Is it allowable to connect some (a group) of three phase fan motors with Dahlander wiring inside in parallel? The whole group will be used for low and high speed according to request. Equal load on each motor. Maybe someone has such an experience? Common fuses for the whole group, not individual (know it is bad). The same power all of them.
Yes. Multiple motors have been connected to essentially "help each other" to drive a load in many ways for many decades. One common variation is to have both a supply and a return fan in a heating, ventilation and air conditioning (HVAC) system. You need to be careful in designing the mechanical system so that one motor is not taking all of the load or even "over driving" the other(s). Load sharing, multiple-motor systems have even been used is situations in which the loads are mechanically coupled together like two motors coupled to the same shaft, multiple motors with driving sprockets on the same chain or driving more than one pinion gear on the same ring gear. In some cases special motor control systems are used. With induction motors, the inherent slip of the motors helps with load sharing.