At the area of the clip opener on the top level is only the mounting of 90mm motors possible. But in this area a high force is needed, so on the bottom level a 135mm motor is mounted. If a 90mm motor and a 135mm are connected parallel, the induced voltage in each motor is not at the same level. This would lead again to an asymmetric current. To avoid this, each segment (top and bottom) is connected to one inverter (single zone).