Generator Voltage Dip Calculator — Motor…

Subtransient motor-start voltage-dip screening model

Screen the first-cycle voltage sag caused by a sudden motor-start or block-load kVA step. This deliberately uses an entered alternator subtransient reactance Xd″ and starting kVA instead of pretending every generator has the same transient capability.

How to use this calculator

Enter the generator or alternator kVA rating on the same base as the published Xd″.

Enter motor locked-rotor/starting kVA or the applicable sudden block-load kVA.

Enter the alternator manufacturer’s subtransient reactance Xd″ in percent.

Enter nominal terminal voltage and your project-specific maximum initial dip target.

Use the result for screening, then confirm the selected genset against manufacturer motor-starting/load-step curves or sizing software.

Where people use it

•Screening whether an existing genset is likely to tolerate a motor start.
•Comparing alternators with different published Xd″ values.
•Finding the approximate starting-kVA ceiling for an entered voltage-dip target.
•Comparing DOL versus reduced-current starter scenarios after calculating their starting kVA.

Example: 500 kVA generator, 400 kVA start, Xd″ 18%

The starting demand is 0.8 pu. The screening estimate is 14.4% initial dip, leaving about 85.6% voltage. On a 415 V bus that is about 355.24 V. At a 20% target, the same inputs imply about 555.56 kVA maximum starting demand.

Verify final transient performance with manufacturer curves

•This is a first-pass subtransient-reactance screening model, not a manufacturer genset-selection curve. Excitation system, AVR response, saturation, engine response and alternator design affect measured transient performance.
•Use Xd″ on the correct generator rating/base. Do not mix reactance published on a different kVA base without converting it.
•Starting kVA must represent the actual starting method. DOL, star-delta, autotransformer, soft starter and VFD starts can impose very different current and kVA steps.
•The approximation is most useful for initial screening. Large predicted dips require manufacturer transient/load-step analysis rather than extrapolating this linear model.
•Acceptable voltage dip is application-specific. Sensitive controls, contactors, lighting, UPS inputs and other connected loads can impose tighter limits; this calculator does not invent a universal pass threshold.
•This result does not calculate frequency dip, acceleration time, starting torque, cable voltage drop, harmonic effects or protective-device coordination.

Frequently asked questions

Why does a generator voltage dip when a motor starts?+

A motor can demand several times its running current at start. That sudden current produces a voltage drop across the alternator internal reactance before excitation and engine controls recover.

What is Xd double prime?+

Xd″ is the alternator subtransient reactance, normally given in per unit or percent on a stated rating base. It characterizes the very early electrical response and should come from manufacturer data.

Should I use running kVA or starting kVA?+

Use the motor or block load starting kVA for the transient event. Running kVA understates a DOL motor start.

What voltage dip is acceptable?+

There is no universal value for every installation. Set the target from the connected equipment requirements, project criteria and generator manufacturer guidance.

Can I use this to select the final generator model?+

Use it to screen alternatives. Final model selection should use manufacturer motor-starting/load-step curves or validated sizing software because excitation, saturation and recovery behavior are model-specific.

How can I reduce motor-start voltage dip?+

Reduce the starting current/kVA with a suitable starter or drive, sequence loads, choose an alternator with stronger transient capability, or increase generator capacity after checking the complete system.

Semantic next steps

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