Soft start technology of asynchronous motor


Generally, the motor needs more current than rated current when starting, which is about 6 times of rated current. Under such current, the motor will be impacted more than normal operation, which will increase the loss of the motor, reduce the life of the motor, and even damage other parts inside the machine when the current is too large. In this case, people began to pay attention to the research of soft start of motor, hoping to enable the motor to start smoothly and smoothly through related technologies.

1. Soft start principle of motor

The research of the current technology for motor soft start is mainly to control the start of three-phase AC asynchronous motor. Through the use of three-phase AC asynchronous motor, the soft start of the motor is realized, and the protection for the motor start and stop is provided. This technology is widely used in the industrial field. It has been used in industry to replace the traditional y/ △ start-up, and good results have been achieved.

SCR can adjust the voltage of soft starter. It is the voltage regulator of soft starter. When the three-phase reverse parallel thyristor is connected to the circuit, it plays a role of connection between power supply and motor stator. When clicking on start, the electric pressure inside the thyristor gradually increases, and the motor will accelerate operation slowly under the action of voltage, When the running speed reaches the required speed, the thyristor will realize full conduction. At this time, the voltage clicked is the same as the rated voltage, so that the soft start of the motor can be realized and the mechanical performance of the machine can be met. Such start-up enables the whole process to run smoothly. In this case, the motor operates normally under the protection of thyristor, which makes the motor suffer less impact and loss, so that the service life of the motor can be significantly improved and the motor can be kept in good working condition.

2. Soft start technology of asynchronous motor

2.1. Soft start of AC voltage regulation of thyristor

The soft start of AC voltage regulation of thyristor is mainly through changing the connection mode of thyristor, and changing the traditional connection mode to connect to three winding, so that the thyristor can be powered in parallel. Thyristor soft starter has strong adjustability, so users can adjust the motor according to their different needs, and make the starting mode of the motor more suitable for their own needs through corresponding changes.

2.2. Regulation principle of three-phase AC voltage regulating soft starter

The three-phase AC voltage regulating soft starter makes full use of the characteristic curve of AC voltage, and starts the motor by the characteristics of AC voltage. The main idea of soft starter is to use AC voltage characteristic curve to realize soft start of motor. The main purpose of this paper is to use three pairs of thyristors in the motor to series the motor, and change the opening time by controlling the trigger pulse and the trigger angle. In this case, the input of the motor can maintain enough voltage to control the start-up of the motor. When the motor is started, the voltage will change to rated voltage, and then the three bypass contactors will be combined, and the motor can realize grid connected operation.

3. Advantages of soft start compared with traditional start

Soft start can not only greatly reduce the starting impact of the transmission system itself, prolong the service life of key parts, but also greatly shorten the impact time of motor starting current, reduce the thermal impact load on the motor and the influence on the power grid, so as to save the power and prolong the working life of the motor. In addition, by using the "soft start" technology, the motor with smaller capacity can be selected in the selection of motor, so it can reduce unnecessary equipment investment. Star delta start is based on changing the wiring of motor winding, so as to change the voltage of motor start-up. The voltage at startup is reduced, so that the starting current becomes smaller, and the impact on bus bar is reduced during startup, so that the voltage drop of bus voltage is within the allowable range (bus voltage drop shall not exceed 10% of rated voltage), The current can also be reduced when the motor starts by the auto coupling and decompression start. It is the voltage obtained by the change of the voltage tap of the autotransformer to reduce the voltage obtained during the motor start, thus reducing the current and reducing the impact on the bus.

For example, the requirements for power grid are required for starting in 4 sets of 36 kW. The normal working current of 36 kW motor is about 70A, and the direct starting current is about 5 times of the normal current, that is, the current required for simultaneous start of four groups of 36 kW motors is 1400A; The requirements of star delta start for the grid are 2-3 times of the normal current and 560-840a for the grid current. However, the starting time will have a relatively large impact on the voltage, which is about 3 times of the normal voltage; The soft start also requires 2-3 times of the normal current, that is 560-840a, but the impact of soft start on voltage is about 10%, which will not have a great impact.