Reasons for overcurrent tripping of the pump at the compressor water station
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The newly built compressor water station is being tested. The motor has a power of 11 KW and a rated current of 20 A. When the outlet valve is not opened, the pump’s current remains at 20 A; however, when the valve is turned two full turns, the current rises to 30 A, causing the protection system to trip. The pump rotates smoothly, and there are no abnormal noises during operation. What could be the cause of this?
If the pump can be turned easily with no problems, then the associated pipelines are too large
If this occurs with a centrifugal pump, you need to analyze the reasons for it. Closing the outlet valve at startup means that no flow is delivered; therefore, the motor’s output power should be low. Yet the actual current drawn by the pump has reached its rated value, which indicates that there is an excessive gap in the outlet valve, bypass valve, drain valve, or the pump’s inlet ring, resulting in significant internal leakage in the pump. It also suggests that the drive mechanism may not be compatible with the pump. It is recommended that you investigate the issue step by step.
Step-by-step troubleshooting: 1. Disconnect the coupling – what is the current drawn by your motor when it rotates alone? Generally, around 8A is considered normal (for bipolar motors, the current should be higher if the number of poles increases); if it exceeds 10A, it can basically be attributed to issues with the motor itself ； 2. If the coupling is in good condition, then it is necessary to check the pump’s test report from the time of manufacture, and to examine the performance curves from those tests in order to confirm the suitability of the pump and the motor together.
It can be checked based on the design parameters to see if there is an issue with the motor matching; otherwise, it should be a leakage problem with the outlet valve.
11KW belongs to a small pump; check the protection setting values of the motor. The starting current of an AC asynchronous motor is very high (5-8 times the rated current), and the current varies during the starting phase. Based on my experience, the protection setting should be set at 1.2 times the rated current, and care should be taken to avoid continuous starting of the motor.
This post was last edited by YouyanBuzijue on 2015-12-2 at 10:49. The testing process you’re using now must be in a loop; without backpressure at the outlet, opening all the valves will cause the system to shut down. The performance tests should have been carried out before leaving the factory, and with a report issued, there should be no problems. During on-site testing, to reach normal operating conditions – that is, with backpressure – the valves must be fully open as required. In the absence of back pressure, fully opening the valve will result in excessive flow, causing the motor to be overloaded; therefore, the valve should only be opened to the rated current, and the operation status of the equipment should be monitored. As long as the rated current is reached and the operating condition is normal, there is no problem.
Oh, I overlooked one thing: with a rated current of 20, the current when starting with the outlet completely closed is still 20! Starting a centrifugal pump with the inlet open and the outlet closed represents starting under minimum load; it is abnormal if the rated current is still reached at this time. If the pump rotates smoothly, it indicates that there are no mechanical issues with the pump. There are probably several possible explanations: 1. The outlet valve isn’t sealing properly, resulting in obvious leakage. 2. Medium density issue. 3. The motor is too small. The possibility of a second issue with the water pump is very low. Check each item individually