The set ramp time (tStart) is not the actual acceleration time of
the drive. This is dependent on the load and the breakaway
torque. The ramp time only controls the change in the voltage.
In the process, the current rises to its maximum and then falls
to the rated current, after the rated motor speed is achieved.
The maximum current now sets to suit the drive (motor plus
load) and cannot be determined in advance. As a result, drives
subject to high loads in conjunction with long ramp times can
lead to highly excessive thermal loading of the thyristors.
If a determined current level is not to be exceeded, a soft
starter featuring a current limit must be selected. This start-up
variant is frequently stipulated by the electricity supply companies,
when large drives are connected to the public supply (e.g.
elevating pumps, fans for tunnel ventilation systems).
Soft starters also enable a time-controlled reduction of the
motor voltages and thus a controlled run down of the motors.