Why Motor Starting Currents Matter For Generator Power
Imagine a hospital chiller failing to restart after a mains outage because the standby generator stalled on the pump start — hours of temperature rise and emergency action follow. If you manage sites, hire plant, or design standby power in the UK, this article helps you avoid that outcome. In our experience at PowerTech Generators, the difference between a reliable installation and one that nuisance-trips is rarely the generator brand and usually the sizing, controls and commissioning.
Read on for practical rules of thumb, the data you must collect before sizing, starter comparisons, commissioning checks and a short checklist to take to site. Links to our tools and guides are included for deeper reading.
kW Vs kVA: The Foundations Of Sizing
kW is real power; kVA is apparent power. Power factor (pf) links them: kW = kVA × pf. Most industrial gensets are rated at 0.8 pf. Motors may run at 0.85–0.9 pf but pf typically collapses during start, inflating kVA demand.
In our experience, nameplate kW alone misleads installers. Always convert to kVA on the generator pf when proposing a set and check starting method and torque requirements before final selection. For quick calculations use our kVA calculator and see our commercial diesel generators pages for typical genset options and the case studies for real deployments.
Motor Starting Current And Torque Explained
Full-load amps (FLA) are the steady running figures. Locked-rotor current (LRC) is the inrush at standstill and is commonly 6–8× FLA for standard induction motors. The driven machine matters: centrifugal pumps and fans need high starting torque that falls as speed rises, while compressors and conveyors can demand sustained torque.
Generator short-term capability is governed by alternator subtransient reactance (X’d’), AVR speed and governor class. A common issue we see is assuming the AVR alone will save a borderline-sized set; excitation and governor response must be specified together. For a straightforward primer, consult our guide to automatic voltage regulators (AVRs).
Comparing Starting Methods: DOL, Star‑Delta, Soft Starters And VFDs
Direct‑on‑line (DOL) is cheapest but produces the highest inrush (6–8× FLA) and full starting torque. Star‑delta reduces line current to roughly one-third of DOL (≈2–3× FLA) but also reduces torque and requires suitable motor wiring. Autotransformers offer adjustable reductions between those ranges.
Soft starters ramp voltage to limit current (commonly 2–4× FLA) and give gentler mechanical starts. Variable-frequency drives (VFDs) control acceleration, often holding inrush to ~1.1–1.5× FLA, with near‑unity pf — but they introduce harmonics and EMC considerations. Choose the method to match the mechanical needs of the load and the transient capability of the genset.
This image was generated with AI and may not always represent the product or service exactly.
How To Size A Generator For Motor Starts
Collect this data before quoting: motor kW, FLA, LRC or NEMA code, starting method, number of simultaneous starts, cable lengths, site altitude/ambient, and any VFD/soft starter settings. Model realistic voltage dip and recovery targets with that data.
- Rules of thumb we use: DOL ≈ 3–4× motor kW (kVA basis); star‑delta ≈ 1.5–2.5×; soft starter ≈ 1.25–1.75×; VFD ≈ 1.1–1.3×.
- Specify PMG or AREP excitation and G2/G3 governor classes when heavy or multiple starts are expected.
- Always verify with witnessed load acceptance testing and step‑load or motor‑start tests.
PowerTech Generators sources Cummins, Perkins, FG Wilson and Baudouin sets so we can match duty without brand bias. For examples of load testing and system integration see our guide on generator load testing and ATS panels.
What Most People Get Wrong
Most teams undersize for transient torque, relying on steady‑state kW and ignoring simultaneous starts, cable voltage drop and AVR/governor selection. That’s why on‑site tests are decisive.
When This Doesn’t Apply
If your site only runs resistive heating or tiny motors (under a few kW) and no VFDs, transient issues are minimal and standard sizing is usually sufficient. Also different rules apply where generators run in parallel with the utility.
Prevent Voltage Dip And Nuisance Tripping
Choose alternators with suitable short‑circuit characteristics and set AVR gains to avoid overshoot. Upsize conductors to reduce voltage drop and use screened neutrals or separate runs where VFD harmonics are present. Coordinate protection: select MCB/MCCB curves and thermal overloads that tolerate inrush without masking real faults.
Sequence large loads after ATS transfer and stagger starts with delays in control logic. In our experience, sequencing plus correct protection settings removes most nuisance trips on standby systems.
Sector Scenarios And Worked Examples
Construction dewatering: 22 kW pump on hire, star‑delta start, estimated start ~2.5× FLA → generator ≈ 40 kVA. Manufacturing: 30 kW compressor on soft starter → generator ≈ 45 kVA for smooth restarts. Healthcare chilled water: 55 kW pump on VFD with filters, PMG alternator → generator ≈ 66 kVA for tight control. See the case studies for similar projects.
This image was generated with AI and may not always represent the product or service exactly.
Commissioning, Testing And Documentation
We survey site cabling, earthing and harmonic content, fit chokes/filters for VFDs where needed, and run loadbank and motor‑start tests to verify recovery times. Witnessed starts under real conditions are the final proof.
You should receive a compliance pack (BS 7671, IEC 60034, ISO 8528), RAMS, method statements and a maintenance schedule as part of handover. In our experience, clear documentation saves weeks of troubleshooting later.
Maintenance For Reliable Motor Starts
Preventative servicing prevents avoidable start failures. Check fuel, cooling, belts, batteries and controller firmware. Recalibrate the AVR and governor as part of annual servicing and run periodic representative start tests after plant changes.
Planned spares, remote monitoring and 24/7 support reduce downtime and speed fault diagnosis. Talk to our team about maintenance plans that match duty cycles and criticality.
Buy, Hire, Export Or Decommission With Confidence
PowerTech Generators offers new units, short‑term hire for trials and cost‑effective used options. We handle export compliance, testing, crating and logistics, and manage decommissioning, isolation and asset recovery with full documentation for insurers and ESG reporting.
Talk To PowerTech Generators
Get a free sizing review and site survey. We are independent, UK‑wide, and deliver export solutions. Speak to our engineering team about controls, commissioning and resilience planning so your motors start first time, every time.
Quick Checklist
- Record motor kW, FLA, LRC/NEMA code and starting method
- Estimate simultaneous starts and sequence requirements
- Specify PMG/AREP and governor class if heavy starts expected
- Plan witnessed motor‑start tests and loadbank recovery
- Document protection curves, AVR settings and maintenance plan
FAQs
Which Starting Method Suits My Process?
Match the method to torque and process control needs. Use DOL for simple, robust applications; star‑delta where reduced inrush is acceptable; soft starters for smoother mechanical starts; and VFDs where speed control or tight process control is needed, remembering to manage harmonics.
How Should I Sequence Multiple Motor Starts On A Single Generator?
Stagger starts by a few seconds with control logic or ATS sequencing, model combined inrush beforehand, and use soft starters/VFDs where available. Verify with on‑site acceptance testing.
How Tight Should My Voltage Dip Target Be?
Typical targets are 10–15% dip with recovery under 1–2 seconds. Critical sites (data centres, healthcare) often require tighter limits—agree thresholds with stakeholders and test to them.
What Tests Prove A Generator Will Handle Motor Starts?
Loadbank tests, step‑load and witnessed motor‑start tests under realistic site conditions prove recovery time and protection coordination. Insist on witnessed tests before handover.