In this article we will like to cover all the three equipment and try to provide you with more clarity regarding them and how they are completely different from each other. To make you understand it better, we have added diagrams as well.
DOL starter -Direct on line Starter:
DOL Starters - Here we have taken the starting current as 6 times of full load current. In this you will see how a bimetallic thermal overlay provides protection against the over load. It will lead to no reduction of starting torque during starting.
Auto Transformer Starters:
The auto transformer is always connected to the motor that is present in the delta connection. Hence, the delta connection starting voltage is reduced by 1.732 times and so is the current. The auto transformer has different tapping points, there is 60%, 70%, and 80%. The moment the motor starts the auto transformer starts adjusting the taping points to limit the starting current. Once the servo motor reaches the full speed then the tapping windings are short circuited and the line voltage is directly proportional to the motor windings.
Soft Starter
Soft starters are also thyristor drives and the rms value of the applied voltage is reduced with the help of the firing angle of power thyristor. When you reach the full speed then the thyristor reaches its full limit. In a situation like such the voltage can vary from zero to maximum value. Similarly, there is a soft-stop facility available so we can stop the motor gradually by reducing the speed.
VFD or VSD:
In VFDs (variable frequency drives) or Variable Speed Drives the ac voltage that has frequency can be converted into DC voltage and can be further converted into AC voltage with variable frequency range. In addition to that, variable frequency can also be produced by the thyristor controller. In this, the frequency can range from 0-200% and this kind of starter is more often than not used in squirrel cage induction motors.
Here’s hoping that this article helped you provide a better understanding of the equipment and in case we missed out anything, then please let us know.