Kick start and soft start are two different ways to start up a motor or fan.
The best acceleration method depends on your application. Applications with high inertia might need a higher torque at start up to avoid stalling of the motor.
Kick start — the motor will accelerate immediately from standstill towards maximum speed.
The full motor torque is almost immediately available. After this start-up period (typically 8 – 10 s), the motor will decelerate towards the requested fan speed setpoint. This starting method is often used to avoid motor stalling at low speed. The disadvantage is the mechanical stress at start-up and a high motor start current.
Soft start — the motor will smoothly accelerate from standstill towards the requested fan speed setpoint.
This starting method gives you the advantage of reduced mechanical stress and lower motor starting currents. Due to the reduced motor torque during start up, this acceleration method is not ideal for high inertia applications.