In short. The starting method sets the current a motor draws and the torque it produces. On a generator, lower starting current means a smaller voltage dip and a smaller generator, but the motor must still develop enough torque to accelerate its load. Choose the method that meets the torque need with the lowest current.
The trade-off
Torque falls with the square of the voltage applied. A method that cuts current by reducing voltage also cuts torque by a bigger factor. A fan or centrifugal pump has a low starting torque need and tolerates reduced-voltage starting; a loaded conveyor, a compressor under load or a crusher may need near full torque at start.
Comparison
| Method | Starting current | Starting torque | Best for | Watch for |
|---|---|---|---|---|
| Direct on line | 5 to 7 × FLC | High | Small motors, strong supply | Voltage dip; generator size |
| Star-delta | About 1/3 of DOL | About 1/3 of DOL | Light-start loads | Transient on changeover; needs six-terminal motor |
| Auto-transformer | Depends on tap | Depends on tap | Moderate-torque loads | Size and cost |
| Soft starter | Adjustable, typically 2 to 4 × FLC | Adjustable | Pumps, fans, conveyors | Heat; bypass contactor; harmonics during start |
| Variable-speed drive | About 1 to 1.5 × FLC | Full torque available | Process control, high-inertia loads | Harmonics; generator interaction; cost |
| Wound-rotor with resistors | Low | High | Special heavy-start duties | Maintenance |
FLC is full-load current. Figures are typical orders of magnitude; use the motor and starter data for design.
Selection steps
- Find the load’s torque demand at start and the acceleration time needed.
- List candidate methods that give enough torque.
- Compare the starting kVA of each with the generator’s capability. See motor-starting capacity.
- Check voltage dip against sensitive loads.
- Consider harmonics, cost, maintenance and control needs.
Generator-specific considerations
- Soft starters can be tuned for generator operation, with a lower current limit than on the grid.
- Drives draw little starting current but their rectifiers can distort generator voltage, so check the alternator and filtering. See harmonics and THD.
- Star-delta changeover can cause a transient, and open transition may produce a second current spike. Closed transition can reduce it.
- Several motors starting together multiply the effect, so sequence them. See restart sequencing.
Worked example (illustrative)
A 110 kW pump motor must start on a 600 kVA generator already carrying 250 kVA. DOL start is about 800 kVA, total 1,050 kVA, too much. Star-delta cuts the start to about 270 kVA, total 520 kVA, which is feasible if the pump’s starting torque need is under about a third of the motor’s DOL torque. A pump curve may show that requirement is met. A soft starter would give similar current with adjustable torque, at higher cost. The choice follows the pump’s torque curve.
Starting methods compared
| Method | Current multiple | Torque | Notes |
|---|---|---|---|
| Direct on line (DOL) | 6 to 8 | Full | Simple, hardest on a generator |
| Star-delta | 2 to 3 | About one third | Needs light-load starts; open transition surge on changeover |
| Autotransformer | 3 to 4.5 | 40 to 65 percent | Taps choose a balance |
| Soft starter | 2 to 4 | Adjustable ramp | Harmonics during start, bypass after |
| VFD | 1 to 1.5 | Full from zero speed | Harmonics in operation, filters may be needed |
Selecting by load type
- Centrifugal pumps and fans: start with low torque, so star-delta or soft start works.
- Reciprocating compressors and conveyors: high starting torque, which limits reduced-voltage options; unloaded start valves help.
- Screw compressors: often star-delta or soft start with unloaded start.
- High-inertia loads: long acceleration with soft start needs thermal checks on the motor.
Generator impact table (illustrative)
For a 110 kW motor on a 500 kVA set: DOL may require a 750 kVA or larger alternator; star-delta may allow 500 kVA; VFD may allow 400 kVA. The reduction in generator cost can pay for the starter several times over. Verify with the maker’s data.
See motor-starting capacity guide and motor and feeder protection.
Common mistakes
- Choosing a reduced-voltage method without checking torque.
- Forgetting that a VFD adds harmonic load to the generator.
- Specifying the starter after the generator is bought.
What to send us
Send the motor ratings, load type and current starting method. Send your application details. See also motor and feeder protection.
Questions engineers ask
Which starting method is easiest on a generator?
A variable-speed drive draws the least starting current, followed by soft starters and star-delta, provided they give enough torque.
Why not use soft starters on everything?
They cost more, generate heat and are not always needed. Small motors may start DOL without trouble.
Does star-delta work for any motor?
It needs a motor wound for delta at the supply voltage with six terminals, and a load that starts at about one-third torque.
Do VFDs affect generators?
They add harmonic current. Check alternator sizing and filtering.
Figures are typical orders of magnitude. Confirm with equipment data.