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The difference between open loop and closed loop soft starter

Dec 21, 2020

The difference between open loop and closed loop soft starter

The soft starter either operates in open loop or closed loop. The open loop design does not provide any current feedback loop. The start phase of the open loop control and the pre-selected voltage profile do not protect the motor. On the other hand, the closed loop design provides current functionality and motor protection. This combination allows the user to select the preferred current level of startup.


The starting voltage curve of the open-loop system follows a predetermined slope, and has nothing to do with the motor speed or current. The most suitable setting is for the motor to provide separation torque when starting. Although these starters make the movement more gentle, they are not able to produce any specific torque. In the case of a closed-loop starter, it monitors the output and automatically adjusts the input voltage to reach the target voltage.



Other closed-loop units only adjust the voltage to maintain a constant acceleration. There is a voltage ramp system to monitor the input current of a stage. The next step is to compare the set values. The last step involves the ramp when generating more current.


When it comes to machines that require different starting torques, such as load conveyors, current-type soft starters are preferable. This device takes the current from an initial value to a predetermined limit within a certain period of time.


In most cases, the use of 75 HP or higher applications will benefit from the soft start procedure. These include grinding machines and papermaking machinery. The equipment needs to reach full load within the specified time to prevent damage to certain equipment in the transmission system. A limited acceleration level is necessary to avoid too much gear tooth stress on the heated cylinder of the paper dryer.