On most industrial sites the generator is not sized by the running load. A diesel generator for motor starting duty is sized by the surge, not the steady state. It is sized by the largest motor. A direct-on-line induction motor can draw five to seven times its full-load current for the first few hundred milliseconds, and the alternator has to supply that surge without letting the bus voltage collapse. Once you know the locked-rotor kVA of the biggest motor and the voltage dip your other equipment can tolerate, the required frame size follows from a fairly short calculation.
Why Motor Starting Dominates the Sizing Decision
A motor at rest produces no back-EMF. Until the rotor approaches running speed, the stator sees little more than winding resistance and leakage reactance, so current is limited mainly by impedance. The result is a large inrush at a power factor typically between 0.2 and 0.4.
Two consequences follow. The alternator must deliver a kVA pulse far above its continuous rating, and the terminal voltage dips in proportion to that pulse. The dip is usually the binding constraint, because contactors, drives and control power supplies on the same bus are far less tolerant than the generator itself.
Start From Locked-Rotor kVA, Not From Horsepower
The reliable input is locked-rotor kVA. If the motor nameplate carries a NEMA code letter, that letter defines a range of locked-rotor kVA per horsepower. The commonly encountered letters are:
| NEMA code letter | Locked-rotor kVA per horsepower (NEMA MG-1) |
|---|---|
| F | 5.0 – 5.59 |
| G | 5.6 – 6.29 |
| H | 6.3 – 7.09 |
| J | 7.1 – 7.99 |
| K | 8.0 – 8.99 |
| L | 9.0 – 9.99 |
| M | 10.0 – 11.19 |
| N | 11.2 – 12.49 |
Multiply the range by the motor horsepower to get locked-rotor kVA, and use the upper bound when you have no reason to do otherwise. Always prefer the actual nameplate locked-rotor current if it is available; the code letter is a fallback, not a substitute.
Estimating the Voltage Dip and Choosing a Starting Method
The dip can be approximated from the alternator's sub-transient reactance, expressed in per-unit on the machine's own base:
Approximate dip (per-unit) ≈ S_start ÷ (S_start + S_rated ÷ X''d)
where S_start is the locked-rotor kVA of the motor plus any load already running, S_rated is the generator kVA rating, and X''d is the alternator sub-transient reactance in per-unit. The expression is an estimate for planning; the manufacturer's motor-starting study is the document of record.
A worked comparison shows how much the starting method matters. Take a 90 kW process pump, roughly 120 hp, code letter H, running at 0.85 power factor.
| Case | Locked-rotor kVA | Generator considered | Approximate dip |
|---|---|---|---|
| Direct-on-line | 120 hp × 7.0 ≈ 840 kVA | 500 kVA, X''d ≈ 0.12 | 840 ÷ (840 + 4,167) ≈ 17% |
| Star-delta (3×) | ≈ 360 kVA | 500 kVA, X''d ≈ 0.12 | 360 ÷ (360 + 4,167) ≈ 8% |
| Soft starter set to 2.5× | ≈ 300 kVA | 400 kVA, X''d ≈ 0.12 | 300 ÷ (300 + 3,333) ≈ 8% |
The same motor therefore fits comfortably on a 400 kVA set with a soft starter, while direct-on-line pushes the dip on a 500 kVA machine into a range that many facilities would reject. That difference is usually far cheaper than buying the larger generator.
Each method trades inrush against torque, cost and complexity. The right choice depends on what the driven machine needs in order to accelerate.
| Method | Inrush reduction | Torque penalty | Where it fits |
|---|---|---|---|
| Direct-on-line | None | None | Small motors on a generously sized bus |
| Star-delta | Roughly one third | Torque falls to about one third of full | Fans and pumps with low breakaway torque |
| Autotransformer | Depends on tap | Scales with the square of the applied voltage | Larger motors where reduced torque is acceptable |
| Soft starter | Adjustable, typically 2.5–3× | Ramped and adjustable | Pumps and compressors; good general choice |
| Variable frequency drive | Lowest | Full torque across the ramp | Where process control justifies the cost |
A reduced-torque start is only viable if the load lets the motor accelerate. A loaded conveyor or a high-inertia fan may simply stall at one third torque, in which case the reduced-current method saves nothing.
What the Alternator and Excitation System Contribute
Two machines with the same kVA rating can behave very differently during a start. The variables that matter are the sub-transient reactance, the excitation type and the AVR response.
- A lower sub-transient reactance raises the short-circuit contribution and reduces the dip, at the cost of higher fault currents that the protection must handle.
- PMG or auxiliary-winding excitation keeps the AVR supply independent of the output terminals, so regulation recovers faster than on a self-excited machine.
- The AVR's response time and its sensing arrangement determine how quickly voltage returns after the dip.
- Alternator frame size matters: a larger frame on the same kVA generally has more thermal and magnetic margin.
In practice this means you should ask for the alternator's sub-transient reactance, excitation type and a motor-starting calculation for the specific frame, rather than comparing kVA ratings across suppliers.
Sequenced Starting and Load Management
Where several large motors exist, the cheapest solution is often control rather than capacity. Starting the largest motor first, with the others interlocked until it is up to speed, keeps the surge to one motor at a time. Unloading a pump before start — closing a valve or starting against a soft starter — reduces both inrush and mechanical stress.
The same logic applies after a mains failure. An automatic transfer scheme that re-energises everything at once creates a far worse transient than one that stages the load. See how an automatic transfer switch works for the transfer side of that sequence.
What Your Supplier Needs, and Where This Fits
Motor starting cannot be studied from a single kW figure. Send the following and the calculation becomes straightforward:
- Motor nameplate data: rated kW or hp, voltage, phases, full-load current, power factor and NEMA code letter
- Starting method for each motor, and whether it is fixed or selectable
- Starting sequence and any interlocks between motors
- Load already running when a motor starts, with its power factor
- Maximum allowable voltage dip and recovery time for the facility
- Site altitude and maximum ambient temperature, since both affect output
The last two items are frequently overlooked. A set that clears the dip at sea level may not do so at altitude, because the engine has less power available during the transient and the alternator's cooling margin is smaller.
Motor starting is one of three checks that a complete sizing exercise has to pass: steady running load, worst-case starting surge, and step load acceptance. Passing one does not imply passing the others. Our guide to diesel generator specification parameters lists the ratings and tolerances that should be on the datasheet before you compare proposals.
Once the required starting kVA is known, you can match it against a specific frame — the Shangchai 600 kW diesel generator set is one example of the output band that typically serves mid-sized industrial motor loads.
FAQ
Can I start a motor across the line if the generator is much larger than the motor?
Often yes, but "much larger" has to be demonstrated rather than assumed. Run the dip calculation for the alternator's actual sub-transient reactance, and confirm that the dip stays within what the rest of the facility can tolerate.
Does a soft starter always solve the problem?
No. It reduces inrush and therefore the dip, but it also reduces available starting torque. If the load needs high breakaway torque, a soft starter may not accelerate it.
Why does the voltage dip matter if the generator recovers?
Because the equipment sharing the bus may not wait. Contactors can drop out, DC control supplies can trip, and drives can fault on undervoltage before the generator recovers.
Do variable frequency drives remove the starting problem entirely?
They largely remove the inrush, yes. The trade-off is harmonic current drawn from the alternator, which becomes the new thing to check, particularly when drives are a large share of the total load.
Can several motors be started together if the generator is large enough?
Only if the combined locked-rotor kVA still produces an acceptable dip. Sequencing is usually the more economical answer.
Reference Sources
