Motor replacement: cable capacity, volt drop and starting
A 7.5 kW, 400 V, three-phase, 50 Hz squirrel-cage motor drives a borehole pump at a water treatment plant. The nameplate gives 14.8 A at 400 V, power factor 0.82 and efficiency 90%. The motor is 55 m from the MCC and is wired in a three-core 4 mm² XLPE/SWA copper cable clipped to a tray with two other loaded cables. The DOL starter has a thermal overload set at 16 A. The cable has a tabulated voltage drop of 9.5 mV/A/m and an installed current-carrying capacity of 32 A. The plant engineer proposes replacing the pump with a 9 kW unit of the same speed and frame, drawing 18.5 A at 0.84 power factor and 91% efficiency. The MCC nominal voltage is 400 V and the permitted running voltage drop for this motor is 5%.
1. Calculate the running volt drop for the existing and proposed motors and state whether the cable is thermally adequate and within the 5% limit.
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Use Vd = (mV/A/m × I × L)/1000. Existing: (9.5 × 14.8 × 55)/1000 = 7.73 V, or 1.93% of 400 V. Proposed: (9.5 × 18.5 × 55)/1000 = 9.67 V, or 2.42%. Both are below 5%, so running volt drop is acceptable. Thermally, the installed capacity is 32 A; the proposed full-load current is 18.5 A, so Iz is adequate, but the protective device must satisfy Ib ≤ In ≤ Iz. The 16 A overload is below the proposed 18.5 A and would trip; it should be replaced with a setting appropriate to the motor, typically 20 A or the next suitable size, and the cable and starter checked. The calculation assumes the tabulated mV/A/m applies at the actual conductor temperature and that the cable grouping is already included in the 32 A figure.
2. Explain why the 16 A overload setting is unacceptable and what else must be checked before changing it.
Read the worked answer
The overload protects the motor against sustained overcurrent. A 16 A setting on a motor drawing 18.5 A will operate in normal service. The correct setting is based on the motor full-load current, the starter manufacturer's table and the motor service factor, not simply on the cable. Before changing it, check the contactor rating, thermal overload range, cable Iz (32 A), upstream short-circuit protection, and the motor's permitted starting time. Also check the MCC fault level and whether the starter is suitable for the higher current. The 20 A device must still be below Iz; 20 A is less than 32 A, so the cable is protected. The overload is not there to protect the cable.
3. The engineer suggests a VSD to address starting. What additional electrical issues would you assess?
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A VSD changes the starting current but introduces harmonics, EMC and high-frequency leakage. I would check: input current and supply capacity; harmonic distortion and any PF correction; whether the MCC has a suitable breaker or fuse; cable length between VSD and motor, because long cables cause reflected-wave overvoltage and require a dv/dt filter or inverter-rated cable; screening and earthing of the motor cable; common-mode leakage current and RCD type (Type B may be needed); motor derating for non-sinusoidal supply; cooling at low speed; and bearing currents. I would also check that the existing 4 mm² cable is suitable for VSD output, not just 50 Hz running.