In the application of soft starters, some users often have the following misconceptions:
1. It is believed that whether the motor can start successfully is related to the performance of the soft starter. Under heavy load conditions, a good brand, or even a foreign brand, should be selected.
2. When the motor fails to 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. Some believe that a certain starting method is particularly suitable for "heavy load" starting.
4. It is believed that the soft starter with the smaller the starting current and the less voltage drop across the power grid, the better its performance.
These misconceptions arise from a lack of understanding of the principles of motor soft starters. We will now analyze each of these issues in detail.
The first misconception: Whether a motor can start successfully is related to the performance of the soft starter. This question needs to be analyzed from two aspects. First, it's true that reputable brands generally have better product quality, and therefore, in terms of reliability, reputable brands are less likely to fail. However, the decisive factor in starting a motor load is the motor's starting torque, which is related to many factors. A motor soft starter, as a voltage regulator, can only change the stator voltage—one control quantity. Motor torque is proportional to the square of the stator voltage; changing the stator voltage changes the motor torque. This fundamental principle cannot be altered, and all brands of soft starters are designed based on this principle, regardless of their product performance or quality. Therefore, whether it's a domestic or foreign brand, the starting torque is essentially the same. Unless the soft starter itself is faulty, if the motor cannot start the load, switching to a better brand will be useless.
The second misconception: Whether a motor can start successfully depends on the capacity of the soft starter. This question actually confuses the capacity of the soft starter with the capacity of the motor, or rather, the capacity with the driving torque. These are two different concepts. For motors, capacity and driving torque have a certain affinity; a larger motor capacity reflects a greater output driving torque. However, this is not the case for soft starters. The capacity of a soft starter only represents its maximum starting capacity; the actual output torque depends entirely on the motor itself. Qualitatively speaking, a soft starter is merely a power source, not a drive mechanism, and cannot provide driving torque. Even with a large soft starter capacity, if the motor capacity is too small, it will still not start successfully. It should be noted that as a reduced-voltage starting device, the starting torque of a soft starter will inevitably be reduced, falling short of the rated torque. This is determined by the motor's operating principle and is unrelated to the brand or control performance of the soft starter. Generally speaking, for loads with less than 60% of the rated torque, a soft starter is suitable; for loads with a soft starter torque greater than 60%, other starting devices are recommended.
The third misconception: that a certain starting method is particularly suitable for heavy-load starting. It should be said that the ability to handle "heavy loads" is unrelated to the soft-start method. "Heavy loads" require the motor to generate a large starting torque at low speeds, which necessitates a sufficiently large current flowing through the motor's stator windings. This is irrelevant to the choice of soft-start method. If the motor current is insufficient, soft starting will fail regardless of the method used. Conversely, as long as the current is large enough, with other control methods and a well-shaped reference input time curve, successful starting with good starting characteristics can be achieved.
The fourth misconception: A starter with a lower starting current and lower grid voltage drop is considered to have better performance. This is essentially the same as the second misconception. The starting current of a motor depends entirely on the load; the starting current must be greater than the current required by the load's driving torque for the motor to start successfully. Furthermore, assuming the grid conditions remain constant, the grid voltage drop depends entirely on the starting current of the motor. Therefore, both of these indicators are irrelevant to the soft starter itself.
