37 kW pump tripping on overload: voltage unbalance, ambient derating, and a proposal to raise the setting
A 37 kW, 400 V, three-phase, four-pole IE3 pump motor drives a cooling-water pump. The nameplate gives a full-load current of 68 A, a service factor of 1.15, class F insulation and a 40 degrees C ambient rating. The motor runs direct-on-line from a motor control centre 60 m away; the overload relay is a class 10 thermal device set at 68 A. After a bearing replacement, the motor trips on overload every 15 to 25 minutes at steady pumping load. Measurements at the motor control centre give line voltages of 415 V, 408 V and 399 V, and line currents of 74 A, 70 A and 66 A. The pump-room ambient is 46 degrees C. Insulation resistance is 800 megohms on all windings, the pump has been re-aligned, and the suction strainer is clean. A maintenance fitter proposes setting the overload relay to 78 A, which is 1.15 times full-load current, to stop what he calls nuisance tripping.
1. Calculate the voltage unbalance and the current unbalance, and explain the relationship between them.
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Voltage unbalance (NEMA definition): average voltage = (415 + 408 + 399) / 3 = 407.3 V. Maximum deviation from the average = 407.3 - 399 = 8.3 V.
Unbalance = 8.3 / 407.3 x 100 = 2.0 percent.
Current unbalance: average current = (74 + 70 + 66) / 3 = 70.0 A. Maximum deviation = 74 - 70 = 4.0 A.
Unbalance = 4.0 / 70.0 x 100 = 5.7 percent.
Relationship. A small negative-sequence voltage produces a rotating field in the opposite direction, which the rotor sees at nearly twice synchronous speed. The rotor resistance to that field is low, so a small voltage unbalance produces a much larger current unbalance - commonly five to ten times larger. That is exactly what the numbers show: 2 percent of voltage has become 5.7 percent of current.
The consequences are a negative-sequence current in the stator and rotor, double-frequency torque pulsation, and additional rotor heating concentrated in a few bars. At 2 percent unbalance a typical motor must be derated to about 0.95 of nameplate output, so the motor is already losing capacity before ambient temperature is considered.
2. Evaluate the fitter's proposal to set the overload relay to 78 A, and state the correct setting basis.
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The proposal should be rejected. A service factor of 1.15 permits operation above nameplate output only under the conditions the nameplate assumes: balanced supply, 40 degrees C ambient, and rated altitude. Two of those assumptions are already broken.
- Ambient derating at 46 degrees C for a class F machine rated at 40 degrees C is about 0.95.
- Voltage-unbalance derating at 2 percent is about 0.95.
- Combined usable output is roughly 0.95 x 0.95 = 0.90, giving an effective full-load current of about 68 x 0.90 = 61 A, not 78 A.
The measured 70 A average already exceeds that derated figure, so the relay is doing precisely what it should: protecting the winding. Raising the setting to 78 A would allow winding temperatures to rise above the class F limit. Insulation life roughly halves for every additional 10 K of sustained winding temperature, so the motor would fail, probably within months, and the failure would be blamed on the bearing job.
Correct basis: the overload setting protects the motor and the cable. It should be set from the motor's nameplate full-load current adjusted for the actual service conditions, or to the manufacturer's documented figure, and it must not exceed the cable's corrected capacity or the motor's permitted winding temperature. The class 10 characteristic is appropriate for a direct-on-line pump with a short start.
3. Set out the fault-finding and corrective actions in priority order, distinguishing what must be fixed from what is a workaround.
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Must be fixed - the unbalance. 2 percent of unbalance is at the edge of acceptability and is the root cause. Check, in order: the motor control centre connections and terminal torques on all three phases; the fuses or MCCB poles for a high-resistance joint or a partially failed pole; the cable for a damaged core; the upstream distribution for large single-phase loads or a capacitor bank with a blown fuse on one phase; then ask the utility to log the supply. A loose lug on one phase commonly produces exactly this pattern of 8 V deviation.
Must be fixed - the thermal environment. 46 degrees C exceeds the motor's rating. Improve ventilation, duct cool air to the motor, or fit a higher-ambient-rated motor. This is not cosmetic; it removes the 0.95 factor.
Verify the mechanical side. A bearing replacement can change rotor position and preload; check shaft end-float, coupling alignment with a laser, and that the pump is not running out on its curve at higher flow and power than before. Confirm running current against the pump curve.
Workarounds that must be resisted: setting the relay to 78 A, fitting a higher class relay, or forcing the contactor closed to "test". Each disables the protection that is correctly detecting an overload.
After correction: re-measure voltages and currents at steady load, reset the overload to the corrected figure, record the values, and re-check the relay's trip time if it is adjustable. Log the before-and-after currents so the effectiveness of the fix is demonstrable.