The main circuit of the soft starter uses the thyristor. By gradually changing the opening angle of the thyristor, the voltage is raised to complete the starting process. This is the basic principle of the soft starter. In the low-voltage soft starter market, there are many products, but the medium-voltage soft starter products are still relatively few.
The basic principle of medium-voltage soft starter is the same as that of low-voltage soft starter, but there are the following differences between them: (1) Medium-voltage soft starter works in high-voltage environment, the insulation performance of various electrical components is better, and the anti-interference ability of electronic chip is stronger. When the medium-voltage soft starter is formed into an electric cabinet, the layout of electrical components and the connection with the medium-voltage soft starter and other electrical equipment are also very important. (2) The medium voltage soft starter has a high performance control core, which can process the signal timely and quickly. Therefore, the control core generally uses high-performance DSP chip, rather than low-voltage soft starter of the MCU core. The main circuit of low voltage soft starter is composed of three inversely parallel thyristors. However, in the high-pressure soft starter, multiple high-voltage thyristors in series are used for voltage division due to the insufficient voltage resistance of a single high-voltage thyristor. But the performance parameters of each thyristor are not completely consistent. The inconsistence of thyristor parameters will lead to the inconsistence of thyristor opening time, which will lead to the damage of thyristor. Therefore, in the selection of thyristors, the thyristor parameters of each phase should be as consistent as possible, and the component parameters of RC filter circuit of each phase should be as consistent as possible. (3) The working environment of medium-voltage soft starter is prone to various electromagnetic interference, so the transmission of trigger signal is safe and reliable.
In the medium-voltage soft starter, the trigger signal is usually transmitted by optical fiber, which can effectively avoid various electromagnetic interference. There are two ways to transmit signals through optical fibers: one is multi-fiber, and the other is single-fiber. In the multi-fiber mode, each trigger board has one optical fiber. In single-fiber mode, there is only one fiber in each phase, and the signal is transmitted to one main trigger board, and then transmitted to other trigger boards in the same phase by the main trigger board. Since the photoelectric transmission loss of each optical fiber is not consistent, the single optical fiber is more reliable than the multi-optical fiber from the perspective of trigger consistency. (4) Medium-voltage soft starter has higher requirements for signal detection than low-voltage soft starter. There is a lot of electromagnetic interference in the environment where the medium-voltage soft starter is located, and the vacuum contactor and vacuum circuit breaker used in the medium-voltage soft starterwill produce a lot of electromagnetic interference in the process of breaking and closing. Therefore, the detected signal should not only be filtered by hardware, but also by software to remove the interference signal. (5) After the soft initiator completes the startup process, it needs to switch to the bypass running state. How to smoothly switch to the bypass running state is also a difficulty for the soft initiator. How to select the bypass point is very important. The early bypass point, the current shock is very strong, even under low voltage conditions, will cause the three-phase power supply circuit breaker trip, or even damage the circuit breaker. The harm is greater under high pressure conditions. The bypass point is late, and the motor jitter badly, which affects the normal operation of the load. Therefore, the bypass signal hardware detection circuit is very, and the program processing should be just right.
Post time: Jun-05-2023