Power quality and networks
Motor starting voltage dip
- where
- motor starting apparent power (MVA)
- starting current ratio Ist/In
- motor rated input apparent power (MVA)
- fault level at the motor bus (MVA)
- source fault level (MVA)
- transformer impedance (%)
- transformer rating (MVA)
- voltage dip (%)
Explain this equation
Estimates the voltage dip at the motor busbar during a direct-on-line start. Use it for an early check against ENA P28 step limits and to decide whether a soft starter, VSD or a stiffer supply is needed.
Sm is the motor’s locked-rotor kVA: rated kVA times the starting-current ratio. Seff is the fault level at the motor bus, combining the upstream source with the transformer impedance; a transformer of rating St and impedance uk on its own could deliver St/uk × 100.
Fault levels combine like impedances in series, so their reciprocals add. During starting the motor behaves as a fixed impedance; the source and motor impedances form a voltage divider and the fraction lost across the source is Sm/(Seff + Sm).
Sense check: a 3 MVA start on a 250 MVA busbar dips only about 1 %; behind a 2 MVA 6 % transformer the busbar fault level drops to about 29 MVA and the dip rises to about 9 %.
Watch out: the dip at the point of common coupling, which P28 limits, is smaller than at the motor bus. The low starting power factor makes this method slightly pessimistic.
Approximate voltage dip at the motor busbar when a motor is started direct on line, from the starting kVA and the source fault level, with an optional transformer in between.
Enter your values to see the result.
Good to know
Sr is the motor’s rated input kVA (kW / (η·pf)). The starting kVA is a constant-impedance load at rated voltage; the dip is the voltage divider between source and motor impedances. ENA EREC P28 limits voltage steps at the point of common coupling (typically 3 % for infrequent events at HV, with higher limits for very infrequent events); compare the dip at the PCC, not at the motor bus. Soft starters and star-delta reduce Sm.
Common mistake
Using the transformer’s own through-fault level without the upstream source impedance, or using motor rated kVA rather than locked-rotor kVA.
Limits
Impedance-magnitude approximation: assumes source and motor impedances have similar angles and ignores the motor’s low starting power factor (which makes the true dip slightly smaller) and pre-start load. For anything marginal run a motor-starting study to IEEE 3002.7.
Sources
- IEEE 3002.7-2018 Recommended practice for conducting motor-starting studies and analysis of industrial and commercial power systems
- ENA Engineering Recommendation P28 Issue 2 (2019): Voltage fluctuations and the connection of disturbing equipment to transmission and distribution networks in the UK
- GE / Alstom Grid, Network Protection & Automation Guide (NPAG), 2011 edn – chapters 6 (CTs) and 9 (overcurrent protection)
For competent high-voltage engineers. Results are calculations from the published methods shown and depend entirely on the data you enter. They are not a substitute for a system study, manufacturer data, the applicable standards and Engineering Recommendations, or your organisation’s safety rules. Never use a result to justify working on or near live HV equipment.