Why Right‑Sizing Generators For VFDs And Motor Loads Matters
Imagine a UK food‑processing line where a single VFD‑driven pump stalls every night on generator power, tripping the switchgear and halting production for hours. If you run or specify backup power for sites with pumps, compressors, conveyors or critical HVAC, this guide is for you. In our experience at PowerTech Generators, the mismatch between alternator stiffness, AVR settings and VFD behaviour is the most common cause of nuisance trips and overheating. Read on to learn practical checks, clear sizing rules and the control choices that stop downtime.
What you’ll get: concise electrical parameters to check, starter and harmonic mitigation options, a worked sizing approach, commissioning steps and procurement options tailored to UK sites.
The Key Electrical Parameters To Check
THDi (current distortion) from 6‑pulse VFDs drives THDv (voltage distortion) on the bus. A common issue we see is assuming a generator behaves like a mains supply; it doesn’t. Alternator subtransient reactance (Xd″) determines voltage dip during fast changes — lower Xd″ (≈8–12%) gives better stiffness. Ensure the AVR is true RMS with three‑phase sensing and PMG/AREP excitation so it remains stable with distorted waveforms. For a practical primer, see our guide to automatic voltage regulators (AVRs).
Starting Methods And How They Affect Generator Size
DOL starts can pull 6–7× full‑load current, causing a rapid kVA step and short voltage dip. Soft starters cut peak current but reduce torque, so longer ramps may be needed. VFDs remove mechanical inrush but introduce a DC‑link charge surge and running harmonics. If you’re specifying for UK plant equipment, sequence starts and allow for transient dip when selecting set kVA.
Managing Harmonics And Power Factor Correctly
First steps: fit input line reactors or DC chokes on 6‑pulse drives. For stricter THDi targets use 12/18‑pulse drives or active front ends (AFEs). Passive or active filters may be applied at the drive or distribution board. A common mistake we encounter is adding standard PFC capacitor banks to a generator — they can resonate with source reactance and worsen harmonics. If PFC is essential, use detuned banks and confirm with a harmonic study during the generator installation and maintenance planning. For installation and ongoing support, see our installation & maintenance services.
What Most People Get Wrong
Most assume generator sizing equals running kW × safety factor. They forget VFD charging surges, THDi heating effects and AVR stability. Those omissions cause undersized alternators and repeated trips.
Alternator And Control Choices That Improve Performance
Specify alternators with Xd″ ≈ 8–12%, suitable winding pitch and skew to reduce harmonic voltage. PMG or AREP excitation with true RMS sensing keeps the AVR stable on distorted loads. Tune UFRO and AVR response during commissioning to prevent hunting. If you plan to parallel sets, discuss droop CTs and kVAR sharing with your engineer early on.
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Practical Sizing Rules For Common Motors
Use these conservative starting points and validate with an engineer or site tests:
- Compressors: DOL ≈ 3–4× motor kW in generator kVA; soft starter ≈ 2–3×; VFD run ≈ 1.2–1.5×. Add 20–30% alternator margin for THDi.
- Centrifugal pumps: DOL ≈ 2.5–3.5×; soft starter ≈ 1.5–2.5×; VFD ≈ 1.15–1.4× with a line reactor.
- Conveyors/fans (high inertia): DOL ≈ 3–5× depending on start time; prefer VFD or soft start; aim for voltage dip ≤15%.
Adjust for altitude and ambient temperature. For quick checks use our kVA calculator. If you need hire or purchase options, PowerTech Generators supplies new sets and flexible generator solutions.
When This Doesn’t Apply
If your site only runs small, isolated VFDs (<5 kW) with AFEs and no parallel gensets, these conservative margins can be relaxed. Always confirm with measurements or an engineer for legacy installations or bespoke controls.
Step‑By‑Step Sizing Method
Follow a simple sequence to remove guesswork:
- Gather nameplate data and starting method for each motor.
- Identify VFD type (6‑pulse, multi‑pulse, AFE) and expected THDi; note any line reactors or filters.
- Sequence starts, apply diversity, and include ambient/altitude derates.
- Check AVR features (RMS sensing, PMG/AREP) and estimate transient dip using Xd″.
- Verify with staged commissioning using a load bank and record THDi/THDv.
Quick Checklist
- Nameplate kW/kVA/PF and start method
- VFD type and THDi target
- Alternator Xd″ and excitation type
- Planned start sequence and allowable voltage dip
Commissioning, Testing And Documentation
Commission with a load bank and staged starts to confirm voltage/frequency dips, THDv/THDi, AVR stability and temperature rise. Document results to ISO 8528 and BS 7671 standards, provide O&M packs and settings records. Planned generator servicing underpins reliability and warranty performance — our teams provide 24/7 support and preventative maintenance.
This image was generated with AI and may not always represent the product or service exactly.
Integration: ATS, Switchgear And Controls
Choose ATS and switchgear rated for ampacity and short‑time withstand. Coordinate transfer logic to avoid back‑feed and manage VFD DC‑link precharge. Set earth‑fault and RCD thresholds to accommodate VFD leakage. Remote monitoring gives resilience and faster fault response from PowerTech Generators.
Procurement Paths And Lifecycle Support
We supply new and quality used generators, fast hire options and turnkey installation including ATS and switchgear. We also manage export, decommissioning and end‑of‑life services. As an independent supplier we source across major manufacturers to match duty, budget and lead time. For project starts, book a free sizing review via our contact page.
FAQs
How Do I Decide If A PMG Alternator Is Necessary?
If your VFDs draw significant harmonic current or you need tight voltage regulation under frequent start cycles, choose PMG/AREP excitation. In our experience it reduces nuisance trips and stabilises AVR response.
What Oversize Should I Budget For When Buying A Generator?
Budget 20–30% alternator margin for sites with 6‑pulse VFDs. Multi‑pulse or AFEs often let you reduce that margin, but confirm with a harmonic study.
How Can I Check If My Process Can Tolerate Voltage Dips?
Define allowable dip for each motor/control and verify during commissioning with staged starts and measurement. Most industrial processes accept 10–15% dip for a few seconds; sensitive controls need tighter limits.
Are Active Front Ends Cost‑Effective For Small Sites?
AFEs lower THDi and improve power factor, often allowing a smaller genset and lower running losses. They have higher upfront cost, so assess payback against reduced generator size and fuel use.
What Information Should I Send For A Sizing Review?
Provide nameplate kW/kVA/PF, voltage, starting method, VFD type, THDi target, start sequence, site altitude/ambient and drawings. The more detail we have, the faster we can produce a reliable recommendation.