What buyers should know first
A motor can draw five to seven times its full-load current during direct-on-line (DOL) starting. On a weak grid that dip is absorbed by the utility. On a industrial generator sets, the dip appears directly at the alternator terminals and can trip contactors, upset controls, or cause other motors to stall. For construction generator sets and factory buyers, this is often the real sizing event, not the steady-state running load.
This guide compares DOL, soft starter, and variable-frequency drive (VFD) starting from a genset sizing perspective, and gives the RFQ language that forces suppliers to size the alternator insulation and temperature-rise guide for your largest motor.
How starting method changes the inrush
The starting method decides how much current the motor draws at start and for how long. The table below is a practical starting point. Actual numbers depend on motor design, load torque, and starter settings.
| Starting method | Typical starting current | Typical ramp time | Genset impact |
|---|---|---|---|
| Direct-on-line (DOL) | 6 – 7 x FLC | 1 – 3 s | Highest dip; needs the largest generator or a dedicated starting set. |
| Star-delta starter | 2 – 3 x FLC | 2 – 5 s | Two current steps; dip is lower but the transition surge still matters. |
| Soft starter (voltage ramp) | 2.5 – 4 x FLC | 3 – 10 s | Adjustable; reduces peak dip but extends the event. |
| Variable-frequency drive (VFD) | 1 – 1.5 x FLC | 5 – 15 s ramp | Lowest current; active front end may add harmonics. |
FLC means full-load current. A 100 kW motor on DOL can momentarily look like a 600 kW load. If the generator set is only sized for the running load, the voltage dip can be 30 % or more.
NEMA code letters and kVA code
A motor nameplate usually shows a NEMA code letter such as H, J, or K. That letter translates to a locked-rotor kVA per horsepower. Code letters around G, H, and J are common for general-purpose motors; lower letters mean lower starting kVA and less stress on the generator.
When asking a supplier to size a generator for a motor load, always provide the motor code letter, rated kW, starting method, and the load inertia. Without these four items, any generator recommendation is an educated guess.
Procurement checklist
- List every motor above 15 kW with its full-load current and NEMA code letter.
- State the starting method for each motor: DOL, star-delta, soft starter, or VFD.
- Identify which motors start simultaneously and which can be sequenced.
- Request the predicted voltage dip for the largest single starting event.
- Ask whether the quoted set uses a close-ratio or extended-capability alternator.
- Confirm the generator can handle the starting kVA while already carrying base load.
Soft starters and VFDs: hidden considerations
Soft starters reduce peak current but lengthen the starting time. The generator sees a lower dip over a longer period, which can still cause overheating if repeated frequently. VFDs draw near-unity power factor during ramp but may produce harmonics that heat alternator windings and transformers. Harmonic filters or line reactors are often needed.
If a site has several motors, the best solution is often a combination: VFDs or soft starters on the largest motors, plus staged starting so the generator never sees more than one large inrush at a time. request a quote can review the motor schedule against Cummins diesel generator sets, Volvo diesel generator sets, and Perkins diesel generator sets starting curves.
RFQ wording that gets useful answers
Instead of ‘size generator for 200 kW load’, ask: ‘Size the generator set to start a [X] kW [NEMA code] motor via [DOL/soft/VFD] while the set is already carrying [Y] kW of base load. State predicted voltage dip, recovery time, and alternator sub-transient reactance used in the calculation.’
Questions buyers often ask
Can a soft starter eliminate voltage dip completely?
No. It lowers the peak dip but extends the starting time. Total energy drawn is similar; repeated starts can still heat the alternator.
Why not just oversize the generator for DOL starting?
Oversizing works but increases capital cost, fuel consumption at low load, and maintenance. A soft starter or VFD is usually cheaper than buying twice the kVA.
What is sub-transient reactance and why does it matter?
X”d is the alternator’s internal impedance during the first few cycles of a fault or motor start. A lower X”d means less voltage dip during starting.
Do I need to consider the load already on the generator?
Yes. A set that can start a motor on its own may fail if it is already supplying lighting, controls, and other motors when the start command arrives.
How does altitude affect motor starting?
Engine power de-rates with altitude, which reduces the generator’s ability to recover frequency. Combine altitude de-rating with motor-starting margin, not one or the other.
Getting the motor-starting calculation right is the difference between a generator sets from 200 kW to 400 kW set that works and one that trips every Monday morning. Bring the motor schedule and starter details to the RFQ stage.