Motor Overload Trips After Pump Impeller Trim
A 75 kW, 400 V, three-phase motor drives a centrifugal cooling water pump. The pump was oversized, so the impeller was trimmed to reduce flow from 180 L/s to 150 L/s. After the trim, the motor draws 148 A against a nameplate full load current of 132 A, and the overload relay trips after about 20 minutes. The relay is set at 132 A. Voltage is balanced within 1 percent. Power factor measured at the starter is 0.86, and the motor is rated 0.88. The pump discharge pressure is 480 kPa, and the design point requires 450 kPa. The motor was rewound 18 months ago. The starter is a direct-on-line contactor with a thermal overload relay. The pump runs continuously and has a soft-start bypass contactor.
1. Determine why the motor is overloaded after the impeller trim and whether the overload relay setting is correct.
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The overload is real, not a nuisance trip. Current is 148 A against 132 A nameplate, about 12 percent over. The relay set at 132 A is correctly set to protect the motor, and it is doing its job.
The surprise is that trimming the impeller should reduce power, not increase it. Power scales with flow and head, so a trim from 180 to 150 L/s should reduce absorbed power by roughly 30 percent if the system curve is unchanged. The fact that current rose means the operating point moved to a higher flow or higher head than expected.
Likely explanations:
- The trim changed the pump curve so the operating point moved out to the right on the system curve, increasing flow and power. Check actual flow, not just discharge pressure.
- The discharge pressure of 480 kPa is above the 450 kPa design point, suggesting the system resistance is higher than assumed or a valve is throttled, which would normally reduce flow and power. This conflicts with the high current and points to a measurement or control issue.
- The rewound motor may have a different efficiency or service factor than the original. Confirm the actual nameplate data and service factor.
- Voltage imbalance within 1 percent can cause current imbalance and extra heating, but not a 12 percent average current rise by itself.
The relay setting is correct. The fix is to find the true operating point and correct the hydraulic or motor issue.
2. Outline the diagnostic steps and the acceptable remedies.
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Diagnostics:
- Measure actual flow with a calibrated meter and confirm the operating point on the new pump curve. Compare absorbed power with the curve prediction.
- Check the discharge valve position and system resistance. Confirm whether the 480 kPa reading is accurate and whether a valve or strainer is partly closed or fouled.
- Verify motor nameplate data, service factor, and rewind records. Confirm the motor is the correct rating for the pump duty.
- Check voltage and current balance on all three phases and confirm the relay is calibrated and correctly sized for the motor cable and ambient temperature.
Acceptable remedies:
- If flow is higher than design, trim the impeller further or add a correct-size orifice or throttle valve, but only after confirming the true operating point.
- If the motor is undersized for the corrected duty, replace it with a correctly sized motor.
- If the system resistance is higher than design, correct the piping or valve issue rather than raising the relay setting.
Do not raise the overload relay setting to 148 A. That removes protection and risks motor damage. The relay should stay at or below the motor nameplate full load current, adjusted for service factor and ambient per code.