Starting a large motor pulls the voltage down across the bus for the length of the acceleration. In the UK how far you may pull it at the supply boundary is a matter for your Distribution Network Operator, and ENA Engineering Recommendation P28 covers the voltage fluctuation limits behind that. What happens inside the site is your own problem, and usually the more expensive one.
BS 7671 · IET Wiring Regulations · HSR25 · IEEE 1584 · IEC 60909
The motor almost always starts. The question is what trips while it does, and whether the DNO notices.
Repeated large starts produce voltage fluctuation at the point of common coupling. ENA Engineering Recommendation P28 covers the limits, and a site starting large plant on a frequent cycle can breach them without any internal symptom.
The dip depends on fault level at your connection, which comes from the DNO for your licence area. On a weak connection it is the network rather than the motor rating that decides whether a direct on line start is viable.
A motor starting into a depressed voltage accelerates more slowly, and on a high-inertia load that can exceed the safe locked-rotor time. The check is calculated acceleration against the manufacturer's thermal limit curve for the actual load torque.
Contactors drop out, drives trip on undervoltage and control systems reset. On a continuous process that is the real cost, and it only appears when the study models the whole bus rather than the starting motor alone.
Request fault level and impedance at the point of common coupling from the DNO for your licence area rather than deriving it from the transformer rating.
Confirm how often the motor starts, since frequent starting is assessed against voltage fluctuation limits rather than a single-event dip.
Motor nameplate and thermal limit curve, and the driven machine's torque against speed, which determines whether a reduced-voltage start can still accelerate.
Simulate the start including other connected load, calculate the voltage profile and acceleration, and compare starting methods where margin is thin.
Internal ride-through risk and the DNO connection position, with the data each conclusion rests on.
Starting problems tend to appear at commissioning, when the switchgear is installed and changing the starting method costs far more than the study would have.
Because two separate limits apply. Inside the site, the voltage dip can trip drives and drop contactors across the plant. At the supply boundary, repeated starting produces voltage fluctuation that your Distribution Network Operator assesses against ENA Engineering Recommendation P28. A motor can be acceptable on one test and not the other.
From your DNO, for the licence area your site sits in. Great Britain has fourteen of them and connection strength varies considerably. Deriving fault level from the transformer nameplate ignores the upstream network and usually overstates supply strength, which makes a marginal start look acceptable on paper.
Substantially. A single start is assessed on the depth and duration of the dip. Frequent starting is assessed on voltage fluctuation, which is what ENA P28 addresses, and a site that starts large plant on a short cycle can breach fluctuation limits while every individual start looks acceptable. We establish the duty before modelling.
It follows from the driven load's torque demand during acceleration rather than from the motor rating. Direct on line gives maximum torque and maximum inrush. Star delta, autotransformer and soft starters reduce inrush but also reduce torque, which on a high-inertia load can push acceleration past the motor's thermal limit. The study compares them on the real torque curve.