Pump Motor Tripping After Valve Replacement
A 15 kW, 415 V, three-phase, four-pole squirrel-cage motor drives a water-works pump. The nameplate full-load current is 28 A at 0.86 power factor and 89% efficiency. Since a discharge valve was replaced, the motor trips on overload after 35–50 s on every start. Measured line currents are 27.5 A, 26.8 A and 28.2 A; line voltage is 398 V; insulation resistance is 200 MΩ at 500 V; winding resistances are balanced within 1%. Ambient temperature is 44 °C. The overload relay has a 20–32 A range, is set at 30 A and is Class 10. The starter is star-delta with transition at 8 s. During stable running the current is 28 A. The discharge pressure is about 10% higher than before the valve work. The operator proposes to set the relay to 32 A and continue running.
1. Calculate the motor's full-load current from its rating and compare it with the relay setting. Is changing the setting from 30 A to 32 A a valid fix? Explain.
Read the worked answer
Using P = √3 V I cosφ η, I = 15,000/(√3×415×0.86×0.89) = 15,000/(1.732×415×0.765) = 15,000/550.1 ≈ 27.3 A. The nameplate 28 A is consistent. The measured 27.5, 26.8 and 28.2 A are balanced and average 27.5 A, about 101% of calculated FLC, so the motor is running at its rating, not in a clear electrical overload.
A relay setting of 30 A is 107% of 28 A, which is normal for a service-factor-1.0 motor; 32 A would be 114% and would allow a sustained overload that heats the winding beyond its insulation class. The trip occurs after 35–50 s, which is thermal, not instantaneous. Before touching the setting, check whether the relay is ambient-compensated: at 44 °C an uncompensated thermal relay trips earlier than at 40 °C. The correct action is to find why the motor takes so long to accelerate or draws full load, not to raise the setting. If the process genuinely needs more torque, a higher-rated motor or a different starting method is required.
2. List the likely causes created by the valve work and the tests you would perform, in order. State what you would do about the star-delta transition time.
Read the worked answer
Likely causes after valve work: a valve left partly closed or installed with the wrong flow direction, a strainer or suction line blocked, changed pump duty point, pump-motor misalignment, or a phase-voltage imbalance. Tests in order: (1) record suction and discharge pressure and flow; compare with the pump curve; (2) check valve position and direction of the new valve; (3) check line currents and voltages at the motor terminals, then at the starter; (4) check insulation resistance and winding resistance again; (5) check pump alignment and bearing temperature; (6) verify the star-delta timer and transition.
The 8 s transition may be too early: if the motor has not accelerated in star, the change to delta produces a high current surge and a long thermal start. Measure the starting current and speed; if acceleration takes more than about 10 s, increase the star period or use a soft starter, but first remove the mechanical cause. Do not increase the overload setting to 32 A to hide the symptom.