Generator voltage drop when motor starts is normal, and a dip of 15-20% is usually acceptable on a general industrial bus. The drop happens because the motor pulls five to seven times its running current for the first few seconds, and the alternator cannot hold terminal voltage against that surge. What matters is how deep the dip goes, how fast it recovers, and whether the frequency falls with it.
If you have watched a voltmeter collapse the moment a pump or compressor kicks in, you know the feeling. The lights flicker. A contactor chatters. Sometimes the set trips outright.
Most operators assume the generator is faulty, and most of the time it is not. It is answering a step load as physics requires.
This guide covers what causes the dip, what limits are acceptable, and the one diagnostic split that tells you whether you need a different alternator or a bigger engine.
Puntos Clave
- A 15-20% voltage dip on motor start is normal for general industrial loads; hold it under about 10% when sensitive electronics share the bus.
- The first, deepest part of the dip is set by the alternator’s subtransient reactance, before the AVR or the engine has responded to anything.
- If voltage and frequency both fall, the engine is losing speed and no AVR adjustment will fix it. If voltage falls and frequency holds, the problem is on the excitation side.
- Motor starting kVA comes from the nameplate locked-rotor current, or from the NEMA code letter when starting current is not printed.
- Measure voltage and frequency together during a start. That single reading separates an engine problem from an alternator problem.
What Causes Generator Voltage Drop When a Motor Starts?
Voltage falls because current rises faster than the alternator can respond. An induction motor at standstill has no back-EMF to oppose the supply, so it briefly behaves like a short circuit. Every generator voltage drop motor starting case traces back to that one fact, and everything else follows from it.
The Motor Asks for Five to Seven Times Its Running Current
A motor running at full speed draws its nameplate current. The same motor at standstill draws corriente de rotor bloqueado, typically five to seven times higher. On a large direct-on-line start, that surge can be the biggest load the generator ever sees, larger than the entire running load that follows.
That surge flows through the alternator winding, and terminal voltage falls by surge current times winding impedance.
Subtransient Reactance Sets the First, Deepest Part of the Dip
The first few milliseconds of the dip are governed by subtransient reactance, written X″d. This is the alternator’s effective impedance in the instant after a sudden current change, before the magnetic field has had time to adjust.
Nothing else has acted yet. The AVR has not sensed the drop, the exciter has not increased field current, and the engine governor has not added fuel. The dip in the first cycle is almost purely a property of the alternator’s design.
This is why two generator sets of identical kVA rating can behave differently on the same motor. A set with lower subtransient reactance holds a shallower dip at the same load, and the nameplate rating does not tell you this. It is also the first thing to check when you ask why does generator voltage drop more on one machine than another of the same size.
The AVR and Excitation Control Recovery, Not the Initial Depth
Once the dip is detected, the automatic voltage regulator raises field current and terminal voltage climbs back. How fast depends on the excitation system.
A shunt-excited alternator draws its field power from the main stator output, so its excitation sags exactly when the output voltage sags. An AREP system uses separate auxiliary windings and holds up better. A permanent magnet generator (PMG) has an independent field source and recovers fastest. Our guide to Especificaciones del alternador del generador Los compara.
La consecuencia práctica: excitation type changes the recovery, not the first dip. If your dip is too deep, a better excitation system helps less than you would hope. If your recovery is too slow, it helps a great deal.
The Grain Dryer That Needed a Starter, Not a Set
Aniko specified a standby set for a wastewater lift station with two 75 kW pumps on direct-on-line starting. Sizing on the running load alone gave 200 kVA, which could not survive the second pump starting while the first was already running.
Adding the starting surge pushed the requirement to 400 kVA and doubled the budget. Fitting soft starters to both pumps brought it back to 250 kVA. The starters cost a fraction of the 150 kVA they removed, and the station has run on that set since.
Measuring a start? Our engineering team can review your motor nameplate data and generator spec together and show you whether capacity or starting method is the cheaper answer. Send us the details →
What Is an Acceptable Generator Voltage Drop When a Motor Starts?
For general industrial loads, a dip of 15-20% is the usual working limit. Tighten that to about 10% when sensitive electronics share the bus, and expect magnetic starters to drop out past roughly 35%. There is no acceptable voltage dip generator figure that fits every bus, so start at 15% and tighten only where the load mix demands it.
Recovery matters as much as depth. Voltage should climb back to 90% de la calificación or better while the load stays connected, which keeps the sustained dip under 10%. A deep dip that snaps back within a second is survivable. A shallow dip that lingers is not.
These figures come from generator application practice rather than a single number in a standard. That is why the rules of thumb you find online disagree, and why the correct limit depends on what else runs on your bus. Ventas de energía de Buckeye sets out the instantaneous-versus-sustained distinction in detail, and it remains the clearest treatment of the subject we have found.
| What is on the bus | Working dip limit | Por qué |
|---|---|---|
| General industrial loads, contactors | 15-20% | Contactors ride through; motors keep accelerating |
| Sensitive electronics, PLCs, UPS | sobre 10% | Power supplies and controls reset below this |
| Magnetic starters only | hasta aproximadamente un 35% | Beyond this the contactor releases instead of riding through |
| Any load, sustained | menos del 10% | The set must recover to 90% with load applied |
Instantaneous and Sustained Dip Are Two Different Numbers
A single dip has two measurements, and confusing them causes most arguments about whether a set is adequate.
Instantaneous voltage dip is the lowest point reached, lasting milliseconds. Sustained voltage dip is where voltage settles once the transient passes. A voltage dip when motor starts has both, and you should log both.
A set can have a deep instantaneous dip and an acceptable sustained one. That is normal. A sustained dip that never recovers means an excitation problem, and it will damage equipment even though the start looks gentle.
Why a Deep Dip Trips the Starter Instead of Stalling the Motor
A deep dip does not always stall the motor. Sometimes it takes the starter out instead. A contactor’s coil holds the circuit closed, and below a certain voltage the coil lets go, so the motor never gets a chance to accelerate.
This is a common misdiagnosis. The operator sees the set trip and assumes it is undersized, when often the running load is fine and the contactor simply could not hold through the start.
Does the Frequency Drop Too? The Generator Voltage Drop Split That Decides the Fix
This is the diagnostic most guides leave out. When generator voltage and frequency drop together, the fault is mechanical. When only voltage falls, it is electrical. Measure both at once.
If voltage falls and frequency holds steady, the engine is maintaining speed and the problem is electrical: excitation, AVR, sensing, or alternator transient capability. If voltage and frequency fall together, the engine is losing speed under the load step.
Voltage Dips, Frequency Holds: The Excitation Side
Steady frequency means the engine governor is holding rated speed. The engine is not the problem, so adding generator capacity will not help.
Look at the excitation system instead. A shunt-excited alternator on a heavy motor start will sag more than an AREP or PMG unit, and recover more slowly. Check the AVR settings, the sensing leads, and whether the AVR is receiving a clean voltage reference during the dip. On a set with marginal excitation, a PMG alternator can hold a motor that a shunt-excited set of equal kVA cannot start, and that difference never shows on a comparison sheet listing only kVA.
Both Dip: The Engine Is Losing Speed
Frequency is set by engine speed. If frequency falls during the start, the engine slowed, and the electrical side cannot fix that. No AVR adjustment will recover a frequency dip, and chasing it with the regulator can make the voltage problem worse by over-exciting an alternator whose engine cannot carry the load. Check these in order:
- Set undersized for the starting kVA. La causa más común, con mucha diferencia.
- Governor response or droop. A slow governor lets speed sag before it corrects, and how governor class holds engine speed explains what the class rating tells you.
- Fuel or air restriction. Clogged filters limit how fast the engine picks up load.
- Load already on the bus. A set at 70% load has little reserve for a motor start.
A Start That Looked Like an Alternator Fault
Bogdan ran a feed mill with a 250 kVA set and one 90 kW hammer mill motor on direct-on-line start. Every start pulled the lights down hard and the contactor occasionally dropped out, so the maintenance team replaced the AVR. Nothing changed.
A technician finally measured both readings during a start. Voltage fell 22% and frequency fell from 50.0 Hz to 47.8 Hz. The engine was losing speed, so the set was too small for that starting demand, not the alternator weak.
The honest fix was a soft starter on the one motor, which cut the starting demand enough for the existing set to carry it. Replacing the AVR had cost a week of downtime and solved nothing.
How to Measure the Dip During a Start
You cannot choose a fix without a measurement, and a standard multimeter will usually miss the event entirely. The dip peaks and recovers in well under a second, faster than most handheld meters update their display.
Use a meter with min/max capture or a power quality analyser, and record four values in one start: the lowest voltage as a percentage of rated, the tiempo de recuperarse to within 90%, the lowest frequency, y carga en funcionamiento already on the bus.
Measure at the generator terminals, not at the far end of a long cable run. Cable voltage drop adds to the generator’s dip and makes a healthy set look worse than it is, and our guide to conexión eléctrica del grupo electrógeno shows the correct test points. If the reading at the generator is fine but the motor still will not start, the problem is the cable, not the set.
Run the start three times with a recovery gap. If the dip deepens on the second or third, the set has a recovery problem, not a capacity problem.
Cómo calcular la potencia aparente de arranque del motor (kVA)
You need the starting kVA before you can size anything. There are two routes, and the nameplate usually gives you one or the other.
From the Nameplate Starting Current
If the motor nameplate lists locked-rotor or starting amps, the calculation is direct:
Starting kVA = √3 × voltage × starting current ÷ 1000
A 400 V motor drawing 620 A at locked rotor needs √3 × 400 × 620 ÷ 1000, about 430 kVA of starting demand. That number, not the motor’s kW rating, is what the generator sees during the start.
From the NEMA Code Letter
When starting current is not printed, the nameplate usually carries a NEMA code letter. The letter maps to a range of locked-rotor kVA per horsepower, which you multiply by the motor’s rating.
Starting kVA = hp × locked-rotor kVA per hp
The full table runs from code A at 0-3.14 kVA/hp to code V at 22.4 and above, and Caja de herramientas de ingeniería publishes the complete range. A code G motor sits at 5.6-6.29 kVA/hp, so a 100 hp code G motor needs roughly 560-629 kVA to start at full voltage. Two 100 hp motors with different code letters can differ by more than 300 kVA in starting demand, and the nameplate kW looks identical on both.
Why Starting kVA, Not Running kW, Sizes the Set
Running load tells you what the generator must carry continuously. Starting kVA tells you what it must survive. These are different requirements and they can differ by a factor of five.
A set sized on running kW alone will start the load once and trip on the second attempt, which is why sizing a three-phase generator when motors are involved starts from the surge rather than the running load. A set sized on starting kVA carries the running load and has reserve for the next start. Our guide to Arranque de un motor trifásico en un generador works through the capacity and cost comparison, including when a soft starter removes a size band.
How to Reduce Generator Voltage Drop on Motor Start
You have four levers, and they differ enormously in cost. Work down this list before buying a bigger generator.
| Método | Corriente de arranque típica | Costo | Mejor cuando |
|---|---|---|---|
| Directamente en línea | 100% of locked-rotor | Más bajo | Small motor on a large set |
| Estrella-delta | alrededor de un tercio | Bajo | Light starts, low inertia loads |
| Arranque suave | 2-4× running current | Moderado | Most pump, fan and compressor starts |
| VFD | 1-1.5× running current | Mayor | Variable speed is also wanted |
| Generador más grande | sin alterar | Mayor | Starting demand genuinely exceeds the set |
1. Sequence the Starts (Free)
Start the largest motor first, before anything else is on the bus. Then stagger the remaining motors so the set never sees two starts at once. This costs nothing and solves more cases than most operators expect. A set that trips on the third motor start usually has a sequencing problem, not a capacity problem.
2. Reduce the Starting Method’s Demand
Direct-on-line is the worst case. Star-delta cuts starting current to roughly a third, though the open transition can produce a second inrush at changeover that trips a marginal set. Soft starters and variable frequency drives reduce it further and hold it down through the whole acceleration. This is the trade behind every soft starter vs bigger generator decision: the starter costs less, but the set is bought once.
The trade is real. A drive also needs a line reactor, often a filter and screened cable, and panel cooling that a soft starter avoids. Those extras can push a drive install past its headline price.
3. Improve the Excitation (When Frequency Holds)
If the frequency reading stays steady and the voltage still dips too deep, the excitation system is the lever. Moving from shunt to AREP or PMG excitation improves both dip depth and recovery on motor-starting duty. This is the cheapest electrical fix and often overlooked, because buyers compare kVA and ignore excitation type on the spec sheet.
4. Increase Generator Capacity (Last)
A larger set is the honest answer when the starting demand genuinely exceeds what the machine can deliver. It is also the most expensive, because you pay for capacity you will use for a few seconds at a time.
When the Dip Is a Symptom, Not the Problem
A dip beyond these limits is worth diagnosing.
| Lo que ves | Lo que suele significar | Comprobar primero |
|---|---|---|
| Voltage and frequency both fall | Engine losing speed: set undersized or governor slow | Starting kVA against set rating |
| Voltage falls, frequency steady | Excitation or AVR problem | Excitation type and AVR sensing |
| Contactor chatters, motor never reaches speed | Dip deeper than the starter can ride through | Terminal voltage during the start |
| Set trips a second after a good start | Star-delta open transition producing a second inrush | Changeover timing, or move to closed transition |
| Starts once but not on the next attempt | Insufficient recovery between starts | Allow a longer gap, verify voltage has returned |
Two of these rows point at the engine and three at the electrical side. Without the frequency reading you cannot tell which group you are in. If the set also shows unbalanced phase currents, the starting load may not be the only issue, and our guide to Balanceo de carga de generador trifásico covers which limit applies.
Preguntas frecuentes
How much voltage drop is normal when a motor starts on a generator?
A dip of 15-20% is normal and acceptable for general industrial loads. Keep it under about 10% if sensitive electronics share the bus. Voltage should climb back to 90% of rated or better with the load still connected.
Why does my generator voltage drop when the motor starts but the frequency stays normal?
That points to the excitation side, not the engine. Steady frequency means the engine is holding speed, so the cause is the alternator’s transient response, the AVR, or the sensing circuit. A PMG or AREP system will usually hold a deeper motor start than a shunt-excited unit of the same kVA.
Is a voltage drop on motor start a sign the generator is too small?
Not necessarily. A dip within the limits above is normal on a correctly sized set. The set is too small only if the starting kVA exceeds what the alternator can deliver, which usually shows up as frequency dropping as well as voltage, or as the set tripping rather than riding through.
How do I calculate the starting kVA of a motor?
Multiply √3 by the line voltage by the locked-rotor current, then divide by 1000. If starting current is not on the nameplate, use the NEMA code letter: multiply the motor’s horsepower by the locked-rotor kVA per horsepower for that letter.
Does a bigger generator always fix a voltage dip on motor start?
No. If the frequency is dropping too, capacity is the right answer. If the frequency is steady and only the voltage dips, the alternator’s transient response or the excitation system is the limit, and a larger set of the same design may behave no better than the one you have.
Generator voltage drop when motor starts is a physics problem before it is an equipment problem. Measure voltage and frequency together during the start, because that one reading tells you whether capacity or excitation is your limit. Most excessive dips are solved by sequencing starts or fitting a soft starter, both cheaper than the next size of generator.
If you are sizing a set around a difficult motor start, our engineering team can work through the starting kVA and the alternator’s transient capability with you before you commit. Habla con nuestros ingenieros →
