Power Circuit
Control Circuit
Circuit Operation
When the start button of the soft start starter is pressed its contacts a3-a4 close, the contactor coil C is energised which causes the main contacts on terminals 1-2, 3-4 and 5-6 to close, it’s auxiliary contact on terminals 13-14 also closes. The auxiliary contact maintains power to the contactor coil when the start button is released. Power is connected from the power supply to the soft start module which then ramps up the voltage depending on the settings of the module. The modified voltage outputs from the soft start module to the motor terminals and the motor will start and accelerate to full speed as dictated by the voltage output of the soft start module.
To stop the motor the stop button is pressed its contacts a1-a2 open which de-energises the contactors, thus stopping the motor by removing its power supply and resetting the control circuit.
There are many variations on the control of the soft start module some require their own auxiliary supplies and control signals, these should be in addition to those shown in the control diagram and will be device specific.
A key feature of a contactor is that when the correct contactor for the motor and electrical system is selected it operates only when power is applied to its coil and even when subjected to through faults, the contacts still operate without welding. When power is removed from it’s coil the normally open contacts open and will have the appropriate isolating distance between the fixed and moving contacts, this is not true of electronic components. Electronic components can fail either open or closed circuit, they therefore do not meet the isolating distances required by regulations. It is also necessary to ensure that the protective device to the soft start module will limit the fault level to the module withstand level at the system fault level where the soft start module is installed.