In short. An induction motor started direct-on-line draws five to eight times its full-load current at a power factor of 0.2 to 0.4 until it reaches speed. On a diesel generator that surge pulls the voltage down and loads the engine suddenly, so the set must be chosen for the starting event, not just the running load. Star-delta, soft starters and variable-frequency drives reduce the demand, and staggering starts is often cheaper than a larger set.
What happens in the first second
At standstill an induction motor behaves like a transformer with a short-circuited secondary. Only the leakage reactance of the windings limits current, so the motor draws its locked-rotor current, typically six times full-load current for a squirrel-cage machine, at a very low power factor. As the rotor accelerates the current falls and the power factor rises until the motor settles at its running current. On a stiff grid this takes a fraction of a second for a small pump and a few seconds for a large fan or crusher.
On a generator, two things make the event harder. The alternator has a high internal reactance compared with a utility transformer, so the starting current depresses the terminal voltage. Torque falls with the square of voltage, so acceleration slows and the high current persists longer. At the same time the engine sees a sudden step in real power, and the governor must open the fuel rack and the turbocharger must spool up before the engine can respond. Voltage dip and frequency dip arrive together.
Inputs that decide the result
- Motor rated kW, voltage, efficiency, power factor and full-load current from the nameplate or data sheet.
- Locked-rotor current as a multiple of full-load current, or the IEC starting code, from the motor data sheet.
- Starting method and, for soft starters and drives, the current limit setting.
- Load torque characteristic of the driven machine and the combined inertia of motor and load, which together set the acceleration time.
- Load already on the generator when the motor starts, and the other motors that may start at the same moment.
- Cable length and size between generator and motor, which adds voltage drop on top of the alternator’s dip.
- The alternator’s transient reactance and the engine’s load acceptance, from the manufacturer’s data.
The starting kVA itself is calculated in the starting kVA guide; the point here is that the number only means something alongside the sequence and the source.
Comparing starting methods
| Method | Starting current (multiple of FLC) | Starting torque | Notes for generator supply |
|---|---|---|---|
| Direct-on-line (DOL) | 5 to 8 | Full, 150 to 250 percent of rated | Simplest and hardest on the set; fine for small motors |
| Star-delta | About one third of DOL, 2 to 3 | One third of DOL | Only suits loads that start light; a second current surge occurs at the changeover to delta |
| Autotransformer | Set by tap, typically 2.5 to 4 | Reduced by the tap squared | Better torque per ampere than star-delta; bulky |
| Soft starter | Adjustable, usually 2.5 to 4.5 | Reduced, ramps up | Best general solution for pumps, fans and compressors on generator |
| Variable-frequency drive | 1 to 1.5 | Full torque from zero speed | Lowest starting demand, but adds harmonic current to the alternator |
The figures are typical ranges to be confirmed from motor and starter data. Two cautions apply. A star-delta starter on a loaded compressor may fail to reach speed in star and then transfer to delta at low speed with a surge almost as large as DOL. A drive solves the starting problem but adds harmonic current, so the alternator must be checked for non-linear load capability instead.
Sequence and simultaneous restart
Three 30 kW pumps starting one after another present the generator with three modest events. The same three pumps starting together after an ATS transfer present one event three times as large, on an alternator that may already be carrying other load. That is often the difference between a 125 kVA set and a 250 kVA set.
Restart after an outage deserves particular attention. Contactors drop out on loss of supply, and when the generator takes the bus every motor with an automatic restart tries to return at once. A sequencer in the PLC or BMS, or simple time-delay relays giving 10 to 30 seconds between groups, converts one large event into several small ones. Where this is not possible, the load management scheme should hold back the heavy motors until the set has settled on the base load. In our experience across industrial sites in Telangana, a restart sequence is the most cost-effective fix for a generator that trips on transfer.
Voltage drop in the motor circuit
The dip at the alternator terminals is not what the motor sees. The cable adds its own drop, and during starting that drop is five to eight times its running value. A 100 metre run that loses 2 percent at full load loses 10 to 16 percent at locked-rotor current. Added to a 15 percent alternator dip, the motor terminal voltage may fall to about 70 percent, where torque is only half of rated, and if load torque exceeds motor torque the motor stalls until protection trips. Long runs to borewell pumps, remote compressors and yard cranes are the usual culprits; see the voltage drop guide.
A worked example (illustrative)
A rice mill runs a 75 kW DOL motor on its main huller with a locked-rotor current of 6.5 times a full-load current of 135 A. Starting current is about 880 A, which at 415 V is roughly 630 kVA at a power factor near 0.3. The mill’s other running load during a start is about 120 kVA. A 250 kVA set that comfortably carries the running load would see a starting demand two and a half times its rating and would either trip on under-voltage or stall the motor. The options are a set of around 500 kVA chosen for its motor-starting capability, or a soft starter limiting current to 3.5 times full-load current, which reduces the event to about 340 kVA and allows a 320 kVA set, provided the huller starts unloaded. The second option costs less and runs the set at a healthier load. Both must be confirmed against the alternator’s published starting kVA at the acceptable dip.
Common mistakes
- Sizing the generator on the motor’s running kW, so the set carries the plant easily until the first large motor is started and then trips on under-voltage.
- Fitting a star-delta starter to a compressor that starts under load, which stalls in star and surges at changeover almost as hard as DOL.
- Letting all motors restart together after transfer, which creates a block load far above anything the set was sized for.
- Ignoring the cable run to a remote motor, so the alternator dip looks acceptable while the motor terminal voltage is too low to accelerate the load.
What to send us
Send nameplate photographs of your largest motors, the type of starter on each and any current-limit settings, what each motor drives and whether it starts loaded, the length and size of the cable to the farthest large motor, and what else is running when the motors start. If the set already trips during starting, send the controller’s alarm log or a photograph of the fault screen. Send your application engineering enquiry.
Questions engineers ask
How large a motor can a generator start direct-on-line?
As a rough screening rule, a standard set can start a DOL motor of about 20 to 25 percent of its kVA rating from no load with an acceptable dip. The real figure depends on the alternator’s transient reactance and the load already on the set, and must be read from the manufacturer’s motor-starting data.
Will a soft starter solve generator tripping on motor start?
Usually, if the driven load can accelerate at the reduced torque the current limit allows. Set the limit as low as the load permits, typically 3 to 4 times full-load current, and confirm the acceleration time does not exceed the starter’s thermal rating.
Why does the motor start fine on grid but not on generator?
The grid is a stiff source with low impedance, so the voltage barely moves during starting. The alternator’s impedance is many times higher, so the same starting current produces a large dip, lower torque and longer acceleration, and the engine must also respond to the sudden load.
Does oversizing the alternator alone help?
Yes, substantially. A larger alternator frame on the same engine has lower reactance per unit of load and gives a smaller voltage dip, which is often the cheapest fix at the specification stage. It does not help with frequency dip, which depends on the engine.
How should motors be sequenced after a power failure?
Start the largest motor first on the lightly loaded set, then add smaller motors in groups with 10 to 30 seconds between them. The order matters because the dip for a given start is smallest when the set is lightly loaded.
Technical content reviewed by Srinivas Reddy, Managing Partner, Reddy Generators. Last updated 19 October 2026.