Motor Starting Voltage Dip Calculator
Computes the voltage dip at the motor bus and at the motor terminals when a large motor is started across-the-line or on a reduced-voltage starter, using the impedance (voltage-divider) method of IEEE 399 clause 9.6.1a. Results are checked against the critical voltage levels of IEEE 399 Table 9-1 and the accelerating-torque reduction that follows from torque varying as the square of applied voltage.
Impedance (voltage-divider) method and Equations (9-2) through (9-5) from IEEE Std 399-1997 clause 9.6.1; critical voltage levels from Table 9-1; study-threshold guidance from clause 9.2.1. Locked-rotor kVA/hp code letters from NEMA MG 1 §10.37.2; running voltage tolerance of ±10 % from §12.44.1(a); torque varying as the square of applied voltage from §12.44.2. This is a single-instant snapshot at the moment of inrush — it does not model generator exciter/regulator response, transformer tap control, motor slip during acceleration, or reacceleration of other running motors. IEEE 399 clause 9.4 calls for a time-domain study where those effects matter. Verify against motor and transformer manufacturer data and have the final design sealed by a licensed PE.
How the Motor Starting Voltage Dip Calculator Works
This calculator implements the impedance (voltage-divider) method of IEEE Std 399-1997 (the "Brown Book"), clause 9.6.1 — Equations (9-2) through (9-5) — the standard hand-calculation approach for estimating the voltage dip that occurs the instant a large motor is started. It models the utility source, the supply transformer, the motor's locked-rotor impedance, and the feeder between them as a single-instant voltage divider: the motor's high starting current pulls down the voltage across every upstream impedance, and this calculator reports how much of that drop lands at the motor bus versus the motor terminals.
Locked-rotor current can be entered three ways — directly as locked-rotor amperes, as a multiple of full-load amperes, or via the motor's NEMA MG 1 §10.37.2 locked-rotor kVA/hp code letter, which this calculator converts to an equivalent starting current using the motor's rated horsepower and voltage. The result is checked against the critical voltage levels of IEEE 399 Table 9-1, and against the fact that motor starting torque varies as the square of applied voltage (NEMA MG 1 §12.44.2) — so a 20% voltage dip at the motor terminals costs roughly 36% of available starting torque, not 20%.
What This Calculation Does Not Cover
IEEE 399 clause 9.6.1 is explicit that the voltage-divider method is a single-instant snapshot at the moment of inrush. It does not model the utility source's voltage regulator or generator exciter response, transformer automatic tap-changer action, motor slip as it accelerates through the starting transient, or the reacceleration behavior of other motors already running on the same bus when a new motor starts. Clause 9.4 calls for a full time-domain simulation study where any of those effects matter — this calculator is the fast, standards-based first-pass check that tells you whether a full study is warranted.
What Is an Infinite Bus, and When Should You Assume One?
An infinite bus is a modeling simplification: the utility source is assumed to have zero source impedance, so its voltage holds perfectly steady no matter how much current a motor pulls from it during starting. Every real utility connection has some finite fault duty (entered here as Utility Short-Circuit Capacity in MVA) and therefore some source impedance — but when that fault duty is very large relative to the motor being started, the utility's own contribution to voltage dip becomes small enough to ignore. Setting this field to 0 tells the calculator to treat the source as an infinite bus: all of the voltage dip is then attributed to the transformer, feeder, and motor impedance downstream of the utility connection, with none coming from the utility itself.
The infinite bus assumption is reasonable for a first-pass check when the utility's available fault current is large compared to the motor's locked-rotor current — common on strong grid interconnections and larger industrial services — and it's a conservative simplification in the sense that it isolates how much dip your own transformer and feeder design contribute, without also needing utility-specific short-circuit data you may not have on hand yet. It understates total voltage dip whenever the utility connection is actually weak (a long rural feeder, a small standby generator instead of a grid connection, or a heavily loaded substation), which is exactly when entering a real MVA value instead of 0 matters most.
Related Calculators
A motor starting study is typically run alongside a voltage drop check on the feeder itself — see the Voltage Drop calculator. For the DC side of a facility's power system, see the Battery DC Short-Circuit Current calculator. The full suite of eleven free calculators is available from the powerengcalc.com home page.
For engineering consulting, motor starting and power quality studies, or forensic investigations, visit pefgconsulting.com.