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Some Summary And Discussion On The Daily Application Of Soft Starter

Sep 15, 2020

Some summary and discussion on the daily application of soft starter

1. What is a soft starter? What is the difference between it and the inverter?

   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 called Soft Starter abroad. 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.

    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 frequency converters 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 motor starting, 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.

2. What is the soft start of the motor? What are the starting methods?

    Use a soft starter connected in series between the power supply and the controlled motor to control the conduction angle of its internal thyristor, so that the motor input voltage gradually rises from zero with a preset function relationship, until the end of the start, the full voltage is given to the motor, which is Soft start, in the process of soft start, the starting torque of the motor gradually increases, and the speed also gradually increases. The soft start generally has the following starting methods.

    (1) Slope boost soft start. This kind of starting method is the simplest. It does not have current closed-loop control and only adjusts the conduction angle of the thyristor to increase it as a function of time. The disadvantage is that due to the unlimited current, in the motor starting process, sometimes a large inrush current will be generated to damage the thyristor, which will have a greater impact on the power grid and is rarely used in practice.

    (2) Slope constant current soft start. This kind of starting method is to increase the starting current gradually in the initial stage of motor starting, and keep constant (t1 to t2 stage) when the current reaches the preset value, until the starting is completed. During the starting process, the rate of current rise and change can be adjusted and set according to the motor load. If the current rise rate is large, the starting torque is large and the starting time is short.

This starting method is the most widely used starting method, especially suitable for starting fans and pumps.

    (3) Step start. Turn on, that is, in the shortest time, make the starting current quickly reach the set value, which is a step start. By adjusting the starting current setting value, a quick starting effect can be achieved.

    (4) Pulse impact start. In the starting stage, let the thyristor turn on with a relatively large current for a short period of time and then fall back, and then linearly increase according to the original set value, and connect to constant current start.

This starting method is rarely used in general loads, and is suitable for starting occasions with heavy loads and large static friction needs to be overcome.

3. What is the difference between soft start and traditional decompression start?

    The traditional decompression starting methods of cage motors include Y-q starting, auto-coupling decompression starting, and reactor starting. These starting methods are all step-decompression starting, with obvious shortcomings, that is, a secondary surge current occurs during the starting process. The difference between soft start and traditional decompression start is:

    (1) No inrush current. When the soft starter starts the motor, by gradually increasing the conduction angle of the thyristor, the motor starting current linearly rises from zero to the set value.

    (2) Constant current start. The soft starter can introduce current closed-loop control, so that the motor maintains a constant current during the starting process, and ensures the smooth start of the motor.

    (3) According to the load conditions and the selection of the power grid relay protection characteristics, it can be freely and steplessly adjusted to the best starting current.

4. What is the soft stop of the motor?

    When the motor is stopped, the traditional control method is completed by instantaneous power failure. But there are many applications where the motor is not allowed to shut down instantly. For example, if the water pump system of a high-rise building or a building is shut down for an instant, it will have a huge "water hammer" effect, which will damage the pipeline and even the water pump. In order to reduce and prevent the "water hammer" effect, the motor needs to be stopped gradually, that is, soft stop. The use of a soft starter can meet this requirement. In the pumping station, the application of soft-stop technology can avoid damage to the pumping station's "slap door" and reduce maintenance costs and workload.

The soft stop function in the soft starter is a process in which the thyristor gradually reduces the conduction angle from full conduction after receiving a stop command, and then transitions to full closure after a certain time. The parking time can be adjusted from 0 to 120s according to actual needs.

5. How does the soft starter achieve light load energy saving?

    The cage asynchronous motor is an inductive load. During operation, the current in the stator coil winding lags behind the voltage. If the working voltage of the motor is unchanged, the power factor is low when it is under light load, and it is high when it is under heavy load. The soft starter can achieve the purpose of light-load energy saving by reducing the motor terminal voltage and improving the power factor, reducing the motor’s copper and iron consumption when the load is light; when the load is heavy, increasing the motor terminal voltage to ensure the normal operation of the motor .

6. What protection functions does the soft starter have?

    (1) Overload protection function: The soft starter introduces a current control loop, so it can track and detect the change of motor current at any time. By increasing the setting of overload current and the inverse time control mode, the overload protection function is realized. When the motor is overloaded, the thyristor is turned off and an alarm signal is issued.

    (2) Phase loss protection function: When working, the soft starter detects the changes of the three-phase line current at any time, and once the current is cut off, it can respond to phase loss protection.

    (3) Overheating protection function: The temperature of the thyristor radiator is detected through the internal thermal relay of the soft starter. Once the radiator temperature exceeds the allowable value, the thyristor will be automatically turned off and an alarm signal will be issued.

    (4) Other functions: through the combination of electronic circuits, other interlocking protections can also be realized in the system. 7. What is the soft start MCC control cabinet?

    MCC (Motor Control Center) control cabinet, namely the motor control center. The soft starter MCC control cabinet is composed of the following parts: (1) Circuit breaker at the input end, (2) Soft starter (including electronic control circuit and three-phase thyristor), (3) Bypass contactor of soft starter, (4) ) The secondary side control circuit (to complete the selection and operation of functions such as manual start, remote start, soft start and direct start), with voltage, current display and fault, running, working status and other indicators.

8. Why are some soft starters equipped with bypass contactors?

    Most soft starters have bypass contactor contacts on both sides of the thyristor. The advantages are:

 (1) The control cabinet has two starting methods (direct start, soft start).

(2) When the soft start is over, the bypass contactor is closed, so that the soft starter will exit operation until it is stopped and put it back in. This prolongs the life of the soft starter, avoids harmonic pollution in the power grid, and reduces soft starters. Heat loss of the thyristor in the starter.

9. What are the extended functions of the soft start MCC control cabinet?

    The soft start MCC control cabinet can be further combined to realize multiple compound functions. For example: adding two control cabinets with control logic can form a "one-use one-standby plan" for the building's fire-fighting system, sprinkler pumps, and life pumps. If equipped with a PC (programmable controller), the fire pump can be automatically detected at a fixed time (such as half a month) and automatically shut down at a fixed time; plus the corresponding control logic, it can be implemented whether the fire pump and each system are operating normally During normal inspections, run at regular low speed and low water pressure (without water outflow); when extinguishing fires, run at full speed and full load. Combining several motors with control logic can form a living pump system or other special systems. Each motor can be turned on one by one according to the required amount, or the motors can be reduced one by one to achieve the best efficiency operation. According to customer requirements, multiple motors can be automatically converted and run each time, so that each motor is in the same operating life.

10. What occasions is the soft starter suitable for?

    In principle, cage asynchronous motors can be applied to various applications where speed regulation is not required. The current application range is AC 380V (or 660V), and the motor power ranges from several kilowatts to 800kW.

    The soft starter is especially suitable for various pump loads or fan loads, where soft start and soft stop are required.

    Similarly, for variable load conditions and long-term light-load operation of the motor, and only short-term or momentary heavy-load occasions, the application of a soft starter (without bypass contactor) has the effect of light-load energy saving.