In general centrifugal pumps, the power always increases with the increase of flow, that is, the power curve is a curve rising with the increase of flow, which will bring a problem to the use of the pump: when the pump operates at the design working point, generally speaking, the pump power is less than the rated power of the motor, and the use of this pump is safe; However, when the pump head decreases, the flow will increase (as can be seen from the pump performance curve), and the power will also increase.
When the flow exceeds the flow at the design operating point and reaches a certain value, the input power of the pump may exceed the rated power of the motor, resulting in motor overload and burning. When the motor is overloaded, either the protection system acts to stop the pump rotation; Either the protection system fails and the motor is burnt. The pump head is lower than that at the design working point, which is often encountered in practice. One is that the pump head is too high when selecting the pump type, while the pump head is reduced in actual use; On the other hand, it is not easy to determine the operating point of the pump in use, in other words, the pump flow needs to be adjusted frequently; Another case is that the pump needs to be used in different places frequently. These three conditions may overload the pump and affect the reliability of the pump. It can be said that for pumps without full head characteristics (including submersible sewage pumps), the scope of use will be greatly limited.
The so-called full head characteristic (also known as no overload characteristic) means that the power curve rises very slowly with the increase of flow. More ideally, when the flow increases to a certain value, the power will not rise again, but will decline. That is to say, the power curve is a curve with a hump. If so, we only need to select the rated power of the motor to slightly exceed the power value at the hump point, In the whole range from zero flow to maximum flow, no matter which operating point you are running on, the pump power will not exceed the motor power and overload the pump. For pumps with this performance, it will be very convenient and feasible to select and use them. In addition, the motor power does not need to be too large, which can save considerable equipment costs.