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Detailed explanation of some mistakes often made when using soft starters daily

Sep 23, 2020

Detailed explanation of some mistakes often made when using soft starters daily

In the application of soft starters, some users often have such misconceptions:


1. It is believed that whether the motor can start successfully is related to the performance of the soft starter, and a good brand or even a foreign brand should be selected under heavier load.


2. When the motor cannot start the load, it is considered that the capacity of the soft starter is insufficient, and the capacity of the soft starter should be increased.


3. It is believed that a certain starting method is particularly suitable for "heavy load" starting.


4. It is believed that the smaller the starting current, the better the performance of the soft starter that reduces the voltage of the power grid.


These wrong understandings are due to the lack of understanding and understanding of the principle of the motor soft starter. Below we will analyze the above problems one by one.


The first wrong concept: Whether the motor can start successfully is related to the performance of the soft starter. This problem needs to be analyzed in two aspects. The first thing to see is that the quality of the products of a good brand is relatively good, so in terms of reliability, the probability of failure of a good brand is relatively small. However, as far as the nature of the starter motor load is concerned, the starting torque of the motor plays a decisive role, and for the motor torque, it is related to many factors. As a voltage regulator, the motor soft starter can only change the control value of the motor stator voltage, and the motor torque is proportional to the square of the stator voltage. Changing the stator voltage also changes the motor torque. This basic principle It cannot be changed. The soft starters of various brands are designed based on this principle, and have nothing to do with their product performance or quality. Therefore, whether it is a domestic brand or a foreign brand, the starting torque is basically the same. As long as the soft starter is not malfunctioning, if the motor cannot start the load, switch to another better brand. The product is useless.


The second wrong concept: Whether the motor can start successfully is related to the capacity of the soft starter. This problem actually confuses the capacity of the soft starter with the capacity of the motor, or confuses the capacity with the driving torque. In fact, these are two different concepts. For the motor, the capacity and the drive torque are of the same nature. The large motor capacity also reflects that it can output a greater drive torque. For soft starters, you can't think so. The capacity of soft starters only represents the maximum starting capacity it can withstand. The actual output torque depends entirely on the motor itself. Qualitatively speaking, the soft starter is only a power source, not a driving mechanism, and cannot provide driving torque. No matter how large the soft starter capacity is, but the motor capacity is too small, it still cannot start successfully. It should be noted that the soft starter, as a reduced voltage starting device, will inevitably reduce the starting torque and fail to reach the rated torque. This is determined by the principle of the motor and has nothing to do with the soft start brand and its own control performance. Generally speaking, for loads with a torque less than 60%, a soft starter should be used, and for loads with a soft starting torque greater than 60%, other starting equipment should be used.


The third misconception: a certain starting method is particularly suitable for heavy-duty starting. It should be said that the ability to be qualified for "heavy load" has nothing to do with the soft start mode. "Heavy load" requires the motor to produce a large starting torque at low speed. For this reason, it must be able to flow a large enough current through the stator winding of the motor, which has nothing to do with which soft start method is selected. If the current of the motor is not large enough, no matter what soft start method is adopted, the soft start will be unsuccessful. On the contrary, as long as the current is large enough, through other control methods, after giving it a better shape of the reference input time curve, it can also achieve successful starting and have better starting characteristics.


The fourth wrong concept: the smaller the starting current, the better the performance of the starter with the lower the voltage drop of the grid. This question is actually the same as the second question. The size of the motor starting current depends entirely on the load. The motor starting current must be greater than the motor current determined by the drive torque required by the load to start successfully. On the premise that the power grid is unchanged, the voltage drop of the power grid depends entirely on the starting current of the motor. It can be seen that the above two indicators have nothing to do with the soft starter itself.