Open Loop Control soft starters (part 2)
Continued from the previous post: Open Loop Control soft starters (part 1), we still focus on this form of soft starter.
On the basic systems, generally the voltage ramps time is referred to as the acceleration ramp time and is calibrated in seconds. But this is not an accurate description as it does not directly control the acceleration of the motor. A lightly loaded motor can accelerate to full speed even with a sixty second ramp selected. And more correctly this should be referred to as the voltage ramp time.
On more comprehensive units, the start voltage is pre-setable, typically from 10% to 70% of full line voltage. This should be set to achieve at least breakaway torque for the motor at start. There is little advantage in the motor sitting, staining to start due to insufficient torque. This will only increase the heat dissipated in the motor.
The start voltage setting is often referred to as the start torque setting and calibrated in percent. This is a nonsense, as although increasing the start voltage is going to increase the starting torque of the connected motor, the actual starting torque is a function of both the start voltage and the motor design.
