Current Limiting Starting
As the name suggests, this method limits the starting current of the motor. It is mainly used for light-load starting to reduce the starting voltage drop. However, it's difficult to know the starting voltage drop during startup, preventing full utilization of the voltage drop range, resulting in a loss of starting torque, which is detrimental to the motor.
Ramp Voltage Starting
The voltage increases linearly from a low value to a high value. In this starting method, the starting voltage gradually increases during the initial stage of motor startup. When the voltage reaches a preset value, it remains constant until the startup is complete. This starting method is the simplest, lacking current closed-loop control; it only adjusts the thyristor conduction angle to increase as a function of time. Its disadvantage is that, due to the lack of current limiting, a large surge current may sometimes be generated during motor startup, damaging the thyristor and significantly impacting the power grid. It is rarely used in practice.
Torque Control Starting
This method linearly increases the motor's starting torque from a low value to a high value. Its advantages are smooth and flexible starting, providing better protection for the drive system and extending its service life. It also reduces the impact on the power grid during motor startup, making it the optimal method for heavy-load starting. The disadvantage is a longer starting time.
Torque with Step-Change Control Starting
Similar to torque control starting, this method is also used for heavy-load starting. The difference is that a step-change in torque is used at the moment of startup to overcome the motor's static torque, after which the torque increases smoothly. This can interfere with other loads, so special attention is required during application.
Voltage Control Starting
Used for light-load starting, this method maximizes the motor's starting torque while ensuring a controlled starting voltage drop, minimizing the starting time. It is the optimal soft-start method for light loads.