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The Working Principle Of Soft Starter Of Three-phase Motor And The Difference Between Soft Starter And Inverter

Sep 17, 2020

The working principle of soft starter of three-phase motor and the difference between soft starter and inverter


The soft starter is a device used to control the start and stop of the AC squirrel cage asynchronous motor, which is a step-down starting method. But what is different from the traditional step-down start method is that the soft start device is a stepless step-down start.


At present, the soft start device with relatively mature technology and most used is the thyristor series soft start device.


The thyristor series soft starter is actually a thyristor AC voltage regulator, using modern power electronic technology and single-chip technology as the core, and its main circuit uses three sets of anti-parallel thyristor series groups, which are connected in series on the motor three-phase power supply line , By controlling its trigger pulse and changing the trigger angle of the thyristor, the AC three-phase power supply can be chopped, the amplitude of the output voltage is controlled, and the input voltage of the motor is changed, so as to control the soft-start process of the motor. When the motor is started, the three-phase bypass contactor is automatically closed, and the motor is put into power frequency operation of the power grid. During the entire start-up process, the output of the soft starter is a smooth boost process (and can have a current limiting function), until the thyristor full conduction motor works at the rated voltage.


Soft starter is a novel motor control device that integrates motor soft start, soft stop, light load energy saving and multiple protection functions. It is the international common term SoftStarter. Its main composition is a three-phase parallel thyristor and its electronic control circuit connected in series between the power supply and the controlled motor. By using different methods to control the conduction angle of the three-phase parallel thyristor, the input voltage of the controlled motor can be changed according to different requirements, and different functions can be realized. Soft starters and inverters are two completely different products. The frequency converter is used where speed regulation is required, and its output not only changes the voltage but also changes the frequency at the same time; the soft starter is actually a voltage regulator, when used for starting the motor, the output only changes the voltage without changing the frequency. The frequency converter has all the soft starter functions, but its price is much more expensive than the soft starter, and the structure is much more complicated.


The soft starter is used in the occasions that need to reduce the voltage to start and stop. The rotation speed of the electrode does not change.


The frequency converter is used in places where speed and constant voltage are required, and the frequency determines the speed.


The biggest difference between the soft starter and the inverter is that the inverter can set the operating frequency arbitrarily, while the starter only plays the role of soft start and soft stop


The inverter changes the output frequency and voltage at the same time, that is, changes the n0 on the motor running curve, so that the motor running curve moves down in parallel. Therefore, the inverter can make the motor start with a smaller starting current, and at the same time make the motor start torque reach its maximum torque, that is, the inverter can start a heavy load.


Soft start only changes the output voltage and does not change the frequency, that is, it does not change the n0 on the motor operating curve, but increases the steepness of the curve to soften the motor characteristics. When n0 does not change, each torque of the motor (rated torque, maximum torque, locked-rotor torque) is proportional to the square of its terminal voltage, so soft start can greatly reduce the motor's starting torque, so soft start is not It is not suitable for heavy-duty starting motors.


The soft starter is a piece of equipment that is not used after the motor is started for a short time. The cost of low power is relatively high. Unless it exceeds 100kw.


In terms of function, the inverter cannot be replaced by a soft start. It is also absolutely different in terms of function because of the difference in the working principle. You can see the relevant technical data to understand.


Here I only mention one very key thing: the inverter can realize constant torque start, that is, at low speed, it can have the same torque as high speed, but soft start cannot be realized.