunt samplers with those that employ high-quality Hall sensors and a nine-point intelligent algorithm, the sampling accuracy and the effectiveness of reactive power compensation can be improved.