You're right, a stall fault on a centrifugal pump seems counterintuitive. Here's a breakdown of potential causes and things to consider

Understanding the Problem

The key is to understand what a "stall" fault means to a VFD. It typically indicates: Sudden and Significant Loss of Motor Speed While Under Load

The drive is trying to maintain speed (based on its command), but the motor's actual speed is dropping dramatically, indicating a mechanical overload or a problem with torque production.

Possible Causes and Troubleshooting Steps1. Mechanical Issues

This is the FIRST place to look, even though it's a centrifugal pump.

Clogged Suction or Discharge

This is the most common culprit. Debris can restrict flow, increasing the load on the pump and causing it to struggle. Even partial obstructions can significantly affect performance. Check suction strainer/foot valve, and discharge piping.

Worn Impeller or Pump Internals

Over time, centrifugal pump components can wear down, reducing efficiency and requiring more torque to maintain flow. Look for cavitation damage, worn clearances.

Binding or Seizing

While less likely in a clean water system, check for any signs of the pump shaft binding or seizing. This could be due to bearing failure, scale buildup, or other debris.

Pump Mismatch

Is the pump actually suitable for the application? Has the system demand changed? Is the pump running too far off its design point (way too little flow, cavitation; way too much flow, overcurrent)?

Coupling Issues

Check the coupling between the motor and the pump for slippage or damage.2.

VFD Settings



Stall Torque/Speed Settings

You're already looking at this. Increasing the stall current threshold (what you did) can mask an underlying problem, but in this case it helped temporarily. Review all stall-related parameters in the VFD manual. Look at the stall acceleration/deceleration settings as well. The VFD might be too sensitive.

Current Limit/Overload

Lowering the current limit can trigger a stall fault if the pump needs more current to overcome a load.

Acceleration/Deceleration Rates

If the acceleration rate is too fast, the motor might not be able to keep up, leading to a stall fault during startup. Conversely, a deceleration rate that's too fast could cause the motor to stall if the load is high. Try slower ramp-up and ramp-down times in the VFD settings.

V/Hz Ratio

The voltage to frequency ratio determines the torque output of the motor. If the ratio is incorrect, the motor might not be able to produce enough torque at lower speeds. Review the VFD manual for proper V/Hz settings.

Motor Data Entry: Review motor voltage, current, speed, and horsepower rating entry to make sure that it matches the nameplate of the motor.3. Electrical Issues



Motor Insulation Failure

A partially shorted winding can draw excessive current and reduce motor torque. Use a megohmmeter to test the motor insulation resistance.

Loose Connections

Check all electrical connections in the motor, VFD, and control panel. Loose connections can cause voltage drops and intermittent faults.

Power Supply Issues

Unstable or low voltage can affect the VFD's performance and lead to a stall fault.

Harmonics

High levels of harmonic distortion on the power line can interfere with the VFD's operation and cause erratic behavior.4.

Pump/System Hydraulics



Air Entrainment

Air in the suction line can drastically reduce pump performance and cause it to draw excessive current as it fights to pump a mixture of water and air. Ensure proper venting and check for air leaks.

Suction Lift/Head

Even with 5 ft of suction head, verify that this is sufficient for the pump at the required flow rate. Too much lift can lead to cavitation and reduced performance.

Discharge Head

Is the pump working against an excessive static head (elevation difference)? Has the discharge piping been modified?5.

Control Issues (Speed Command)



Erratic Speed Command

The control system sending the speed command to the VFD could be malfunctioning, causing the pump to suddenly slow down or stop, triggering a stall fault. Monitor the speed command signal and compare it to the actual pump speed.

Feedback Loops

If the speed command is based on a feedback loop (e.g., pressure), there might be oscillations or instability in the loop that are causing the pump to hunt and stall.

Troubleshooting Steps and Recommendations1. Gather Data



Operating Logs

Review any operating logs or trend data to see if there's a pattern to the stall faults. When do they occur? Under what conditions?

VFD Fault History

Download the fault history from the VFD to see the exact parameters at the time of the stall.

System Conditions

Note suction/discharge pressures (if available), flow rate (if available), motor current, and motor speed at various operating points.2.

Isolate the Problem



Run the Pump at Fixed Speeds

Bypass the automatic control system and run the pump at various fixed speeds using the VFD keypad. This can help determine if the stall fault is related to the control system or the pump itself.

Disconnect the Load

If possible, disconnect the pump from the driven load and run the motor unloaded. This will help determine if the problem is with the motor or the pump. Be extremely careful doing this with a pump. Running a centrifugal pump without flow is generally a bad idea!



Monitor Current and Speed

Use a clamp meter and a tachometer (or the VFD's display) to monitor the motor current and speed while the pump is running. Look for sudden drops in speed or spikes in current.3.

Inspect the Pump and System



Visual Inspection

Carefully inspect the pump, piping, and valves for any signs of damage, leaks, or obstructions.

Pump Performance Test

Perform a pump performance test to determine the pump's flow rate and head at various speeds. Compare the results to the pump's performance curve to see if the pump is operating correctly.4.

Adjust VFD Settings (Carefully)



Document Original Settings

Before making any changes to the VFD settings, document the original settings so you can easily revert back if needed.

Stall Settings

Slightly increase the stall current and/or speed threshold. But remember, this is a band-aid, not a fix.

Acceleration/Deceleration

Adjust the acceleration and deceleration rates to see if that resolves the issue.

V/Hz Ratio

Consult the VFD manual to ensure that the V/Hz ratio is properly set for the motor.

Specific to your situation



Lack of Pressure Indication Between Pumps

This is a HUGE problem. You need to know the pressure the priming pump is delivering. Without this, you're flying blind. The priming pump might be under-performing and causing cavitation in the main pump.

Other Fingers in the Pot

This is always problematic. Insist on a controlled and documented troubleshooting process to avoid conflicting changes. Have a meeting with all involved.

Stall Fault after Current Limit Adjustment

Lowering the current limit is almost always the wrong thing to do without investigating the root cause of the high current. It suggests the underlying issue is increasing the motor load, not a drive problem.

Centrifugal Pump and Stall Faults: Centrifugal pumps are flow devices. Head (pressure) is generated from the velocity the impeller imparts to the fluid. It is very possible that you have something in the discharge line of the pump that is causing too much pressure. As you start the pump the fluid has nowhere to go and the motor sees it as a mechanical overload and gives you a stall fault.

In summary: Focus on mechanical issues FIRST. Then, ensure proper VFD settings (especially stall settings and acceleration/deceleration), and finally, address any electrical issues. Most importantly, get a pressure gauge installed between the pumps. You can't diagnose without knowing what is going on in between the pumps. Flag for review