In short. A motor started direct-on-line draws six to seven times its running current at very low power factor, and the alternator’s voltage dips by an amount set by starting kVA, transient reactance and how fast the excitation system responds. A dip beyond about 20 percent risks dropping contactors and stalling the motor. The alternator, not the engine, is usually the limit in the first second, and a larger frame or PMG machine is often the cheapest fix.
What happens in the first second
At the instant a motor is switched on, its rotor is stationary and it looks to the supply like a transformer with a shorted secondary. The current is the locked-rotor value, typically six to seven times full-load current for a standard induction motor started direct-on-line, at a power factor of 0.2 to 0.4. Nearly all of it is reactive, and reactive current is what pulls an alternator’s voltage down, because it acts directly through the machine’s reactance. The voltage falls within a cycle, the regulator drives the exciter towards its ceiling, and over the next few hundred milliseconds the voltage recovers as excitation builds and the motor accelerates. Only as the motor approaches speed does its power factor rise, at which point the engine sees the real-power demand and the governor has its own, slower, transient to manage. The engine side is covered under frequency dip; the alternator side is the subject here.
Estimating the dip
A first estimate of the voltage dip uses the alternator’s transient reactance and the motor’s starting kVA.
ΔV ≈ (X’d × Sstart ÷ Salt) ÷ (1 + X’d × Sstart ÷ Salt)
Here ΔV is the initial voltage dip as a fraction of nominal, X’d is the alternator’s per-unit transient reactance on its own base, Sstart is the motor’s starting kVA and Salt is the alternator’s rated kVA. As a worked example (illustrative), a 55 kW motor at 0.85 power factor and 92 percent efficiency draws about 70 kVA at full load; with a locked-rotor multiple of 6.5 its starting kVA is about 455 kVA. On a 500 kVA alternator with X’d of 0.20, the product is 0.20 × 455 ÷ 500 = 0.182, and the dip is 0.182 ÷ 1.182 ≈ 15 percent. On a 320 kVA alternator with the same reactance the product is 0.284 and the dip is about 22 percent, enough to worry about. The estimate ignores regulator response and pre-load, and should be checked against the manufacturer’s motor-starting curves or a transient study for anything marginal. Our starting kVA guide explains how to find the starting multiple from the motor nameplate or code letter.
How much dip is acceptable
| Dip at the switchboard | Typical consequence | Usual verdict |
|---|---|---|
| Up to 10 percent | Barely visible, lighting flickers briefly | Acceptable for all loads |
| 10 to 20 percent | Noticeable flicker; most contactors and controls ride through | Acceptable for industrial sites; check sensitive electronics |
| 20 to 30 percent | Some contactors release, UPS may transfer to battery, motor torque falls to half | Marginal; acceptable only with agreement and testing |
| Above 30 percent | Contactors drop out, motor may stall, under-voltage protection operates | Not acceptable |
Motor torque falls with the square of voltage, so at 80 percent voltage the motor develops only 64 percent of its torque at every speed. A loaded conveyor, a crusher or a positive-displacement pump may fail to accelerate at all, drawing locked-rotor current until its overload trips, while a centrifugal pump or fan starting against a closed valve accelerates with ease. The load’s torque curve is as important as the motor’s current. The dip criteria are discussed under voltage dip.
Alternator choices that reduce the dip
- A larger alternator frame on the same engine. Starting kVA is reactive and demands little engine power. Raising Salt from 500 to 625 kVA in the example above, keeping the same engine, cuts the dip from 15 to about 13 percent, and a frame with lower transient reactance does better still. This is the standard remedy and costs far less than a larger engine.
- Excitation support. A permanent magnet generator or auxiliary-winding machine keeps full regulator power through the dip and reaches field ceiling faster, so the recovery is quicker and the minimum voltage is less deep. The architectures are compared under PMG and excitation.
- Lower transient reactance. Within a manufacturer’s range, frames differ in X’d, and the motor-starting curves published for each frame show the starting kVA it will accept for a given dip. Reactance values and their use are explained under reactance and short-circuit performance.
Reducing the demand instead
Where the alternator cannot be changed, the starting event can be made smaller. A star-delta starter reduces starting current and torque to a third of the direct-on-line values, with a transient at the changeover that can itself cause a dip. A soft starter limits current to a set multiple, commonly two and a half to four times full load, at the price of reduced starting torque and a longer acceleration. A variable frequency drive starts the motor at little more than full-load current, but introduces harmonic current that the alternator must be sized for, as discussed under alternators and non-linear loads. Sequencing is the cheapest measure of all: start the largest motor first, when the set carries the least load and has the most margin, and interlock large motors so that two cannot start together. On an automatic changeover, the controller’s load-step sequence should be designed with this in mind rather than reconnecting every feeder at once.
Testing the installation
A calculated dip is a prediction; the commissioning test is the proof. Record voltage and frequency at the switchboard with a recorder that resolves individual cycles, start the largest motor with the agreed pre-load connected, and note the minimum voltage, the recovery time to within 5 percent of nominal and the behaviour of every contactor and drive. The motor must start against its normal load; a test with a different starting method, a different pre-load or an unloaded shaft does not represent the installation.
Common mistakes
- Sizing the set on running kVA and discovering the starting problem at commissioning; the only remedies then are a larger alternator or a change of starter.
- Using the motor’s running current multiplied by a guessed factor; the nameplate locked-rotor current or code letter gives the real starting kVA, which can differ by 30 percent.
- Buying a larger engine to cure a voltage dip; the dip is an alternator and excitation problem, and the engine helps only with frequency.
- Testing with the motor uncoupled; the current falls quickly on an unloaded shaft and the test proves nothing.
What to send us
Send the motor nameplate with kW, full-load current, locked-rotor current or code letter, and the starting method; a description of the driven load and whether it starts loaded or unloaded; the alternator nameplate and, if available, its transient reactance; and the load that will already be connected when the motor starts. Send your application details and we will give you a dip estimate and a recommendation on alternator frame and excitation, or use the motor-starting generator capacity calculator for a first pass.
Questions engineers ask
Why can the alternator be the limit when the engine has plenty of power?
Because the starting current is almost entirely reactive and demands little real power from the engine. It flows through the alternator’s reactance and pulls the voltage down; the engine only feels the load as the motor accelerates and begins to do work. An oversized alternator on an ordinary engine is a normal configuration for motor-heavy sites.
How much starting kVA can a generator of a given size accept?
As a rough screening figure, a standard shunt-excited alternator accepts a starting kVA about equal to its own rating for a dip near 20 percent, and a PMG machine with low reactance somewhat more. The manufacturer’s motor-starting curves give the actual figure for each frame and dip, and they should be used for any real decision.
Does a soft starter solve the problem completely?
It reduces the starting current to the set limit, which usually brings the dip within bounds, but it also reduces starting torque in proportion to the square of the current reduction. A load that needs high breakaway torque may not start on a soft starter at all, and a bypass contactor closing at the end of the ramp can give a second, smaller transient.
What about several small motors starting together after a changeover?
Their starting kVA adds. Ten 5 kW motors starting at once look like one 50 kW motor and can dip the voltage as badly. Automatic changeover sequences should stage the reconnection of motor feeders over a few seconds, which the controller or ATS logic can usually be set to do.
Technical content reviewed by Srinivas Reddy, Managing Partner, Reddy Generators. Last updated 19 October 2026.