In short. Starting a motor draws several times its rated current, which makes a large momentary demand and drops the generator voltage. A generator sized only for running load may stall or trip on a big start. Screen the starting kVA against the generator’s capability, then reduce it with a better starting method if needed.
What happens when a motor starts
A direct-on-line (DOL) induction motor can draw about five to seven times its full-load current at low power factor for a few seconds. The generator voltage dips, the engine slows momentarily, and other loads may drop out if the dip is deep. A larger or more capable generator, a gentler starting method or both can fix it.
Starting kVA
The motor data sheet gives locked-rotor current or locked-rotor kVA. Using a starting multiple, a 75 kW motor at about 0.88 power factor and 0.93 efficiency has a rated input of about 92 kVA. At six times, starting kVA is about 550.
Starting methods and their effect
| Method | Typical starting current | Notes |
|---|---|---|
| Direct on line | 5 to 7 × full load | Simple; hardest on the generator |
| Star-delta | About one-third of DOL | Closed-transition issues; torque limited |
| Auto-transformer | Depends on tap | Reduced current and torque |
| Soft starter | Typically 2 to 4 × full load | Smooth; adjustable |
| Variable-speed drive | About 1 to 1.5 × full load | Lowest; adds harmonics |
See starting methods and generator impact.
A screening method
- List the largest motors and their starting kVA.
- Add the running load already on the generator, in kVA.
- Compare the total with the generator’s short-term kVA capability at the allowed voltage dip. Many sets can supply roughly two to three times their rating briefly at reduced voltage; the manufacturer gives the real curve.
- Check the voltage dip against the limits of contactors and other loads, often around 15 to 20 per cent for contactors and less for sensitive equipment.
- Consider frequency dip and recovery.
Worked example (illustrative)
A 500 kVA generator carries 200 kVA. A 75 kW pump starts DOL at about 550 kVA. The total is 750 kVA, 1.5 times the generator rating. The manufacturer’s curve might show the voltage falling more than 25 per cent at that demand, which would drop contactors. A soft starter cutting the start to 3 × full load (about 275 kVA) gives 475 kVA total, within the curve. The method changed the result more than a bigger generator would have, which is why starting method belongs in the sizing decision.
Where the screening stops
Real behaviour depends on the alternator reactance, excitation system, engine governor and load inertia. For large motors, or many starts in sequence, a transient study or a manufacturer’s check is needed. See large-motor starting on diesel generators.
A quick screen for the largest motor
Starting kVA = rated kW ÷ (efficiency × running PF) × (locked-rotor current ÷ rated current)
| Starting method | Typical starting current multiple |
|---|---|
| Direct on line | 6 to 8 times |
| Star-delta | 2 to 3 times |
| Autotransformer | About 3 to 4.5 times (tap dependent) |
| Soft starter | 2 to 4 times (set by current limit) |
| VFD | 1 to 1.5 times |
Worked example (illustrative)
A 75 kW compressor motor, efficiency 0.93, PF 0.86: running kVA = 75 ÷ (0.93 × 0.86) = 93.8 kVA. Direct on line at 7 times = 656 kVA starting. Voltage dip estimate on a generator with 25 percent transient reactance per its own kVA rating: dip ≈ starting kVA ÷ (starting kVA + generator kVA ÷ X’d). For a 400 kVA set, short-circuit kVA ≈ 400 ÷ 0.25 = 1,600, so dip ≈ 656 ÷ (656 + 1,600) = 29 percent. Too high for most loads. With star-delta (2.5 times): 235 kVA, dip ≈ 235 ÷ (235 + 1,600) = 12.8 percent, acceptable for many motors but not for sensitive electronics.
Check outputs
- Voltage dip against the limit for contactors and other loads.
- Whether the motor reaches speed at the reduced voltage.
- Whether other loads trip.
Alternator selection (larger frame, lower X’d) often fixes a dip more cheaply than a larger engine. See motor starting and large-motor starting studies.
Common mistakes
- Sizing the generator for the running kW of the motor only.
- Ignoring the load already running.
- Using ratios for the wrong starter type.
- Neglecting that several motors may restart together after an outage.
What to send us
Send the motor list with starting methods and the generator details. Send your application details. See also motor starting.
Questions engineers ask
How much bigger must a generator be than the motor?
It depends on the starting method and other load. DOL starts often need a generator several times the motor’s kW rating, while soft starters or drives need less.
What voltage dip is acceptable?
It depends on the loads. Contactors and electronics often tolerate perhaps 15 to 20 per cent or less. Check the equipment data.
Does the alternator type matter?
Yes. Oversize or special winding alternators and good excitation support give better starting performance.
Are VFDs always better?
They give gentle starts but add harmonics and may need a larger alternator or filtering.
Figures are illustrative. For final design use the manufacturer’s starting curves or a transient study.