Documented Motor Starting-Current Arithmetic Worksheet
Calculate balanced three-phase starting line current from either a documented current ratio or documented starting apparent power. The worksheet does not assign DOL,star-delta,soft-starter or VFD multipliers and does not estimate rated current from guessed power factor or efficiency.
Document the starting quantity
Use voltage,current ratio and apparent power for the same motor,connection,supply and starting configuration. Define whether the source quantity is locked-rotor,maximum RMS,ramp-limited or measured over another interval.
Starting line current
Starting apparent power
Rated line current
Start/rated ratio
Line-to-line voltage
Calculation basis
Configuration record
First-cycle peak
Starting duration
Equations and scope
| Documented basis | Equation | Required evidence |
|---|---|---|
| Current ratio | Istart=k·Irated Sstart=√3 ULL Istart/1000 | Rated line current and applicable RMS start/rated ratio for the same configuration. |
| Starting apparent power | Istart=Sstart·1000/(√3 ULL) | Balanced three-phase starting kVA and line-to-line RMS voltage. Rated current is optional and used only to derive a ratio. |
These are general arithmetic relations,not a standard-assigned DOL,Y-Δ,soft-starter or VFD multiplier. Apparent power does not by itself state power factor,active power,torque,duration or first-cycle peak.
Standards and source boundary
IEC60034-12:2024,Edition4 specifies starting-performance parameters for eight designs of single-speed three-phase50/60Hz cage induction motors up to1000V,intended for DOL or star-delta starting and based on S1 duty. Its exact design values,test methods and tolerances are NEEDS_LICENSED_SOURCE;the public scope does not justify universal6–8× or2–3× values.
IEC60034-1:2026,Edition15 is the current motor rating/performance standard. IEC61800-2:2021,Edition3 treats an adjustable-speed AC power drive as a motor/converter/control/protection system with selected performance and functional attributes. IEC61800-5-1:2022,Edition3 covers PDS electrical,thermal,fire,mechanical and energy hazards;this page is not a safety design.
NEMA locked-rotor code-letter ranges were removed because no current licensed NEMA MG1 edition and exact table were verified. Do not infer a letter midpoint or current range from an unsourced online table.
Interpretation safeguards
- DOL current depends on motor locked-rotor characteristics,voltage/frequency,connection and supply impedance;it is not universally6–8× rated current.
- Star-delta behavior requires a motor intended to run in delta at the line voltage and a compatible load/transition. Current and torque relations do not prove successful acceleration.
- Soft-starter current depends on voltage/current-limit,ramp,kick-start,bypass and load settings. VFD current depends on the complete PDS control,limits,speed/torque curve and overload rating.
- Starting current varies with time. RMS locked-rotor,current-limit,maximum-cycle and first asymmetric peak values are not interchangeable.
- Rated current should come from the applicable manufacturer rating. Reconstructing it from output power with guessed efficiency and power factor compounds uncertainty.
What was corrected
The former page assigned default current multipliers7,2.5,3 and1.2 to DOL,star-delta,soft starter and VFD,then presented generic use/power ranges and torque claims as if they selected the method. Those values were not tied to a motor design,connection,supply,starter settings or licensed table.
Defaults,presets,guessed rated current from output/PF/efficiency,method auto-fill,NEMA code-letter example,URL/history persistence,automatic FAQ advice and unsafe protection links were removed. The replacement accepts only documented start quantities,uses strict parsing and reports no peak,duration,torque or protection result.
