Generally the current required to start the ac induction motor is much higher than the rated current, about 6 times the rated current.
At such a current, the electric motors will be subjected to greater shocks than during normal operation, which will increase the induction motor’s losses, reduce ac electric motors life and even cause damage to other parts of the machine when the current is too high.
In such circumstances, people are beginning to attach importance to the study of soft starting of electric motors, hoping that the relevant technology will enable induction motor to start smoothly and gently.
1. The principle of electronic soft starter
Existing technology for soft starting of electric motor is mainly to control the smooth starting of three-phase AC asynchronous motors, through the use of three-phase AC asynchronous motors to achieve soft starting of ac induction motor and to provide protection for motor starting and stopping of motors.
Such technology is widely used in industry, where it has been used to good effect to replace the traditional Y/△ start.
The three-contrast shunt thyristor (SCR) regulates the voltage supplied of the soft starter and is the regulator of the soft starter.
When the SCR is connected to the circuit, it acts as a link between the full power supply and the motor stator.
When the smooth acceleration start is clicked, the starting voltage inside the thyristor gradually rises, and the motor will then slowly accelerate under the action of the full voltage.
When the running speed reaches the required full speed, the thyristor will achieve full conduction, at which point the voltage of the click is the same as the rated voltage.
This enables the soft start of the motor and is in line with the mechanical properties of the machine, and such a start allows the whole process to run smoothly.
In this way, the ac motor runs normally under the protection of the thyristor, allowing the electric motor to be subjected to smaller shocks and losses, which can significantly improve the service life of the induction motor and allow it to maintain good working conditions.
2. Soft start technology for asynchronous motors
2.1, Thyristor AC voltage regulated soft start
Thyristor AC soft starters is mainly through changing the connection of the thyristor, the traditional connection to the three windings, so that the thyristor power supply in parallel.
The thyristor soft starter is highly adjustable, so users can make appropriate adjustments to the electric motor according to their different needs and make the motor acceleration start in a way that is more suitable for their needs.
2.2. Adjustment principle of three-phase AC regulator soft starters
The three-phase AC voltage regulator soft starter makes full use of the characteristic curve of the AC voltage to start the motor through the characteristics of the AC voltage, and the idea of using the AC voltage characteristic curve to achieve a soft start for the electrical motor is the main idea of the motor soft starter.
The main idea is to use three pairs of thyristors in series with the electric motor protection and to change the opening time by controlling the size of the trigger pulse and trigger angle.
In this way, the electrical motor input maintains sufficient line voltage to control the start of the electric motor.
Once the variable frequency drives motor has started, the initial voltage will change to the rated voltage, at which point the three bypass contractors will come together and the motor will be able to run on the grid.
3、The advantages of soft start over traditional start
“Soft start” can not only significantly reduce the start-up impact on the drive system itself and extend the service life of key components.
But also greatly reduce the impact time of the electrical motor start-up current, reduce the thermal shock load on the electric motor and the impact on the grid, thus saving power and extending the working life of the motor.
In addition, by using “soft start” technology, smaller capacity motors can be used in the selection of motors, thus reducing unnecessary investment in equipment.
Star-delta starting relies on changing the wiring of the motor windings, thus changing the voltage at which the motor starts.
The line voltage applied at start-up is reduced so that the starting current becomes smaller and the impact on the busbar is reduced.
So that the supply voltage drop on the busbar at start-up is within the permissible range (requiring the busbar voltage drop to be no more than 10% of the rated voltage).
This reduces the impact on the busbar.
For example, in the case of 4 groups of 36 kW starters, the normal operating current of a 36 kW motor is about 70 A.
The direct start current is about 5 times the normal current, i.e. four groups of 36 kW motors start at the same time and the required current is 1400 A.
Star-delta start for the grid requirements is 2-3 times the normal current, the requirements for the grid current is 560-840A, but the start for the voltage will have a relatively large impact, equivalent to about 3 times the normal voltage.;
And soft start for the grid requirements is also 2-3 times the normal current, that is, 560-840A, but the soft start for the voltage impact is about 10%, basically no big impact.
This will not have a major impact.
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