The voltage rating of the soft start module must be equal to or greater than the electricity supply voltage.
The soft start module must have type 2 coordination when used in combination with the selected fuses, contactors and overloads.
The soft start module must have a minimum current rating equal to or greater than the motor full load current.
The soft start module must have a short time withstand (Icw) in excess of the fuse time-current characteristic.
The over riding consideration is the type 2 coordination as this will give the minimum soft start module rating, the motor full load current, starting method and the projected frequency of operation should then be taken into account to determine the module size to be used.
The soft start module must be of the type that does not utilise the protective earth conductor, or other earth connection as a circuit conductor, nor shall the protective conductor be used as a reference point for the modification of the waveform, unless an isolating transformer is incorporated in such manner that the supply phases are galvanically isolated from the soft start module. The maximum earth leakage from the soft start module and any RFI or EMC filters forming part of the starter power circuit shall not exceed one ten thousandth of the nominal current rating of the soft start module. For example if the rated current is 65A then the maximum permitted leakage current is 6.5mA per phase to earth.
Soft start modules that use waveform modification to control the motor current will introduce harmonics into the motor supply, and those that use pulse techniques will also produce harmonics but to a lesser extent, those harmonics will lead to increased heating within the windings of the motor and may reduce the motor life.
It should be noted that electronic components can fail either open or closed circuit, and 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 soft start module fault withstand level at the system fault level where the soft start module is installed.
It is also necessary to be mindful of the need to keep the heat gain within the starter from the soft start module to a minimum. As with all electronic devices the soft start module does not operate at 100% efficiency, those losses are reflected as heat gains within the starter.
By using a soft start module that is in series with the contactor and one that is self powered the standard control system can be employed.
Where the module requires external inputs for control these should be derived in addition to the contactor control circuits. Under no circumstances should the stop or emergency stop circuits be routed via the soft start module these circuits must act directly on the contactor.
Great care needs to be taken when it is determined to use soft start as the starting method as it indicates an unusually large motor for the size of installation or a particularly weak PES supply. It should not be used as the method of choice to avoid proper consideration being given to other starting methods.