Why Motor Starting On Generators Matters For Pumps, HVAC And Compressors
Induction motors draw 3–8x full-load current when starting. With direct-on-line (DOL) starts this is typically 6–7x. The surge pulls generator voltage and frequency down until the engine governor and automatic voltage regulator (AVR) recover. Pumps, air handling units (AHUs), chillers and screw compressors are most affected. So are building controls: programmable logic controllers (PLCs), building management systems (BMS), uninterruptible power supplies (UPS) and contactors can trip on brownout.
In construction, data centres, manufacturing, healthcare and utilities these dips cause downtime, nuisance alarms and premature wear. Correct specification prevents trips, protects assets and keeps operations resilient. PowerTech Generators provides engineering reviews, design support and 24/7 response across the UK. For a quick assessment, contact our team.
Understanding Inrush Current And Starting kVA
Locked-rotor current (LRC/LRA) is the inrush drawn when the rotor is stationary. Typical multipliers by load type:
- Centrifugal pumps and fans: ~5–7x full-load current (FLC)
- Positive displacement compressors: ~6–8x FLC
- High-inertia loads (e.g., large blowers): can exceed these values
Starting power factor is low (about 0.15–0.30), so kVA demand is high even if kW is modest.
Estimating starting kVA:
- Identify motor full-load kVA (from nameplate or kW ÷ efficiency ÷ PF).
- Apply the expected inrush multiple (e.g., 6–7x for DOL).
- Allow for low starting PF to understand alternator kVA stress.
Design targets (aligned with ISO 8528 practice): aim for 10–15% site voltage dip and <5% frequency dip. Engine recovery is typically 1–3 seconds due to governor and turbo lag—critical for contactor hold-in and protection coordination.
Generator Characteristics That Drive Motor-Start Performance
- Alternator subtransient reactance (X”d): lower X”d reduces the initial voltage sag.
- AVR transient response: a fast, stable AVR shapes recovery and prevents hunting.
- PMG or AREP excitation: decouples the AVR from bus voltage during inrush so field strength is held when it matters.
- Three-phase sensing: improves regulation on unbalanced and distorted loads.
- UFRO (under-frequency roll-off): avoids over-excitation and instability as engine speed dips.
- Good stability/gain settings and RFI filtering: minimise nuisance roll-off and oscillations.
For tough starts and non-linear loads, a PMG alternator with a fast AVR (e.g., Stamford MX321, Leroy-Somer D510) is proven. Pair with engines offering strong step-load acceptance and stable governors from Cummins, Perkins, FG Wilson or Baudouin. See our practical guide to AVRs for common settings and benefits.
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Choosing DOL, Star–Delta, Soft Starters Or VFDs
- DOL: ~6–7x FLC, full starting torque.
- Star–delta: ~2–3x FLC, reduced torque (not suited to high static torque loads).
- Autotransformer: adjustable current, moderate torque.
- Soft starter: ~2–4x FLC with a ramped torque profile.
- VFD: ~1–1.5x FLC with full speed/torque control.
Select the method that meets process torque needs without overstressing the generator.
Soft starters and VFDs introduce harmonics and fast voltage changes that can upset AVRs. Mitigate with line reactors or filters and prefer PMG-equipped alternators. As a rule of thumb: pumps often suit soft starters; fans typically benefit from VFDs for energy savings; screw compressors frequently require DOL or heavy-duty soft start. Set EMC filters correctly and confirm ramp profiles on site.
Sizing Methodology And Oversizing Margins That Avoid Voltage Dip
Gather the following before sizing:
- Motor power (kW/HP) and full-load current (FLC)
- LRC/LRA or NEMA code letter
- Starting method (DOL, star–delta, soft starter, VFD)
- Line voltage and starting PF
- Load type/inertia and any process torque at start
- Restart frequency, ambient temperature and altitude (for de-rating)
Use motor data and alternator curves to model dip and recovery. For quick estimates, try our kVA calculator.
Worked example (30 kW, 400 V pump):
- DOL at ~6x inrush: typically a 125–160 kVA set with PMG and a fast AVR to stay within a 15% dip.
- Soft starter at ~3x: an 80–100 kVA set often suffices.
- VFD at ~1.2–1.5x: 60–80 kVA is common.
Sequence multiple motors and shed non-essential loads during starts to stabilise the bus. Always finalise against manufacturer curves for the specific alternator and engine.
Specifying Alternator And Control Features For Stable Starts
Prioritise AVR and alternator features:
- PMG supply or AREP excitation
- Fast transient response with adjustable gain/stability
- Three-phase sensing and UFRO
- RFI filtering and over-excitation protection
Coordinate protections and controls:
- Set undervoltage/underfrequency thresholds and delays to ride through expected dips.
- Confirm overcurrent and reverse power settings versus starting profiles.
- Include droop CTs for paralleling readiness and remote monitoring for O&M.
Correct settings and documentation improve repeatability and resilience.
Protecting Sensitive Loads And Integrating With Building Systems
- Coordinate transfer timing, capacitor/drive pre-charge, and staggered restarts.
- Pre-energise transformers or use soft-loading to manage magnetising inrush.
- Agree acceptable voltage/frequency windows with UPS and critical controls.
Commissioning matters: prove step-load and motor-start performance to ISO 8528, record AVR/governor tuning, and run harmonic checks on drive-heavy sites. For a structured approach, read understanding generator load testing. Keep test records for BS 7671 compliance and future maintenance.
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Procurement And Delivery: New, Used Or Hire For Motor-Start Projects
- Buy new for bespoke PMG/AVR specifications, factory options and warranties.
- Choose used for cost-effective availability—verify alternator and AVR motor-start capability.
- Hire for temporary capacity during upgrades, outages or seasonal demand.
PowerTech Generators delivers survey, design, generator installation, ATS panels, cabling, fuel systems, commissioning and training. We offer 24/7 support, servicing and spares. Explore new generators, quality-checked used generators and flexible generator hire to match your project and budget.
Common Pitfalls And Troubleshooting On Motor Starts
Typical symptoms include:
- Voltage dip >15% and excessive frequency swing
- Contactor chatter or nuisance trips on PLCs, BMS or UPS
- Soft starter or VFD faults during acceleration
- AVR hunting or instability under non-linear load
Likely causes are undersized alternators, slow/unsuitable AVRs, poor settings or the wrong starting method.
Fixes to consider:
- Upgrade to PMG excitation or a faster AVR; tune UFRO and gain/stability.
- Fit line reactors/filters; adjust soft-start ramps or VFD current limits.
- Improve start sequencing and shed non-essential loads.
- Upsize alternator/engine if dips remain outside limits.
Make changes safely, document settings and verify with witnessed tests.
FAQs On Generator Motor Starting
Do we need PMG for soft starters and VFDs? Strongly recommended. PMG stabilises the AVR during inrush and harmonic distortion, reducing dip and hunting. Tough DOL starts also benefit.
Can one generator start multiple motors together? It’s rarely advisable. Sequence starts and shed non-essential loads to keep voltage and frequency within limits.
What voltage dip is acceptable on commercial sites? Design for 10–15% and coordinate with equipment tolerances. Confirm with site standards and the UPS playbook for critical loads. Share motor data for a quick sizing check from PowerTech Generators.
FAQs
How Do I Know If My Generator Is Big Enough For A DOL Start?
Check the motor LRC and calculate starting kVA. Compare against alternator motor-start curves and engine step-load acceptance. If dip exceeds 15%, upsize the set or change the start method.
Will A Soft Starter Or VFD Reduce My Fuel Use?
Soft starters cut inrush, not running energy. VFDs reduce energy on variable-torque loads (fans, pumps) by matching speed to demand—most gains appear at partial load.
Why Does My AVR Hunt When A VFD Starts?
Drives inject harmonics and fast current changes. Use PMG excitation, fit line reactors/filters and tune AVR stability. A fast, robust AVR usually resolves hunting.
What Tests Should We Perform Before Handover?
Run step-load and motor-start tests to ISO 8528, confirm transfer coordination, and capture AVR/governor settings. Include harmonic checks for sites with many drives.
Can You Support Sites Outside Normal Hours?
Yes. PowerTech Generators provides 24/7 support, nationwide. We also offer proactive maintenance to minimise unplanned outages.
Do You Supply Both New And Used Sets For Motor-Start Projects?
Yes. We specify the right alternator, AVR and engine package across new and used options, and we can provide short or long-term hire to bridge upgrades.