Soft Start on Ground Power Units – What is it and do I need this option?
This is a limiter that can be either added into the power cart during manufacturing or at a later date. It limits the output current of the unit and will allow the voltage to automatically decrease as more current is drawn from the unit.
Its purpose is to soften the inrush of current when the starter of a turbine engine is first engaged to protect the starter from being over torqued and the solenoid contacts from arcing. Starting current will be limited to approximately 1000 amps when using the “soft start”.
On inrush, the “soft start” drops the voltage as needed to limit the current to around 1000 amps in order to stay connected. As the engine spools up, the voltage will return to 28 volts. On some aircraft, the battery must be turned on in order to be able to inject external power into the start system. In this type of arrangement the aircraft batteries and power cart are in parallel. On a start, the engine draws power from both the batteries and the power cart. If the “soft start” is switched on the batteries will furnish about 80% of the power and the power cart about 20%. With the “soft start” switched in normal position, or out of the circuit, the reverse is true, 80% will come from the power cart and 20% from the aircraft batteries as it should. On aircraft where the start can be supplied from external, the power cart will supply 100% of the power.