The third pump killed it.
3:40 on a Tuesday afternoon. A municipal water plant — the kind with three 90 kW booster pumps in duty/standby/assist rotation — fed by a 200 kVA standby genset I’d signed off on six months earlier. Pumps one and two were already running. Then the level switch called pump three, direct-on-line, and the whole plant went dark. Not a fault. Not a failure. The generator did exactly what the physics said it would.
The load math said we were fine: 150 kW of running load on a 160 kW machine. The load math lied — because the load math ignored the first four seconds. Motor starting is what really sizes a diesel generator. The running load is the easy part.
Most generator quotes start with a load list. Add up the kW, multiply by 1.25, pick the next frame size up. That works for lighting, heaters, ovens — loads that draw what the plate says from the first instant. Motors don’t. For the first few seconds, a motor isn’t a 90 kW load. It’s a 600 kVA problem wearing a 90 kW nameplate.
Here’s why. A motor at standstill has no back-EMF — nothing pushing back against the supply — so the windings draw locked-rotor current, typically six to eight times full-load current, until the rotor gets up to speed. On the utility grid, nobody notices; the grid is an ocean and your motor is a bucket. On a genset, that inrush is a near-short-circuit the alternator has to ride through. Terminal voltage dips. The governor stumbles as the engine takes the hit. And if the dip goes deep enough, breakers trip — or contactors drop out and chatter, which is worse. I’ve watched a 400-volt system sag to barely 310 and sit there for the four longest seconds of a commissioning day.
The honest number is locked-rotor kVA. Every motor nameplate carries a NEMA code letter, and that letter translates to kVA per horsepower at locked rotor. Multiply it out and you get the starting kVA the generator actually has to deliver. No code letter handy? For a direct-on-line start, six to seven times the motor’s kW gets you close enough to argue with. One thing most load lists never show: that starting kVA is what the alternator feels, not the engine. The engine cares about kW. The alternator cares about kVA and the voltage dip — two different machines inside one frame, with two different opinions about your motor.
I learned this the expensive way on a grain-handling site years ago. The previous engineer had sized a 250 kVA set for “180 kW of motors.” Looked generous on paper. First start of the 110 kW main conveyor — direct-on-line — the voltage dipped past 25%, the undervoltage relay tripped the main breaker, and the whole line sat dead while the farmer stared at me like I’d sold him a tractor with no wheels. The locked-rotor kVA of that one motor was bigger than the entire running load. We ended up with a 400 kVA set and a soft starter. His money, my embarrassment. I checked the code letter on every motor plate for a year after that.
The starter bolted onto the motor decides how big the generator has to be — sometimes more than the motor itself does. Here’s the honest comparison, from harshest to kindest, the way the generator sees it.
Direct-on-line: the cheapest starter and the generator’s most expensive friend. Full voltage slams into a stalled rotor — six to eight times current, full starting torque, over in a few seconds if the generator survives it. For the genset, the working rule is roughly 2.5 to 3 times the motor’s kW in generator kVA to hold the dip under 20%. Alternator makers publish a “maximum motor starting kVA” for each frame, and a permanent-magnet excitation system or a deliberately oversized alternator buys dip margin the engine alone can’t give you. DOL is perfectly fine when the motor is small relative to the set — say under a third of the generator’s kVA. Beyond that, you’re gambling the whole plant’s voltage on every start.
Star-delta: the old-school compromise that only works when nobody’s watching the torque. Start in star at 58% voltage and the current drops to a third — and so does the torque, because physics doesn’t do discounts. Once the motor is near speed, it slams into delta for full voltage. On paper the generator sees a gentle two-to-two-and-a-half-times current start. In practice, two gotchas. First, a third of the torque means the motor must start essentially unloaded — a fan, a pump with the discharge valve shut. Put star-delta on a loaded conveyor and the motor stalls in star; then the delta transition hits almost as hard as a direct-on-line start, except now everybody’s surprised. I watched exactly that at a quarry: the transition spike tripped the genset breaker and the operator blamed the generator for a week. The generator was innocent. The application was guilty. Second, open-transition starters briefly disconnect the motor entirely, and the reconnection surge can be ugly. It’s a fine cheap trick for unloaded starts. For anything that starts under load, it’s worse than useless — it’s deceptive.
Soft starter: the middle ground most big motors on gensets should probably live in. It ramps the voltage up over a few seconds, holding current to typically two to four times full load, with torque following the ramp. The generator sees a controlled shove instead of a punch, and sizing gets friendlier — roughly 1.5 to 2 times the motor kW in genset kVA, depending on the current limit you set. A bypass contactor shorts the electronics out at full speed, so it runs clean afterward. The catch I keep running into is ramp time. Set it long to be gentle — twelve, fifteen seconds — and the generator sits in a voltage dip for the whole ramp while the governor hunts. I had a 200 kW pump on a 350 kVA set with the ramp at fifteen seconds “to be safe.” Voltage sagged to 340, frequency wobbled, and the engine sounded like it was gargling. Cut the ramp to six seconds and the thing started like it meant it. Gentle is good. Endless is not.
VFD: the kindest start imaginable, with a catch that shows up after the start. A variable frequency drive spins the motor up from zero hertz — current barely above running current, full torque available from standstill. From the generator’s perspective, the motor practically doesn’t exist during starting. So why doesn’t everyone do this? Two reasons. First, a VFD is a nonlinear load: it draws current in gulps, distorting the generator’s voltage waveform with harmonics. On a stiff utility, nobody cares. On a genset — especially a small one — that distortion can confuse the AVR, overheat alternator windings, and make the voltage wander for no reason anyone can see on a meter. The fix is to oversize the generator, commonly 1.5 times the drive kVA and sometimes double, or pay for a 12-pulse or active-front-end drive that draws cleaner current. (We go deeper on what harmonics do to generators here.) Second, money: a big VFD with line reactors can cost more than upsizing the generator one frame size. My take — and the old-timers at my shop argue about this over lunch — for one or two large motors, a VFD is brilliant: soft start and speed control in one box. For a plant full of motors, do the math honestly. Sometimes a bigger genset with plain starters is cheaper than a wall of drives.
Here’s the number that actually runs the sizing: how much voltage dip you’ll tolerate while that motor starts. Most plants live fine with 15 to 20% — the lights flicker, nobody dies. But contactors drop out somewhere around 20 to 25% dip and chatter back in, which is how one motor start becomes a cascade. Sensitive controls, UPS systems sitting on bypass, anything with a picky undervoltage relay — they set a tighter budget, sometimes 10%. So the real sizing sequence goes like this: locked-rotor kVA of the largest motor with its starter factored in, plus whatever base load is already running, checked against the dip the plant can survive. Then — the step that gets skipped most often — ask the genset supplier for the alternator’s maximum motor-starting kVA at your dip budget. Reputable manufacturers publish it as a curve. If your supplier can’t produce that number, you’re not buying engineering. You’re buying a box.
A quick worked example, because rules of thumb deserve at least one real number. Take a 110 kW motor — call it 150 HP — with a code letter G, roughly 6 kVA per HP at locked rotor. Direct-on-line, that’s about 900 kVA of starting demand. Put a soft starter on it limited to three times full-load current and you’re looking at roughly 450 to 500 kVA equivalent. Add 200 kVA of base load already running, and the set has to ride through around 700 kVA momentarily at an acceptable dip — which lands you in 500 to 600 kVA genset territory, verified against the alternator’s starting curve. That is why “1.25 times the running kW” fails motors so reliably. It was never the running that mattered.
Next time someone hands you a genset quote built from a load list, ask this: how does the biggest motor start? If the answer is a shrug, the quote is a guess. Direct-on-line, star-delta, soft starter, VFD — each one writes a different generator size next to the same motor, and the difference can be a full frame size or two. The starter is part of the sizing. Treat it as an afterthought and you’ll do the locked-rotor math at 3:40 on a Tuesday afternoon, in the dark, instead of at a desk six months earlier.
Size for the start and the running takes care of itself. The other way around is how plants end up staring at a dead switchboard wondering why the “big enough” generator wasn’t. If you’ve got a motor list and a generator to size — or a site where the lights brown out every time the big pump kicks in — talk to our team. We’d rather do the starting math now than the apology math later. And if you’re still building the load list from scratch, our sizing walkthrough and the prime-versus-standby guide cover the rest of it.
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