Additional start unloading (SU)

The starting modes described above often reduce the torque of the motor to a value that allows starting only with small pressure differences. Therefore, start unloading (SU) may be necessary (in addition to reducing the starting current).

Even when operating with a frequency inverter, start unloading may be necessary, e.g. in case of heavy starting conditions or if the frequency inverter lacks the necessary overload capacity (see the recommended factors in the documents KT-420, ST-420 and EST-420, chapters "Selection with the Bitzer Software"). Varipack frequency inverters provide sufficient overload capacity, and the compressor start has been optimised also for heavy conditions. For these, no additional start unloading is necessary, and the safe start is accounted for when choosing the frequency inverter in the Bitzer Software.

Start unloading is implemented differently for reciprocating and screw compressors, and it can be integrated in the compressor or implemented externally. Under certain conditions, an integrated capacity control can also be used as start unloading.