“…When the operating point reaches the left of point B which is the highest point of the curve, the outlet pressure and the air volume lose correspondence, and the fan becomes unstable, it will lead to the counter-rotating fan's surge phenomenon and reduce equipment operation safety. Meanwhile, considering the grid voltage fluctuation may cause the speed decrease, thus we require the pressure in the operating point must not exceed the 90% of the maximum static pressure [14][15]. In addition, the Figure 12 also shows that the counter-rotating fan's stable operating zones are different in different setting angles.…”