In short. Voltage dip is the momentary fall in generator terminal voltage when a large load, usually a starting motor, is applied. It is set mainly by the alternator’s transient reactance relative to the load step and by how fast the excitation recovers. Most industrial loads tolerate a 15 to 20 percent dip; 30 percent is a common upper limit for motor starting. A larger or lower-reactance alternator, reduced-current starting or staged loading reduces it.
Why the voltage falls
An alternator is a voltage source behind an internal impedance, and when current surges through that impedance the terminal voltage falls. In the first few cycles the relevant impedance is the sub-transient reactance, then for roughly the next second the transient reactance, which for standard industrial alternators lies around 15 to 25 percent on the machine’s own rating. A starting motor drawing a current equal to the alternator’s full-load current therefore pulls the voltage down by a large fraction of that reactance before the automatic voltage regulator has acted.
The AVR then raises field current to restore the voltage. A self-excited alternator that derives field power from its own terminals has less to work with precisely when the voltage has dipped, and may take a second or more to recover. A permanent magnet generator or auxiliary winding supplies full field power regardless of terminal voltage and recovers faster, with a shallower dip for the same step. The PMG and excitation guide explains why two alternators of the same kVA can behave very differently on the same motor.
Estimating the dip
Dip (per unit) is approximately (X’d x S) / (1 + X’d x S), where S = starting kVA / alternator kVA
Here X’d is the alternator’s transient reactance in per unit on its own rating and S is the starting kVA of the applied load expressed as a fraction of the alternator rating. The expression is a screening approximation that assumes a near-zero power factor load applied to an unloaded machine before AVR action. For an alternator with X’d of 0.2 and a motor whose starting kVA equals the alternator rating (S = 1), the initial dip is about 0.2 / 1.2 = 17 percent. For S = 2 the dip rises to about 29 percent, and for S = 0.5 it falls to about 9 percent (illustrative). Manufacturer motor-starting curves, which include excitation response, should replace this estimate for any real selection.
Load already on the set makes matters worse, because the AVR has already used part of its ceiling. Cable impedance between the alternator and the motor adds further drop that the AVR cannot see or correct, and the voltage at the motor terminals, which decides whether it accelerates, is the alternator dip plus the cable drop at starting current.
What dip is acceptable
| Equipment or situation | Typical tolerable dip | Consequence if exceeded |
|---|---|---|
| Standard contactors and relays | 20 to 30 percent for under 1 second | Contactors drop out, other motors trip |
| Starting motor itself | Terminal voltage above about 75 to 80 percent | Insufficient torque, stall or very long acceleration |
| Variable-speed drives and PLCs | 10 to 15 percent, device dependent | Under-voltage fault, process trip |
| UPS input | Set by UPS tolerance window, often plus or minus 10 to 15 percent | UPS transfers to battery, battery cycles on every start |
| Lighting and general loads | Visible flicker above 5 to 10 percent, usually tolerated | Complaint rather than failure |
| ISO 8528-5 class G2 generating set | Transient voltage deviation within about 20 percent on load increase | Performance class not met |
The figures are typical and should be replaced by the actual tolerance of the site equipment. ISO 8528-5 defines performance classes G1 to G4 for the set itself, and a specification should state the class required rather than quoting a dip percentage with no basis. For most industrial plant in India G2 is adequate; sites with sensitive drives, medical equipment or UPS usually need G3.
Reducing the dip
At the alternator
Selecting the next frame size of alternator on the same engine reduces the per-unit size of the load step, which is frequently the least expensive remedy at the specification stage. An alternator with lower transient reactance, or with PMG excitation and a fast digital AVR, improves both depth and recovery. Oversizing the whole set also works but brings the low-load problems covered in the generator sizing guide.
At the load
Reducing the starting current with a soft starter or drive cuts the dip roughly in proportion, as described in the starting kVA guide. Starting the largest motor first on the unloaded set, and holding other motors back by 10 to 30 seconds, means each start sees the alternator at its strongest. Interlocking power factor correction out of circuit on generator removes a source of instability often mistaken for a dip problem.
In the distribution
Short, generously sized cables to the largest motors limit the drop the AVR cannot correct, and a dedicated feeder to a distant motor keeps the dip from reaching other equipment. The voltage drop guide covers the calculation.
Measuring and proving it
A dip claim is only evidence once it has been measured on the installed set under the agreed condition. Record the applied load, the load already on the set and the ambient. Use a power quality recorder with several samples per cycle; a panel voltmeter averages the event away and shows a dip half as deep as the real one. Capture the minimum voltage, the time to recover within 3 percent of nominal and any overshoot on load rejection. Agree the acceptance criteria before the test so the result produces a decision rather than an argument. The frequency dip is recorded at the same time.
A worked example (illustrative)
A cold storage near Shamshabad has a 400 kVA set with a self-excited alternator of X’d near 0.2. Its largest compressor starts DOL at about 280 kVA with 120 kVA of other load already running. Treating the combined step as roughly the alternator rating, the screening estimate gives an initial dip of 15 to 20 percent at the alternator, and the 70 metre feeder adds 6 to 8 percent at starting current, so the compressor terminals see about 75 percent voltage. The compressor accelerates slowly and the PLC occasionally faults on under-voltage. A soft starter at 3.5 times full-load current reduces the event to about 150 kVA, the alternator dip to under 10 percent and the terminal voltage to about 88 percent, and the PLC no longer faults. The actual decision used the alternator’s published starting curve.
Common mistakes
- Quoting a dip percentage without a basis, so each supplier interprets it differently and the installed set meets none of the expectations.
- Reading dip from a panel meter, which averages the transient and reports an acceptable figure while drives and UPS respond to the real one.
- Judging the dip at the alternator terminals only, so the motor at the end of a long cable sees a much lower voltage and stalls.
- Oversizing the entire set when a larger alternator or a soft starter would solve the problem at lower cost and without the low-load penalty.
What to send us
Send the generator nameplate, the alternator type and, if you have it, its data sheet with transient reactance, the starting kVA or motor nameplate of the largest load, the load on the set when it starts, the cable length and size to that load, and the voltage tolerance of any drives, PLCs or UPS on the bus. If the problem already exists, a power quality recording of one start is worth more than any description. Send your application engineering enquiry.
Questions engineers ask
What is an acceptable voltage dip for a generator?
For general industrial load, 15 to 20 percent for less than a second is widely accepted and 30 percent is a common upper limit for motor starting. Sensitive drives, PLCs and UPS may need the dip held to 10 to 15 percent. State the limit in the specification and tie it to an ISO 8528-5 class.
Why does a bigger generator not always fix voltage dip?
The dip depends on the alternator’s reactance and excitation, not only on kVA. A larger set with a low-cost high-reactance alternator can dip more than a smaller set with a well-specified one. Check the alternator data, not just the set rating.
Does PMG excitation reduce voltage dip?
It reduces the depth slightly and the recovery time considerably, because the field power does not collapse with the terminal voltage. It also sustains short-circuit current for protection. For sites with repeated motor starts it is worth specifying.
How is voltage dip different from voltage drop?
Voltage dip is a transient event at the generator caused by a sudden load change and corrected by the AVR within a second or so. Voltage drop is the steady-state loss along a cable at a given current and persists as long as the current flows. Both add together at the motor terminals during starting.
Technical content reviewed by Srinivas Reddy, Managing Partner, Reddy Generators. Last updated 19 October 2026.