Questions about AB soft start

An bypass circuit must be provided outside the frequency converter of critical equipment; otherwise, if the frequency converter fails, the equipment will not be able to operate, which could pose a risk to the safety of the equipment or personnel.

Most soft starters have bypass contactor contacts on both sides of the thyristors, and the advantages are: (1) the control cabinet offers two starting methods (direct start, soft start). (2) Once the soft start is complete, the bypass contactor closes, taking the soft starter out of service; it is reactivated only when shutdown occurs. This not only extends the lifespan of the soft starter, but also prevents harmonic pollution in the power grid, as well as reducing the heat generation resulting from the thyristors in the soft starter. Without a bypass contactor, it is mainly used for various pump loads or fan loads that require soft starting and soft stopping (to address the water hammer effect). In situations where the load varies, or when the motor operates at light load for extended periods with only short-term or intermittent full-load operation, the absence of a bypass contactor results in energy savings due to lower load operation.

I agree quite a bit with the view above. There are fans everywhere here, which is a bit frustrating; some of them have bypass systems while others don’t – it’s likely that they weren’t made by the same company!

It is generally not recommended to use it without a bypass, as it is detrimental to long-term operation! It’s best to specify it when placing the order!

How can a soft starter function without a bypass? After the soft start process is completed, it must be switched to bypass operation at line frequency! Has the poster confused the inverter circuit with the soft start circuit?

There are many examples of soft starters operating without a bypass. For example, a soft starter can drive three water pumps or fans at once; once startup is complete, it can switch directly to operate under another control circuit. In such cases, there is definitely no need for a bypass contactor.

After startup is complete, it directly switches to operate using another control circuit; and this “another control circuit” is essentially equivalent to a bypass, right? A soft starter without a bypass contactor is no longer a soft starter in the traditional sense; it remains in operation even when the motor is running normally, rather than being taken out of service. In my opinion, its function is likely to act as a buffer to smooth out sudden changes in load. But I personally think this application isn’t very useful!

First, let’s discuss the differences between direct starting and soft starting: Direct starting: 1. The starting current is over 800% of the motor’s full-load operating current. 2. The high starting current causes a drop in grid voltage, affecting other loads. 3. Electrical components such as contactors and overload protectors must be selected to be able to handle such high surge currents. 4. In the mechanical system, the lifespan of gearboxes, fan belts, motor shafts, and water pump shafts is reduced, and wear increases. 5. Fragile products may break, crack, or scatter, and the positioning of these products may be affected. Full-voltage starting has many drawbacks. Traditional starting methods include Y-△ starting, starting with a series reactor for voltage reduction, starting with an autotransformer for voltage reduction, and extended delta starting. In all of the aforementioned methods, there is a process of coil voltage switching during startup, which thus causes a \"secondary impact\" on the power grid. A soft starter combines power electronics technology with microcomputer control technology (including computer technology). During startup, electronic circuits control the conduction angle of the thyristors, allowing the output voltage to increase gradually and thus enabling the motor to accelerate slowly. The starting voltage is raised in a continuous and smooth manner until the thyristors are fully conductive, at which point the rated voltage is reached, allowing the motor to start smoothly without any shocks ； During shutdown, the turn-off speed of the thyristor is controlled so that the terminal voltage of the motor gradually decreases from its full value to zero, thereby achieving a soft shutdown ； It can also completely solve the problem of \"secondary shock\" caused by traditional starting methods. It is recommended to use a soft starter if possible.