Why is it necessary to reduce the motor starting current by closing the outlet valve when starting a centrifugal pump?
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When the valve is closed, resistance increases – shouldn’t that lead to an increase in current?
There is a flow power curve; the power is at its minimum when the flow is zero
The load on a centrifugal pump is related to its flow rate; closing a valve reduces the flow rate, which in turn means that the load on the motor decreases, and as a result the current drops
Don’t ask why; this is the truth of practice
The flow rate is low when the power is low; this is a characteristic of centrifugal pumps
If the outlet is small, the flow rate is low, less work is done, and naturally the current decreases
When starting a centrifugal pump, the outlet valve is closed to reduce the motor’s starting current; once the motor current level is monitored, the valve is gradually opened further. The degree of valve opening is determined based on the motor’s rated current, in order to prevent the motor from being overloaded. This is the most basic requirement for factory pump operators.
The direct purpose of closing the outlet valve when starting a centrifugal pump is not to reduce current, but rather because only by closing the outlet valve to a certain degree can the centrifugal pump draw in the fluid it is meant to handle.
When starting a centrifugal pump, the outlet valve is closed to reduce the motor’s startup time; this does not lower the motor’s startup current. If the outlet valve is not closed, the startup time may increase, and the motor may overcurrent and trip as a result. After the outlet valve is closed and the startup process is complete, the motor current is at its no-load value, which is quite low. By monitoring this motor current value, the opening degree of the pump’s outlet valve can be increased gradually, thereby applying load slowly and preventing overcurrent. Thus, the pump startup is completed.