Starting method of electric motor

1. The difference between soft starters and frequency converters
A 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 a soft starter abroad. Its main composition is a three-phase anti 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 three-phase anti parallel thyristors, the input voltage of the controlled motor can vary according to different requirements, thus achieving different functions.

Soft starters and frequency converters are two completely different products. A frequency converter is used in places where speed regulation is required, and its output not only changes the voltage but also the frequency; A soft starter is actually a voltage regulator used when starting a motor, where the output only changes the voltage without changing the frequency. The frequency converter has all the functions of a soft starter, but its price is much higher and its structure is much more complex than that of a soft starter.

2. Soft starting of electric motors and several starting methods.

By using a soft starter connected in series between the power supply and the controlled motor, the conduction angle of the internal thyristor is controlled, so that the input voltage of the motor gradually increases from zero in a preset function relationship until the start is completed. The full voltage of the motor is given, which is called soft start. During the soft start process, the starting torque of the motor gradually increases, and the speed also gradually increases. Soft start generally has the following starting methods.
(1) Ramp boosting soft start. This starting method is the simplest and does not have current closed-loop control. It only adjusts the conduction angle of the thyristor to increase it in a certain functional relationship with time. Its disadvantage is that due to the lack of current limitation, during the motor starting process, sometimes a large surge current is generated, causing damage to the thyristor and having a significant impact on the power grid, which is rarely applied in practice.

(2) Slope constant current soft start. This starting method involves gradually increasing the starting current during the initial stage of the motor starting, and maintaining a constant value (t1 to t2 stages) when the current reaches the pre-set value until the start is complete. During the starting process, the rate of current increase and change can be adjusted according to the motor load setting. If the current rises rapidly, the starting torque is high and the starting time is short.

This starting method is the most commonly used starting method, especially suitable for starting loads such as fans and pumps.

(3) Step start. Start up, which means quickly reaching the set value of the starting current in the shortest possible time, is called step start. By adjusting the setting value of the starting current, a fast starting effect can be achieved.

(4) Pulse shock starting. At the starting stage, let the thyristor conduct at a high current for a short period of time before falling back, and then linearly increase according to the original set value, connected to constant current starting.

This starting method is less commonly used in general loads and is suitable for heavy-duty starting situations that require overcoming large static friction.

3. Differences between Soft Start and Traditional Pressure Reducing Start Methods
The traditional vacuum starting methods for cage motors include Y-q starting, self coupling vacuum starting, reactor starting, etc. These starting methods all belong to stepwise reduced pressure starting, which has obvious drawbacks, namely the occurrence of secondary surge current during the starting process. The difference between soft start and traditional reduced pressure start is:

(1) No surge current. The soft starter gradually increases the conduction angle of the thyristor when starting the motor, causing the starting current of the motor to increase linearly from zero to the set value.
(2) Constant current starting. A soft starter can introduce current closed-loop control to maintain a constant current during the starting process of the motor, ensuring smooth starting.
(3) According to the load situation and the characteristics of the power grid relay protection, it can be freely and steplessly adjusted to the optimal starting current.

4. Soft stop of electric motor.
When the motor stops, the traditional control method is achieved through instantaneous power outage. But in many applications, it is not allowed to shut down the motor instantly. For example, if the water pump system in high-rise buildings or buildings is shut down instantly, it will produce a huge "water hammer" effect, causing damage to pipelines and even water pumps. To reduce and prevent the "water hammer" effect, the motor needs to gradually stop, that is, soft stop. The use of a soft starter can meet this requirement. In pumping stations, the application of soft parking technology can avoid damage to the pump station's "flap door", reduce maintenance costs and workload.
The soft stop function in a soft starter is to gradually reduce the conduction angle of the thyristor from fully conducting after receiving a stop command, and then transition to fully closing after a certain period of time. The parking time can be adjusted from 0 to 120 seconds according to actual needs.