Why can the starting current be reduced by closing the outlet valve when starting a centrifugal pump?

When the centrifugal pump starts, closing the outlet valve does not reduce the starting pressure. The pressure of the pump here should be the largest. Closing the valve reduces the load during startup.

Closing the outlet valve is to reduce the starting current and protect the motor from overload tripping. When the outlet valve is closed, the operating current is the smallest and the starting current is also the smallest. If the outlet valve is opened, the corresponding starting current will also increase.

The poster can take a look at the characteristic curve of the centrifugal pump. Generally, the shaft power of a centrifugal pump is minimum when the flow rate is 0. The purpose of starting by closing the outlet valve is to make the starting current relatively minimum, mainly to protect the electrical system, and also to start the pump shaft at the minimum torque.

According to the flow curve of the centrifugal pump, when the outlet is closed, the centrifugal pump is at the highest pressure head point. At this time, the motor power is minimum, and the voltage is constant. P=UI, so the current is minimum. According to the principle of mechanics, when the outlet valve is closed, the liquid in the pump cavity will not be lifted, but will make a circular motion in the cavity. The impeller only needs to overcome the frictional resistance of the liquid in the cavity, and does not need to provide a linear speed for the liquid. Therefore, the shaft power is minimum, the motor power is also minimum, and the current is also minimum. Of course, if the outlet valve is fully open, the motor power will be maximum. If the motor power exceeds the current water pump configuration, it will cause problems with the seal and the shaft.

The poster looked at the chemical engineering principles. Turning down the outlet valve is mainly to ensure that the starting current is not too large:)

To put it simply, the least work is done at this time

1: From the perspective of energy transfer, no work is done, so the current will be very small. ； 2: When the outlet is closed, the pump is idling, and the shaft power is theoretically 0, so the current is very small. ； 3: From the perspective of motors, the current coil of the stator generally generates electromagnetic action to drive the rotor. The rotor shaft power is 0, so the electromagnetic force required to overcome the rotation of the rotor is very small. Naturally the power in the stator is very small. Naturally, the power of the motor is very small. This is the same principle as the motor idling by itself. ； I don’t know if that’s right or not, it’s my personal opinion.

According to the characteristic curve of the centrifugal pump, when the pump flow rate is 0, the power consumption is minimum and increases with the increase of flow rate. At the moment when the centrifugal pump starts, affected by the instantaneous dry friction of the rotor, the starting current will be too large. If the outlet valve is fully opened and operated at full flow, it may cause an overcurrent trip.