Monitoring Pump Motors,

Monitor Current Draw: Place a a current sensing relay or switch over a lead powering the pump driver and set it so a contact closes or opens when the current draw of the pump exceeds a limit. The contact can trigger an alarm, or Open the Contactor Operating Coil in an over-current condition caused by a jammed impeller, failing bearings, or corrosion of any rotating component. Conversely, a current sensing relay or switch can sense an abnormal reduction in current caused by an open discharge, broken coupling between motor and pump, or a blocked intake line.
Monitor Change in Current Over Time: Use an analog output sensor with a constant signal proportional to the primary current to monitor the change in current over time. As the pump system wears, current use will increase indicating a problem. Maintenance can be performed before the problem becomes catastrophic.
Monitor Phase Loss: Use a voltage monitor to detect phase loss, whether it is at the motor terminals or at the source. Starting a three-phase motor in a single-phase condition will likely damage the motor windings quickly, resulting in a rewind or replacement. Monitor Power or Wattage: Most motors draw at least 50% of the nameplate full load current in an "open-shaft" or free spinning condition. Add the torque required to turn the pump mechanically and the pump will be draw over 50% of nameplate current, increasing in a very linear manner proportional to the load. If it is a critical load, or uses an over-sized motor for the load, use a power monitoring device to monitor power or wattage. Since watts are calculated by measuring the voltage, current and power factor, the transducer output will always be directly proportional to the load. Monitor for Earth Leakage: Monitor earth leakage with a ground fault sensor to prevent damage to equipment or injury to personnel from phase to earth faults. The output contact can be selected to close, operating a Shunt Trip Circuit Breaker, or a contact can open the circuit to a contactor coil. Both would shut down the monitored load so maintenance can be performed.