The rotor current at the start of the three-phase asynchronous motor is usually 5 ~ 8 times of the rated state. Since the rotor current is induced from the stator windings, the current in the stator windings is correspondingly increased to 4 ~ 7 times of the rated value.Such a large starting current will have the following adverse consequences: it will cause the grid voltage to fluctuate (especially when the larger capacity motor starts). Then what are the differences between the soft start of the motor and the traditional reduced voltage start?
(1) no impulse current.When the motor is started by the soft starter, the starting current of the motor increases linearly from zero to the set value by gradually increasing the thyristor conduction Angle.(2) constant current start.The soft starter can introduce the current closed-loop control to make the motor maintain constant current during the starting process and ensure the motor to start smoothly.(3) according to the load situation and the selection of the relay protection characteristics of the power grid, it can be adjusted to the optimal starting current freely and continuously.→→ soft starter is better than star - triangle starter and self - coupling decompression starter, is because it has many functions, have "intelligence", can achieve man-machine "dialogue", through the keyboard, computer or manual remote operation, not only can achieve, soft stop, but also has a lot of protection functions.These are the requirements of the new humanized device, so it is highly recommended.
Soft start runs have the following characteristics.
(1) the starting voltage of the motor can rise steadily with a constant slope, the starting current is small, and there is no impact current on the power grid, thus reducing the mechanical impact on the load.
(2) the starting voltage rising slope can be adjusted to ensure the smoothness of the starting voltage. The starting voltage can be adjusted continuously within the range of (30% ~ 70%) gong (gong is the rated voltage) according to different loads.
(3) startup time can be set according to different loads.
(4) in order to adapt to modern production, reduce the impact on the power grid and mechanical wear, reduce the maintenance of the start-up device, the soft starter also has SCR short-circuit overload protection, phase fault protection, overheating protection and other protection functions.
Overload protection function: the soft starter adopts the current control ring to track and detect the changes of motor current at any time. The overload protection function is realized by increasing the setting of overload current and the reverse time control mode. When the motor is overloaded, the thyristor is turned off and the alarm signal is sent.
Lack of phase protection function: when working, the soft starter can detect the change of three-phase current at any time.
Overheating protection function: the temperature of thyristor radiator is detected by the thermal relay inside the soft starter. Once the temperature of the radiator exceeds the allowable value, the thyristor is shut off automatically and an alarm signal is sent.
Other protection functions: through the combination of electronic circuits, other kinds of interlocking protection can be realized in the system.
Generally speaking, the soft starter can be used in various applications where the asynchronous motor does not need to adjust the speed. The soft starter is suitable for various pump loads or fan loads.For variable load conditions, when the motor runs under light load for a long time and only has a short or instantaneous full load, the application of soft starter (without bypass contactor) can save energy under light load.
There are three types of soft starter for three-phase motor.
(1) power saving: online operation, suitable for motor load rate is not high or intermittent load of the drive system, such as shearing machine, press, press, etc., its active power power saving rate can be up to 20% ~ 40%.The lower the stamping frequency, the higher the active power saving rate. If the load rate of the motor exceeds 40%, the power saving effect is not obvious.The panel is equipped with LED display light and operating keyboard, which can display and modify system operating parameters, which is convenient and fast.
(2) bypass: is a unit as the main control device composed of soft start control equipment, its wiring mode for the down line, down line.Based on the single-chip microcomputer control technology, the w melon power-saving soft starter dynamically adjusts the voltage and current in the motor operation process through the built-in special optimization control software.Without changing the speed of the motor, the output torque of the motor is guaranteed to match the load demand, and the active power saving rate of the motor is up to more than 50%.It not only has perfect soft start and soft stop functions, can ensure the motor continuous smooth start, to avoid the motor to start the current and mechanical impact;It also has the protection functions of phase fault, overcurrent, overload, three-phase unbalance, thyristor overheat, power supply inverse equalization, and the functions of fault state output (BK), operation state output (TR), which can be used to control other interlocking devices.
(3) cell type: using the thyristor on line operation mode, is the three-phase asynchronous motor soft start control of the most basic unit;The output terminal of bypass signal is provided, that is, the contact of bypass relay is closed after the start and completion, and the bypass operation of contactor is controlled;Because of the patented technology of "electronic arc extinguisher", no arc is generated when the contactor is absorbed and divided.Equipped with fault signal output terminal, the system will stop automatically in case of system failure and fault relay contact will be closed, and other interlocking devices will be controlled to realize protection.The connection form is the up and down wire.When this unit is selected, the user needs self-matching current closed-loop sampling transformer, bypass contactor, start and stop buttons to be used normally.