How to test a pump underload detector before it controls a motor

A running pump can draw less current when it loses load. A current sensor can flag that change, but a low reading by itself cannot tell you whether the pump is dry. The useful output is suspected underload, along with enough context to investigate it.

Decide when a reading counts

Take measurements only after the motor is confirmed running and the startup window has passed. Ignore the startup transient when evaluating underload. If your system has both a run command and contactor feedback, store both so you can spot a mismatch. Treat missing, stale or invalid current data as a sensor fault. Zero current and no current reading are different observations.

Use RMS current over a documented sampling window. Give the alert its own persistence timer so one noisy sample cannot trigger it. If the reading returns to the normal band, reset the timer. If it sits between the entry and clear thresholds, do not count that time as continuous low current.

Separate the monitoring logic from protection

The following pseudocode returns an event for a log or alert dashboard. It does not issue a motor command. The startup window and thresholds are configuration values that you validate for the pump and its normal operating range.

if motor is not confirmed running:
    reset underload timer
    report STOPPED
else if current data is stale or invalid:
    reset underload timer
    report SENSOR_FAULT
else if elapsed time since motor start < startup window:
    reset underload timer
    report STARTING
else if RMS current < low current threshold:
    add measured valid interval to underload timer
    report UNDERLOAD only after persistence time is met
else:
    reset underload timer
    report RUNNING

For a latched alarm, define its clear threshold and reset behavior separately. Keep any motor-stop logic, restart rules and protection hardware independent of this telemetry example.

Replay these cases before field testing

Test input	Expected result
Motor stopped, current near zero	Stopped
Motor just started, current fluctuates	Starting
Current dips briefly, then recovers	Running, with no sustained alert
Current stays below the threshold with valid samples	Underload after the configured delay
A missing sample interrupts a low-current sequence	Sensor fault, with the timer reset
Run command is on but feedback never confirms running	Report the mismatch separately

Replay sample traces with simulated input first. Then compare the output against known normal operation at more than one operating point. Log the sample timestamp, RMS current, motor state, threshold and fault reason so you can distinguish a setting issue from a process change. Depending on the installation, an independent level, flow or pressure reading can provide stronger evidence about the actual water condition.

The motor protection technical reference discusses underload and startup delays. The VUKAR pump-control video library includes a TPM-A dry-run trip-delay demonstration if you want to see how a finished controller presents a delay to the operator.

Written with AI assistance.