In short. Starting a large motor is the hardest load a generator sees. The alternator must supply a large kVA surge, which dips voltage, and the engine must supply the real power, which dips speed. A set that carries the running load comfortably can still fail to start the motor.
Two limits to check
| Limit | Governed by | Symptom |
|---|---|---|
| Voltage dip | Alternator reactance, excitation, AVR response | Contactors drop out, motor torque falls, other loads trip |
| Frequency (speed) dip | Engine power reserve, governor, inertia | Speed falls, can stall engine, protection trips |
The alternator limit is about kVA; the engine limit is about kW. Both must pass.
How to screen
- Compute the motor’s starting kVA from locked-rotor current and voltage.
- Compare with the alternator’s motor-starting capability, usually given as a kVA curve against allowed voltage dip. See alternator motor starting.
- Estimate real power during start and compare with engine reserve. Starting real power is lower than kVA because starting power factor is low, but it rises as the motor accelerates.
- Check the load already on the set. Motor start on a lightly loaded set is easier than with a base load.
- Run a dynamic simulation or consult the manufacturer for final confirmation.
What helps
- Reduced-voltage or soft starting to limit current.
- VFD for controlled start, with harmonic checks.
- Alternator with lower reactance or oversized frame, giving more kVA surge capability.
- Excitation with short-circuit support (PMG) for stronger voltage recovery. See PMG excitation.
- Governor and AVR tuning and under-frequency roll-off settings.
- Sequencing so the motor starts first, or loads are shed during the start.
- BESS or parallel sets to supply the surge. See load-step response.
Ratings of thumb versus studies
Quick rules, such as a generator needing a multiple of the largest motor’s kW, vary a lot with starting method and acceptable dip. Treat them only as a starting point and confirm with the alternator data.
Illustrative example
A 160 kW pump motor with direct-on-line starting at six times full-load current is proposed on a 500 kVA set. The kVA screening shows the dip would exceed the allowance, and the engine reserve is thin at start. Options studied: a soft starter reducing current to about three times, a 750 kVA set, or sequencing so the pump starts before other loads. The soft starter plus sequencing is chosen as the lowest-cost route that passes.
Two limits act together
Starting a large motor on a diesel generator tests the alternator (voltage dip, driven by reactance) and the engine (frequency dip, driven by available power and governor response). Both must be inside limits.
| Limit | Controlled by |
|---|---|
| Voltage dip | Alternator transient reactance, AVR response, oversizing of alternator |
| Frequency dip | Engine turbo response, governor, step-load acceptance, inertia |
| Motor acceleration | Torque available at reduced voltage |
Motor torque falls with voltage squared
Torque ∝ V², so at 80 percent voltage torque is about 64 percent of the full-voltage value
If the load torque at start exceeds the reduced motor torque, the motor will not accelerate. Compressors and pumps with high starting torque are especially sensitive.
Rule-of-thumb check (illustrative)
A common first screening is that the generator kVA be several times the motor’s starting kVA divided by a permissible dip factor. Use it only to shortlist. The decision needs the alternator’s X’d and the engine’s step-load data from the maker.
Solutions
- Soft starter or VFD to cut starting current to 2 to 3 times.
- Oversize the alternator, not necessarily the engine.
- Start the large motor first, before other loads connect.
- Unload the generator via sequencing before the start.
See motor starting and frequency dip.
Common mistakes
- Sizing from running kW only.
- Not considering base load on the set at the time of the start.
- Ignoring governor and AVR response.
What to send us
Send motor and generator datasheets and the starting method. Send your application details. See also motor starting studies.
Questions engineers ask
Why does my generator stall on a motor it can carry running?
Starting needs more kVA and a short burst of power. Running load can be far lower.
Does a bigger generator always fix it?
It helps, but soft starting or sequencing can be cheaper.
What is under-frequency roll-off?
An AVR feature that reduces voltage when speed falls, easing the load on the engine.
Can the motor be started with load connected?
Sometimes, but starting with minimal load is easier and safer for the set.
This guide gives general principles. Confirm with manufacturer data and simulation.