Across-the-Line Motor Start Causes Voltage Dip
A 480 V, three-phase plant has a 500 kVA transformer with 5% impedance. Existing load is 200 kVA at 0.9 lagging power factor. A new 150 hp induction motor will start across the line; its full-load current is 180 A and locked-rotor current is 6 times FLA. The utility allows no more than 10% voltage dip at the service point during motor starting. The plant wants the lowest-cost solution that preserves the existing process; the motor only needs to start unloaded and run at fixed speed. A soft starter and a VFD are both quoted, but the VFD is 2.5 times the cost and adds 5% input current distortion. The transformer is dedicated to this plant, and the source impedance is negligible compared with the transformer.
1. Estimate the voltage dip for an across-the-line start and determine whether it meets the 10% limit.
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The transformer short-circuit capacity is S_sc = 500 kVA / 0.05 = 10 MVA. The motor locked-rotor current is 6 × 180 A = 1080 A, so starting kVA is √3 × 480 V × 1080 A ≈ 898 kVA. Add the existing 200 kVA load: total apparent load during start is about 1098 kVA. Using the approximate dip S_total/S_sc gives 1098/10,000 ≈ 11%. That exceeds the 10% limit, and the actual dip may be worse because the motor starting power factor is low and the existing load is not purely resistive. The across-the-line start is therefore not acceptable without mitigation. A more detailed motor-start model should be used for final verification, but the approximation is already over the limit.
2. Compare a soft starter and a VFD for this application, including the effect on voltage dip.
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A soft starter reduces starting current, typically to about 3 times FLA. That gives 540 A and about 449 kVA; with the 200 kVA existing load, total is about 649 kVA and dip is roughly 6.5%. It is lower cost and adequate because the motor starts unloaded and runs at fixed speed. A VFD can limit starting current to near FLA, so starting kVA is only about 160 kVA and dip is roughly 3.6%, but it costs 2.5 times more and adds 5% input current distortion. The VFD is justified only if the process needs speed control, high starts per hour, or tighter flicker limits. For this fixed-speed, unloaded start, the soft starter meets the limit with less cost and less harmonic impact.
3. What recommendation would you make, and what checks are needed before approving it?
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Recommend the soft starter, provided the motor manufacturer allows reduced-voltage starting and the load can tolerate the reduced starting torque. Verify the actual locked-rotor current and starting power factor from the motor data sheet, because the 6×FLA assumption drives the dip estimate. Check that the existing 200 kVA load is realistic at the worst starting time and that other motors are not started simultaneously. Confirm the soft starter settings limit current to about 3×FLA and that the resulting dip stays below 10% at the service point. If the utility requires stricter flicker limits or the motor fails to accelerate unloaded, revisit the VFD; otherwise the soft starter is the lowest-cost compliant choice.