50 n SENSORS AND ENCODERS July/August 2026 www.drivesncontrols.com Using a motor as a sensor to detect pump faults Current monitoring is often the goto method for protecting threephase pump motors. Unfortunately, at light loads, it is also one of the least reliable. A current monitor cannot distinguish between a lightly loaded pump and one that is running dry. At low shaft loads, the current measurement no longer reflects what the motor is actually doing. Understanding why this is the case points to a better approach: treating the motor itself as a process sensor for measuring active power, rather than line current. Three-phase induction motors draw two types of current simultaneously. Active current is proportional to the mechanical load. Reactive current is determined by the motor's magnetising inductance and applied voltage, not by what the pump is doing. The problem with current monitors is that they read both as a single vector sum. At full load, power factor typically sits at 0.85–0.90 and active current dominates. As the shaft load falls, the magnetising current stays essentially constant at around 30–50% of the full-load current, while the active current collapses. At no-load, power factor falls to around 0.20. The total line current drops only modestly as the shaft load drops sharply. A pump running dry looks almost identical to one running at a light duty point. For an overload relay tripping on current, these two states produce essentially the same signal. Active power does not have this problem. It is directly proportional to the shaft load across the full operating range, because the reactive component is already accounted for. As the load falls, active power falls in proportion; the measurement sensitivity does not degrade at light loads as it does with current. This makes the motor a self-contained fault detector. Nothing needs to be installed inside the pump or pipework. All of the measurement parameters required are on the motor supply cables, with a load monitor installed conveniently in the motor control centre. The motor is the sensor; the pumped fluid is what it senses. The protection logic follows directly. Define a normal operating power band at the duty point. Set an underpower threshold: published motor-protection design guidance recommends a separation of 15–20% between minimum-flow active power and the dry-run level for reliable detection. Outside of that band, a fault condition is in progress. Fault signatures Centrifugal pump faults produce several distinct active power signatures: n Dry running, where the pump has lost prime, is the clearest. Active power drops Current monitoring has limitations when protecting three-phase motors. An alternative approach using the motor itself as a sensor can detect faults that current monitors cannot, as members of Charter Controls’ technical team explain. A motor can act as a process sensor to detect pump faults using active power at the motor’s power supply
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