Borewell Pump Tripping Its MCB After Thirty Seconds
A 1 HP, 230 V single-phase borewell pump motor is installed on a farm. The motor starts, runs for about 30 seconds and then trips the 16 A type C MCB at the board. The pump was working normally until the capacitor was replaced by a local mechanic with a used unit of unknown rating. Measurements taken at the site: supply 228 V; main winding resistance 2.4 Ω; auxiliary winding resistance 5.8 Ω; insulation resistance to body 50 MΩ; capacitor marked 36 µF but measured 9 µF; pump rotates freely by hand. The motor nameplate gives 1 HP, 230 V, insulation class B, continuous rating. Assume efficiency 0.75 and power factor 0.80. The MCB has a thermal setting of 16 A and a type C magnetic trip of 5–10 times rated current.
1. Calculate the full-load current and the likely starting current, then explain why the MCB trips after about 30 seconds rather than instantly.
Read the worked answer
Output power = 1 HP = 746 W. Input power = 746/0.75 = 995 W. Input current = P/(V × cos φ) = 995/(228 × 0.80) = 5.45 A. A single-phase capacitor motor takes roughly 4–6 times full-load current at starting, so about 22–33 A. The type C magnetic trip needs 5–10 × 16 = 80–160 A, so it will not trip magnetically; the starting surge is below that. The 16 A thermal element trips on sustained overcurrent, typically around 1.45 × 16 = 23 A in under an hour, faster at higher current. A healthy motor's start current falls to running value within a second or two, so the breaker holds. Here the motor never accelerates, so it stays near locked-rotor current of about 25–30 A, and the thermal element heats and trips after roughly 30 seconds. The timing is the signature of a failure to start, not a short circuit.
2. Using the measured data, identify the most probable fault and explain the effect of the 9 µF capacitor.
Read the worked answer
The winding resistances are plausible and balanced enough for a 1 HP motor, and 50 MΩ insulation rules out an earth fault. The decisive reading is the capacitor: 9 µF against a marked 36 µF is about 25% of rated value. The starting capacitor's job is to produce a large phase shift in the auxiliary winding and therefore high starting torque. With only 9 µF the phase shift and auxiliary current are far too low, so the motor cannot overcome the pump load and remains at standstill. At standstill the impedance is low, current stays at locked-rotor level, the winding heats, and the MCB's thermal element trips. The free rotation by hand shows the pump and bearings are not seized, supporting the electrical diagnosis. The used capacitor is the prime suspect, but the auxiliary winding and the centrifugal switch, if fitted, should also be checked.
3. State your repair plan, including the checks you would make after fitting the correct capacitor and the reason for not simply raising the MCB rating.
Read the worked answer
Fit a capacitor of the rating specified by the motor manufacturer, with a voltage rating of at least 400 V AC and a suitable duty class; verify its capacitance before fitting. Check the auxiliary winding continuity and the centrifugal switch or start relay, clean the terminals, and confirm the pump turns freely. Then run the motor and measure current: it should settle near 5.5 A, not 25 A, and the MCB should hold. Check that the thermal overload, if fitted, is set to the nameplate current. Do not raise the 16 A MCB to 20 A or 25 A: the breaker protects the cable and the motor against sustained overload, and a larger breaker would allow the winding to burn while the fault remains. If starting current is genuinely high on a long cable, the correct fix is a larger cable and a motor-rated contactor, not a bigger breaker.