Star-Delta Changeover Trip on a 7.5 kW Borewell Motor
A 7.5 kW, 415 V, 14.8 A IE3 induction motor drives a borewell pump through a star-delta starter. The overload relay is set to 14.8 A and the ambient temperature is 42 °C. The motor starts in star, runs for 15–20 s, changes to delta, and the overload trips within about 5 s. The pump shaft turns freely by hand. Supply line voltages are 415 V, 414 V and 412 V. At the motor terminal block, winding resistances are U1-U2 2.1 Ω, V1-V2 2.2 Ω and W1-W2 6.8 Ω. Insulation resistance to earth is greater than 100 MΩ. With the pump uncoupled and the starter in star, line currents are 5.8 A, 5.7 A and 2.9 A. The starter contactor was replaced last year, the overload relay is new, and no terminal is visibly burnt. The operator has tried restarting five times.
1. Interpret the resistance and star-current readings. Can supply imbalance explain them? What fault do they point to?
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The supply imbalance is only (415 − 412)/415 = 0.7%, so it cannot explain the readings. The winding resistance imbalance is (6.8 − 2.1)/2.1 = 224%, far beyond a normal 2–5% tolerance. That indicates a high-resistance joint or partially open circuit in the W phase, not a normal winding.
In star, the line currents are also unbalanced: 2.9 A is about half of 5.8 A. A healthy motor on light load should have reasonably balanced currents. The high-resistance phase limits current in that line and shifts the neutral point, so the motor produces reduced and unbalanced torque. The fault could be in the winding, the terminal link, the cable or a contactor pole; the resistance test at the motor terminals includes the links and leads, so those must be separated before condemning the winding.
2. Explain why the motor can start in star but trips soon after changing to delta.
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In star, each winding sees 415/√3 = 240 V, so starting current and torque are reduced. The motor can accelerate a free pump even with one phase weak, and the overload may not trip during the short start. At changeover to delta, each winding sees full 415 V and the motor is effectively single-phased. The two healthy windings carry a large current, the high-resistance phase carries little, and the rotating field is severely distorted.
Torque collapses, slip rises, current in the healthy phases rises sharply, and the overload trips in a few seconds. The hum is consistent with negative-sequence braking. Restarting repeatedly will overheat the healthy windings and may turn a repairable joint into a burnt winding. The overload setting should not be increased to mask the fault.
3. State the next tests and the repair decision criteria.
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Disconnect the motor links and measure each winding separately at the terminal studs. Measure contact resistance of each delta and star contactor pole and inspect for pitting or loose terminals. Check cable continuity and insulation on all three phases.
If the winding itself reads 6.8 Ω while the others are about 2.1 Ω, the motor is faulty and should be rewound or replaced. If the high reading disappears when the links are removed and the winding is balanced, replace the links, terminal studs or contactor pole. After repair, run uncoupled and record line currents and voltage; set the overload relay to the motor FLC corrected for the 42 °C ambient, and verify that the star-delta timer allows full acceleration before changeover.