I have a problem with one of the submersible pump in my water priject. the pump operating point is 100m3 at 315m , the normal operating current reading is 32 Amp . the well pump is connected with soft start . In the last two month we have seneral power outages and interruption in the power supply line. Yesterday we tried to run the pump but it ran for few seconds and then it tripped due to OVER CURRENT ,the water reached the wellhead during this trial,  during the trial to run the pumps the current rise up to starting current through the ramp time and then it went down to the normal current value (32 amp ) , and then in few seconde the vaule of current started to rise up gradually from 32 amp to , 34 , 35 , 36 ,37, .. and then it tripped due to overcurrent.

We checked the ohm readings for the motor windings and they were within the accepted range .

Befor I pullout the pump ,is there are any other check I need to do , and what is the expected cause for this curent gradual riseup during starting ?????

You stated that it was a submersible well pump, I'm going to assume that you have a standard sub motor like Byron Jackson, Gould, Hitachi, or Franklin.  The brand doesn't matter, they all work on the same principle.  They all have a fluid lubed thrust bearing in the bottom of the motor.  This bearing has minimum speeds that must be maintained.  The reason being that (in most cases) if the bearing doesn't get to min. speed there is no speration between bearing faces and then just grind the bearing into mush.  Then min. speed isn't reached the fluid isn't present to prevent wear.

The problem that you discribed to me is a clasic description of this.  As the load increases the bearing creates more drag and spikes the amps.  I would check the ramp time on the soft start and see what that is.  In most cases the manufactuer requires that a motor get to min. speed in 2-3 sec.  This is why I'm not a fan of soft starts on well pumps.  You have not way to tell at what speed the motor is turning and when during the ramp time. 

Please tell us what brand of motor it is an I may be able to help more.  Also as the amps climbed, did the operating pressure also climb? I sent the broken shaft to a lab to detect if we have any manufacturer defect with the shaft , the result is that the material of the shaft match exactly the specified construction material which is Alloy Steel (ASTM No. A519,GR 4130).

The important thing is that the lab expects that we had a galling ( Cold welding phenomenon  ) between the shaft and the coupling that likely caused the shaft braek.

My question is : If we had a hard starting for the pump that may cause a high temperatue and overheat for the shaft, can we have this galling between the threaded male side of the shaft and the coupling , while both the shaft and the coupling are from diffrenet material as shown below:

Shaft Material   : Alloy Steel(ASTM No. A519,GR 4130).
Coupling Material: Nickel Chrome Steel(ASTM No.582,Type 303).

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