Hi Ali

On large semi hermetic compressors some of the cylinders are unloaded during start up to reduce the inrush current.

After that the unloaders are energised to apply capacity control. The unloader is just a solenoid operated valve that causes the cylinder to be open so that compression cannot take place.

As far as wiring diagrams go it's just a case of supplying the solenoid valve with a live and neutral at whatever voltage you are using