Soft Starter Starting Method
Starting methods of soft starter mainly include: Current-limiting start, voltage ramp start, step start, current ramp start and voltage-current-limiting double closed-loop start.
Current-limiting start mode
When the motor starts, the output voltage increases rapidly until the motor current reaches the set current-limiting value and then keeps below the set value. As the output voltage gradually increases, the motor speeds up. When the motor reaches the rated speed, the bypass contactor attracts, the output current drops rapidly to the rated current or below, the starting is thereof completed.
When the motor load is lighter or the setting value is rather larger, the maximum starting current may not reach the set current-limiting value. Current-limiting mode is generally applicable for situations that has strict restrictions on starting current.
Voltage ramp start
When the motor starts, the soft starter output voltage rises rapidly to the starting voltage at start-up under a premise that the motor current does not exceed 4 times of the rated value, then output voltage gradually rises according to the set starting parameters, the motor accelerates steadily with the rise of voltage. When the voltage reaches the rated voltage and the motor reaches the rated speed, the bypass contactor attracts, the starting is thereof completed.
Generally, the voltage ramp start mode is applicable for situations that don't have strict demand for starting current but have higher requirements for stable starting.
In some heavy load occasions, the motor can't start due to mechanical friction, the step mode can be thereof selected. When starting, add a higher fixed voltage to the motor for a certain of period, so as to overcome the static friction force of motor load and rotate it, then start the motor by current-limiting start mode or voltage ramp start mode.
Before using this mode, non step start mode shall be adopted firstly to start the motor. If it can't turn for large static friction force, choose this method then. Otherwise, this method shall be avoid to reduce unnecessary high current impact.
Current ramp start
This mode has strong acceleration ability and is suitable for two-stage motor, it can shorten starting time within a certain range.
Voltage-current-limiting double closed-loop start
Voltage ramp and current-limiting double closed-loop control is a comprehensive start mode requiring both stable starup and strict current-limiting, prediction algorithm for estimating motor working state is adopted.
The soft starter output voltage waveform under this mode will vary according to the motor and load conditions.