Written by: Start Pac
Ground power units are essential tools for an aviator and his or her maintenance team. On these power units are unique circuits called limiters that can be added during the manufacturing process or engineered into the system when needed. These limiters allow the voltage of the power unit to automatically decrease as more power is drawn from the aircraft maintenance equipment piece.
Why Add a Limiter?
The limiter’s essential function is to protect the engine from becoming “over torqued” upon ignition and with a wide current load being produced from the unit, it’ll minimize the risk of arcing or malicious contact. Currents will also be limited when the starter of a turbine engine is engaged to around 1000 amps when “soft starting”.
For example, upon inrush, the limiter will soft start the turbine engine and limit the current to around the designated 1000 amps to stay connected. When the engine begins to spool up, the limiter allows the current to reach back up to the normal 24 to 28 volts.
In some cases, there are aircraft designs that demand the battery to be turned on to allow the intrusion of external power. In this case, the aircraft will draw power from both the ground power unit and the aircraft battery in a parallel fashion. Meaning, the airplane will initially draw power from the battery upon start then allow the ground power unit to provide an electrical current in a 50/50 ratio the entire time.
Why is “Soft Starting” Important?
Upon inrush of the turbine engine, the “soft start” is necessary as it controls the amount of voltage. This places less emphasis on the internal battery and draws more power from the portable GPU.
Because aircraft models are built differently, “soft starting” can vary. For example, on some aircraft, the battery requires the battery to be turned on to utilize the external power unit. In this case, the internal battery and the power battery pack will work in conjunction with each other – external power won’t be fully used.
Another example would be if the “soft start” is set to the normal position, over half of the power would be coming from the power cart and the rest from the internal battery. This external power “emphasis” prolongs the internal battery and draws more power from the external source. This method is recommended for longevity purposes. Additionally, there are aircraft where the start will be fully supplied from the external power source – 100% of the power.