Starting a three-phase motor on a generator is usually filed as a sizing question. It is a buying question. The set has to deliver the motor’s starting demand rather than its running load, and that demand can be several times the steady figure. But on most sites, cutting the demand with a starter costs less than buying a set large enough to absorb it.
“Buy a bigger generator” is the most expensive sentence in this field, and it’s often the wrong purchase.
You already know the shape of the trap. The load list adds up to one number, the set is ordered against it, and the first motor start trips the whole plant. What follows is the part that is harder to find. We will cover what the set actually does during a start, the changeover that trips more installations than the initial surge does, the arithmetic that compares a starter against generator capacity, and what to do on the day a start still fails.
We build three-phase sets up to 3,000 kW, and we regularly tell buyers they need a smaller one.
Send us the motor nameplate and your start method, and we will tell you whether the set or the starter is the cheaper fix. Talk to our engineering team →
Key Takeaways
- The starter is usually the cheaper purchase: soft starters run roughly 30-60% below a comparable VFD, and a fraction of the price of the generator capacity they save.
- Voltage dip is the real limit, not current. Most industrial loads tolerate 15-20%; past roughly 35% contactors drop out.
- The star-delta changeover can produce a second inrush larger than the first. This is what trips marginal sets.
- Starting the largest motor first and shedding other load costs nothing and can remove a set size.
- Sometimes the honest answer is a bigger set. This article tells you when.
Why Starting a Three-Phase Motor on a Generator Is a Buying Decision
The sizing arithmetic is public and short, and it is not repeated here. Our three phase diesel generator guide works through the starting kVA calculation, the NEMA code letter, and what each starting method does. If you have not done that arithmetic yet, start there.
This article picks up where it left off. Once you know the starting demand, you have two ways to satisfy it. You can buy a set that absorbs the surge, or you can reduce the surge and buy a smaller set.
Those two routes have different prices, and the difference is often larger than the price of the starter. The rest of this article is the comparison.
For the current your switchgear must carry during a start, our three-phase generator amps guide has the figures.
What Happens When You Start a Three-Phase Motor on a Generator
In practice, the set does not simply see a bigger load. It sees a step, and how it answers that step decides whether the start succeeds.
How Much Voltage Dip Is Acceptable When Starting a Motor?
Voltage at the terminals falls sharply for the first moments of a start, then recovers. The depth is the number that matters.
- 15-20% is the usual working limit for general industrial loads
- About 10% when sensitive electronics share the bus
- Past roughly 35%, magnetic starters may drop out rather than ride through
Recovery matters as much as depth. The set must return to at least 90% of rated voltage with the load still applied, which puts the sustained dip below 10% (Buckeye Power Sales). A deep dip that snaps back in a second is survivable. A shallow dip that lingers is not, and how quickly the set recovers is a property of the alternator, covered in our generator alternator specifications guide.
These figures are the reason the “rules of thumb” you will find disagree with each other. Every one of them is a voltage-dip limit wearing a disguise, and the correct limit depends on what else is on your bus.
When the Starter Drops Out Instead of the Set
A deep dip does not always stall the motor. It can drop out the starter.
Contactors held closed by magnetism release when the voltage collapses far enough. The motor then loses its supply mid-acceleration, coasts, and either restarts when the voltage returns or sits stalled drawing locked-rotor current. Mechanically-held starters avoid this, which is why they cost more and why they are specified on hard-starting loads.
The same chatter appears at the load end when cables are undersized, but the cause is different. That is a conductor problem, and our generator set electrical connection guide covers it. This article is about the dip at the set.
The Changeover Most People Miss
Here is the failure that catches people, and it is rarely written down.
A star-delta starter runs the motor on a reduced-voltage connection until it approaches speed, then switches to the running connection. On the common open transition version, the motor is briefly disconnected during that switch.
For that instant, the motor is a spinning generator with no supply. When the contacts close again, it can be reconnected out of step with the supply, and the resulting current can exceed the initial start. The set that survived the start then trips on the changeover.
A closed-transition starter keeps the motor connected throughout and avoids it. If your set trips a second or two after a successful initial start, suspect the changeover before you suspect the set.
What It Costs to Cut the Starting Demand
What Each Method Costs to Buy
The bands below are market-reported for a motor in the 7.5-37 kW range and vary widely by brand and by the panel work the site needs. Treat them as orders of magnitude, not quotes.
| Method | Typical device cost | Capacity it tends to save |
|---|---|---|
| Direct-on-line | baseline, a contactor and overload | none |
| Star-delta | low, a panel and a timer | moderate |
| Soft starter | roughly $200-800 | large |
| VFD | roughly $400-1,800 | large, plus speed control |
The device price understates the gap in a real installation. A VFD usually needs a line reactor, sometimes a filter and shielded cable, and panel cooling that a soft starter does not. Those adders can push a drive installation 20-40% above the bare unit price, and two to three times a soft starter for the same duty.
What each method does to the starting demand is set out in the manufacturers’ application data, and Cummins’ application manual documents the reducing factors.
The comparison that matters is not soft starter against VFD, though. It is either of them against the generator capacity they remove.
The Set You Get to Shrink
Reducing starting current does not just change the starting figures. It can change the machine you buy.
Take an illustrative case. A motor with a large direct-on-line starting demand pushes the required set into the next size band. A soft starter cuts the surge substantially, and the same motor now fits a set one band smaller. If that step is worth more than the starter costs, the starter is free.
This is the complete calculation process; it is worth working it out on paper before placing an order. A case study involving a water treatment project illustrates a comparison of similar options on a larger scale: for the same startup conditions, the generator-based solution costs 180,000, whereas the soft starter solution costs only 75,000.
Dmitri priced a standby set for a grain dryer with a single large fan motor. The direct-on-line start demanded a set two bands above the running load, and the quote for it stalled the project. A soft starter on that one motor brought the required set back down, and the combined package came in below the larger generator alone. Nobody had offered him that option, because the generator supplier was not selling starters.
How Many Starts Before the Starter Pays
Take the extra cost of the larger set and divide it by the annual cost the starter alternative introduces. A soft starter costing a few hundred dollars, against a set size worth several thousand, pays back on the first day, because the generator is bought once and so is the starter.
The calculation flips when you have many motors. One starter per motor adds up quickly, and a slightly larger set may then be the cheaper route.
Starting Several Three-Phase Motors on One Generator
Sequence and Load Shedding
The sequence rule is the cheapest capacity you’ll ever buy, because it’s free.
Start the largest motor first, with everything else off the bus, then bring the rest on one at a time. Simultaneous starts add their surges together and demand a set size for a moment that lasts a second, as the FHWA sizing guide works through. Staggered, the set only ever sees the largest single start plus the running load.
Load shedding is the same idea applied on purpose. Dropping a non-essential load for the seconds a big motor accelerates frees capacity you already own, and it costs nothing but a control signal.
One caution on sequence. A set that starts a motor once may not start it twice in quick succession, because the engine needs time to recover its speed and the alternator its voltage. Leave a gap between starts, and check the set has recovered before the next motor comes on.
Nadia ran a materials handling site where two conveyors started together and tripped the set most mornings. The fix was a sequencing timer and a shed non-essential load, with no capital spend at all. The set that “needed replacing” did not need replacing.
Starting Techniques That Cost Nothing
Before any capital is committed, check what the driven equipment can do for you.
- Start the motor unloaded. A compressor with an unloader valve, or a machine with a clutch, starts against almost no load. Starting torque demand falls with it.
- Disengage the driven equipment for the first run, then connect it once the set is proven.
These cost nothing and sometimes remove the problem entirely. A compressor that starts against a closed unloader valve is a different proposition from one that starts against full pressure.
Our phase converter vs generator comparison covers the wider route decision if you are weighing alternatives to a three-phase set.
When It Still Will Not Start
Work through the symptom rather than guessing:
| What you see | What it usually means | Check first |
|---|---|---|
| The set trips the instant the motor is switched on | The starting demand exceeds what the set can deliver | Measure the voltage at the terminals during the start |
| The set trips a second or two after a good start | Star-delta changeover, where the open transition can produce a second inrush | Move to a closed-transition starter, or check the changeover timing |
| A contactor chatters and the motor never reaches speed | The dip is deeper than the starter can ride through | Fit a mechanically-held starter, or reduce the starting demand |
| It starts once, but not on the next attempt | Not enough recovery time between starts | Leave a gap, and confirm the voltage has recovered |
If the starting demand exceeds what the set can deliver, no sequencing or starter trick will fix it. That answer is real, and sometimes it’s the right one.
Rafael specified a set for a sawmill from a load list that assumed a soft start. The soft starter was value-engineered out during procurement, and the set could not start the main motor direct-on-line. The honest fix was the starter the specification had called for, but a larger set would also have solved it, at several times the cost.
Our diesel generator range covers the sizes these decisions usually land in.
Frequently Asked Questions
Is it cheaper to buy a soft starter or a bigger generator?
Usually the soft starter. A soft starter for a small industrial motor costs a few hundred dollars, while the generator capacity it can remove is worth considerably more, and the generator is bought once. The exception is a site with many motors, where one starter per motor adds up and a slightly larger set can be the cheaper route.
Why does my generator trip when a motor starts?
Most often the voltage dip is deeper than the bus can tolerate, and the contactor drops out rather than the set stalling. On a star-delta starter, the trip may come at the changeover instead, where an open transition can produce a second inrush larger than the initial start. Measure the voltage at the set during the start to tell the two apart.
Can I start a large motor on a small generator?
Sometimes, and the start method is what decides it. A reduced-voltage starter or a drive cuts the starting demand enough that a much smaller set can bring the motor up, particularly if the motor starts unloaded. Where the set cannot deliver the starting demand even with a starter, no sequence or setting will rescue it.
What is the cheapest way to start a motor on a generator?
Start it unloaded, start it before anything else on the bus, and stagger any other motors after it. Those three cost nothing. If the set is still marginal, a soft starter is usually the cheapest equipment fix, and it can remove a set size rather than merely managing the surge.
Conclusion
Starting a three-phase motor on a generator comes down to one comparison, and it is a money comparison. You can buy a set large enough to absorb the starting surge, or you can spend less on equipment that reduces the surge and buy a smaller set.
The voltage dip sets the limit, not the current. Keep it inside what the bus can tolerate, watch the star-delta changeover, and stagger the motors so the set never sees two starts at once. Those cost nothing and they solve more cases than most people expect.
Where they do not, a soft starter usually costs less than the capacity it saves. And where even that will not do, a larger set is the honest answer, not a sales failure.
